| Filesystem Library Version 3

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# Reference Documentation

Headings and synopsis for recently added functionality is show with a green background.

## Table of Contents

Introduction
Definitions
Conformance
Header `<boost/filesystem.hpp>` synopsis
Error reporting
Class `path`
`path` conversions
`path` conversions to native format
`path` conversions to generic format
`path` encoding conversions
`path` requirements
`path` constructors
`path` assignments
`path` appends
`path` concatenation
`path` modifiers
`path` native format observers
`path` generic format observers
`path` compare
`path` decomposition
`path` query
`path` iterators
`path` deprecated functions
`path` non-member functions
`path` inserters and extractors
Class `filesystem_error`
`filesystem_error` constructors
`f` `ilesystem_error` path1
`filesystem_error` path2
`filesystem_error` what

Enum `file_type`
Enum `perms`
Class `file_status`
`file_status` constructors
`file_status-modifiers` observers
`file_status-observers` modifiers
Class `directory_entry`
`directory_entry` constructors
`directory_entry` observers
`directory_entry` modifiers
Class `directory_iterator`
`directory_iterator` members
Class `recursive_directory_iterator`
Operational functions
`absolute
canonical
copy
copy_directory
copy_file
copy_symlink
create_directories
create_directory
create_hard_link
create_symlink
current_path
exists
equivalent
file_size
hard_link_count
initial_path
is_directory
is_empty`

`is_other
is_regular_file
is_symlink
last_write_time
permissions
read_symlink
relative
remove
remove_all
rename
resize_file
space
status
status_known
symlink_status
system_complete
temp_directory_path
unique_path
weakly_canonical
` File streams
Path decomposition table
Warning: Long paths on Windows and the extended-length \\?\ prefix
Acknowledgements
References

## Introduction

This reference documentation describes components that C++ programs may use to perform operations involving file systems, including paths, regular files, and directories.

> C++11 Support This reference documentation is written as if all compilers supported C++11. Where possible, the implementation falls back to C++03 if a C++11 feature is not available.
> C++11 Feature | Action if not supported by compiler
> `noexcept` | Keyword omitted.
> `constexpr` | Keyword omitted.
> R-value references | Function signature omitted.
> New character types | The `boost::filesystem` interface doesn't use the new types directly. It does use `u16string` and `u32string` in namespace `boost`. These are typedefs to `std::u16string` and `std::u32string` for C++11, or to `std::basic_string<boost::u16_t>` and `std::basic_string<boost::u32_t>` for C++03.
> Defaulted and deleted functions | Workaround replacement functions provided.
> Initializer lists | Not currently used.
> Variadic templates | Not currently used.
> Range-based for statements | Supporting functions always provided; they do no harm even for C++03 compilers.

## Conformance [fs.conformance]

### ISO/IEC 9945 conformance [fs.conform.9945]

Some behavior in this reference documentation is specified by reference to ISO/IEC 9945. How such behavior is actually implemented is unspecified.

> [Note: This constitutes an "as if" rule for implementation of operating system dependent behavior. In practice implementations will usually call native operating system API's. —end note]

Implementations are encouraged to provide such behavior as it is defined by ISO/IEC 9945. Implementations shall document any behavior that differs from the behavior defined by ISO/IEC 9945. Implementations that do not support exact ISO/IEC 9945 behavior are encouraged to provide behavior as close to ISO/IEC 9945 behavior as is reasonable given the limitations of actual operating systems and file systems. If an implementation cannot provide any reasonable behavior, the implementation shall report an error in an implementation-defined manner.

> [Note: Such errors might be reported by an #error directive, a `static_assert`, a `filesystem_error` exception, a special return value, or some other manner. —end note]

Implementations are not required to provide behavior that is not supported by a particular file system.

> [Example: The FAT file system used by some memory cards, camera memory, and floppy discs does not support hard links, symlinks, and many other features of more capable file systems. Implementations are only required to support the FAT features supported by the host operating system. —end example]

The behavior of functions described in this reference may differ from their specification in the presence of file system races. No diagnostic is required.

If the possibility of a file system race would make it unreliable for a program to test for a precondition before calling a function described in this reference documentation, Requires is not specified for the condition. Instead, the condition is specified as a Throws condition.

> [Note: As a design practice, preconditions are not specified when it is unreasonable for a program to detect them prior to calling the function. —end note]

### Operating system dependent conformance [fs.conform.os]

Some behavior is specified in this reference documentation as being operating system dependent ([fs.def.osdep]). The operation system an implementation is dependent upon is implementation defined.

It is permissible for an implementation to be dependent upon an operating system emulator rather than the actual operating system.

> [Example: An implementation uses Cygwin, a Linux® API emulator for some Windows® operating system versions. The implementation would define Cygwin as its operating system. Users could refer to the Cygwin documentation to find details of the operating system dependent behavior. —end example]

> It is user and conformance test detectable that such an implementation is running on Cygwin. Users would be misled and conformance tests would fail if the implementation defined Linux or Windows rather than Cygwin as the operating system, since real behavior is a blend of the two.

## Definitions [fs.definitions]

The following definitions shall apply throughout this reference documentation:

### operating system dependent behavior [fs.def.osdep]

Behavior that is dependent upon the behavior and characteristics of an operating system. See [fs.conform.os].

### file [fs.def.file]

An object that can be written to, or read from, or both. A file has certain attributes, including type. File types include regular files and directories. Other types of files, such as symbolic links, may be supported by the implementation.

### file system [fs.def.filesystem]

A collection of files and certain of their attributes.

### filename [fs.def.filename]

The name of a file. Filenames `"."` and `".."` have special meaning. The follow characteristics of filenames are operating system dependent:

- The permitted characters. See [fs.os.examples].

- Specific filenames that are not permitted.

- Additional filenames that have special meaning.

- Case awareness and sensitivity during path resolution.

- Special rules that may apply to file types other than regular files, such as directories.

### path [fs.def.path]

A sequence of elements that identify the location of a file within a filesystem. The elements are the root-nameopt, root-directoryopt, and an optional sequence of filenames. [Note: A pathname is the concrete representation of a path. —end note]

### absolute path [fs.def.absolute-path]

A path that unambiguously identifies the location of a file without reference to an additional starting location. The elements of a path that determine if it is absolute are operating system dependent.

### relative path [fs.def.relative-path]

A path that is not absolute, and so only unambiguously identifies the location of a file when resolved relative to an implied starting location. The elements of a path that determine if it is relative are operating system dependent. [Note: Paths `"."` and `".."` are relative paths. —end note]

### canonical path [fs.def.cannonical-path]

An absolute path that has no elements that are symbolic links, and no `"."` or `".."` elements.

### pathname [fs.def.pathname]

A character string that represents the name of a path. Pathnames are formatted according to the generic pathname grammar or an operating system dependent native pathname format.

### native pathname format [fs.def.native]

The operating system dependent pathname format accepted by the host operating system.

### normal form path [fs.def.normal]

A path with no redundant current directory (dot) or parent directory (dot-dot) elements. The normal form for an empty path is an empty path. The normal form for a path ending in a directory-separator that is not the root directory is the same path with a current directory (dot) element appended.

### link [fs.def.link]

A directory entry object that associates a filename with a file. On some file systems, several directory entries can associate names with the same file.

### hard link [fs.def.hardlink]

A link to an existing file. Some file systems support multiple hard links to a file. If the last hard link to a file is removed, the file itself is removed.

> [Note: A hard link can be thought of as a shared-ownership smart pointer to a file. —end note]

### symbolic link [fs.def.symlink]

A type of file with the property that when the file is encountered during pathname resolution, a string stored by the file is used to modify the pathname resolution.

> [Note: A symbolic link can be thought of as a raw pointer to a file. If the file pointed to does not exist, the symbolic link is said to be a "dangling" symbolic link. —end note]

### file system race [fs.def.race]

The condition that occurs when multiple threads, processes, or computers interleave access and modification of the same object within a file system.

## Generic pathname format [path.generic]

pathname:
root-nameopt root-directoryopt relative-pathopt

root-name:
An operating system dependent name that identifies the starting location for absolute paths.

> > [Note: Many operating systems define a name beginning with two directory-separator characters as a root-name that identifies network or other resource locations. Some operating systems define a single letter followed by a colon as a drive specifier - a root-name identifying a specific device such as a disc drive. —end note]

root-directory:
directory-separator

relative-path:
filename
relative-path directory-separator
relative-path directory-separator filename

filename:
name
`"."`
`".."`

preferred-separator:
An operating system dependent directory separator character. May be a synonym for `"/"`.

directory-separator:
`"/"
"/"` directory-separator
preferred-separator
preferred-separator directory-separator

Multiple successive directory-separator characters are considered to be the same as one directory-separator character.

The filename `"."` is considered to be a reference to the current directory. The filename `".."` is considered to be a reference to the parent directory. Specific filenames may have special meanings for a particular operating system.

## Operating system dependent examples (Informative) [fs.os.examples]

Certain features are specified in this reference documentation as being operating system dependent. The following table shows the application of those specifications for operating systems that use the ISO/IEC 9945 or Windows® application program interfaces (APIs).[footnote1]

Feature | Section | ISO/IEC 9945 POSIX® API | Windows® API | Notes
`path ::value_type` | [class.path] | `char` | `wchar_t` |
`path::preferred _separator` | [class.path] | `'/'` | `L'\\'` (single backslash) |
`path("/") .is_absolute() path("c:/") .is_absolute()` | [path .query] | `true false` | `false true` |
`path` argument disambiguation between generic format and native format | [path.arg .fmt.cvt] | Not required | Not required | There is no need to distinguish between the generic format and native format for these operating systems.
`path` argument format conversion | [path.arg .fmt.cvt] | No conversion performed | No conversion performed | The generic format is already acceptable to the native API of these operating systems.
`path ("/cats/jane") .c_str() path| ("/cats/jane/") .c_str()` | [path.arg .fmt.cvt] | `"/cats/jane" "/cats/jane/"` | `L"/cats/jane" L"/cats/jane/"` | These operating systems accept the same native separator between directory names and a final file name, so no format conversion is performed. Other operating systems might require conversion.
Format conversion by `path` native format observers | [path.native .obs] | No conversion performed | No conversion performed | For efficiency, `path` objects are required to store pathnames in the native format regardless of operating system.
Format conversion by `path` generic format observers | [path .generic .obs] | No conversion performed | Backslashes converted to slashes |
`p. make_preferred()` | [fs.path .modifiers] | No change | Slashes converted to backslashes |
Characters prohibited in filenames | [fs.def .filename] | 0x00, `'/'` | 0x00-0x1F, `'"'`, `'*'`,`'*'`, `'<'`, `'>'`, `'?'`, `'\\'` (single backslash), `'/'`, `'|'` | Many operating systems prohibit the ASCII control characters (0x00-0x1F) in filenames.
Initial imbued `path` locale | [path .imbued .locale] | `std:: locale("")` [footnote 2] | Implementation supplied locale using `MultiByte ToWideChar` and `WideChar ToMultiByte` with a codepage of `CP_ACP` if `AreFileApisANSI` is true, otherwise codepage `CP_OEMCP`.[footnote 3] | Apple OS X®: Implementation supplied locale providing UTF-8 `codecvt` facet.[footnote 4]

[footnote1] OS X® and Windows® are examples of commercially available operating systems. This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of these products.

[footnote 2] Rationale: ISO C specifies `std::locale("")` as "the locale-specific native environment", while ISO/IEC 9945 says it "Specifies an implementation-defined native environment."

[footnote 3] Rationale: This is the current behavior of C and C++ standard library functions that perform file operations using narrow character strings to identify paths. Changing this behavior would be surprising and at variance with existing code, particularly where user input is involved.

[footnote 4] Rationale: Vendor's documentation states "All BSD system functions expect their string parameters to be in UTF-8 encoding and nothing else."

## Header `<boost/filesystem.hpp>` synopsis [filesystem.synopsis]

    namespace boost
    {
      namespace filesystem
      {
        class path;

        bool lexicographical_compare(path::iterator first1, path::iterator last1,
          path::iterator first2, path::iterator last2);
        void swap(path& lhs, path& rhs);
        std::size_t hash_value(const path& p);

        bool operator==(const path& lhs, const path& rhs);
        bool operator!=(const path& lhs, const path& rhs);
        bool operator< (const path& lhs, const path& rhs);
        bool operator<=(const path& lhs, const path& rhs);
        bool operator> (const path& lhs, const path& rhs);
        bool operator>=(const path& lhs, const path& rhs);

        path operator/ (const path& lhs, const path& rhs);

        std::ostream&  operator<<( std::ostream& os, const path& p );
        std::wostream& operator<<( std::wostream& os, const path& p );
        std::istream&  operator>>( std::istream& is, path& p );
        std::wistream& operator>>( std::wistream& is, path& p )

        class filesystem_error;
        class directory_entry;

        class directory_iterator;

        // enable c++11 range-based for statements
        const directory_iterator& begin(const directory_iterator& iter);
        directory_iterator end(const directory_iterator&);

        // enable BOOST_FOREACH
        directory_iterator& range_begin(directory_iterator& iter);
        directory_iterator range_begin(const directory_iterator& iter);
        directory_iterator range_end(const directory_iterator&);

        class recursive_directory_iterator;

        // enable c++11 range-based for statements
        const recursive_directory_iterator&
          begin(const recursive_directory_iterator& iter);
        recursive_directory_iterator
          end(const recursive_directory_iterator&);

        // enable BOOST_FOREACH
        recursive_directory_iterator&
          range_begin(recursive_directory_iterator& iter);
        recursive_directory_iterator
          range_begin(const recursive_directory_iterator& iter);
        recursive_directory_iterator
          range_end(const recursive_directory_iterator&);

        enum file_type
        {
          status_error, file_not_found, regular_file, directory_file,
          symlink_file, block_file, character_file, fifo_file, socket_file,
          type_unknown
        };

        enum perms
        {
          no_perms,
          owner_read, owner_write, owner_exe, owner_all,
          group_read, group_write, group_exe, group_all,
          others_read, others_write, others_exe, others_all, all_all,
          set_uid_on_exe, set_gid_on_exe, sticky_bit,
          perms_mask, perms_not_known,
          add_perms, remove_perms, symlink_perms
        };

        class file_status;

        struct space_info  // returned by space function
        {
          uintmax_t capacity;
          uintmax_t free;
          uintmax_t available; // free space available to non-privileged process
        };

        enum class copy_option
        {
          none
          fail_if_exists = none,
          overwrite_if_exists
        };

        enum class symlink_option
        {
          none
          no_recurse = none,
          recurse
        };

        // operational functions

        path         absolute(const path& p, const path& base=current_path());

        path         canonical(const path& p, const path& base = current_path());
        path         canonical(const path& p, system::error_code& ec);
        path         canonical(const path& p, const path& base,
                       system::error_code& ec);

        void         copy(const path& from, const path& to);
        void         copy(const path& from, const path& to,
                       system::error_code& ec);

        void         copy_directory(const path& from, const path& to);
        void         copy_directory(const path& from, const path& to,
                       system::error_code& ec);

        void         copy_file(const path& from, const path& to);
        void         copy_file(const path& from, const path& to,
                       system::error_code& ec);
        void         copy_file(const path& from, const path& to,
                       copy_option option);
        void         copy_file(const path& from, const path& to,
                       copy_option option, system::error_code& ec);

        void         copy_symlink(const path& existing_symlink,
                       const path& new_symlink);
        void         copy_symlink(const path& existing_symlink,
                       const path& new_symlink, system::error_code& ec);

        bool         create_directories(const path& p);
        bool         create_directories(const path& p,
                       system::error_code& ec);

        bool         create_directory(const path& p);
        bool         create_directory(const path& p, system::error_code& ec);

        void         create_directory_symlink(const path& to,
                       const path& new_symlink);
        void         create_directory_symlink(const path& to,
                       const path& new_symlink, system::error_code& ec);

        void         create_hard_link(const path& to, const path& new_hard_link);
        void         create_hard_link(const path& to, const path& new_hard_link,
                                      system::error_code& ec);

        void         create_symlink(const path& to, const path& new_symlink);
        void         create_symlink(const path& to, const path& new_symlink,
                                    system::error_code& ec);

        path         current_path();
        path         current_path(system::error_code& ec);
        void         current_path(const path& p);
        void         current_path(const path& p, system::error_code& ec);

        bool         exists(file_status s) noexcept;
        bool         exists(const path& p);
        bool         exists(const path& p, system::error_code& ec) noexcept;

        bool         equivalent(const path& p1, const path& p2);
        bool         equivalent(const path& p1, const path& p2,
                       system::error_code& ec);

        uintmax_t    file_size(const path& p);
        uintmax_t    file_size(const path& p, system::error_code& ec);

        uintmax_t    hard_link_count(const path& p);
        uintmax_t    hard_link_count(const path& p, system::error_code& ec);

        const path&  initial_path();
        const path&  initial_path(system::error_code& ec);

        bool         is_directory(file_status s) noexcept;
        bool         is_directory(const path& p);
        bool         is_directory(const path& p,
                       system::error_code& ec) noexcept;

        bool         is_empty(const path& p);
        bool         is_empty(const path& p, system::error_code& ec);

        bool         is_other(file_status s) noexcept;
        bool         is_other(const path& p,);
        bool         is_other(const path& p, system::error_code& ec) noexcept;

        bool         is_regular_file(file_status s) noexcept;
        bool         is_regular_file(const path& p);
        bool         is_regular_file(const path& p,
                       system::error_code& ec) noexcept;

        bool         is_symlink(file_status s noexcept);
        bool         is_symlink(const path& p);
        bool         is_symlink(const path& p, system::error_code& ec) noexcept;

        std::time_t  last_write_time(const path& p);
        std::time_t  last_write_time(const path& p, system::error_code& ec);
        void         last_write_time(const path& p, const std::time_t new_time);
        void         last_write_time(const path& p, const std::time_t new_time,
                                     system::error_code& ec);

        path         read_symlink(const path& p);
        path         read_symlink(const path& p, system::error_code& ec);

        path         relative(const path& p, system::error_code& ec);
        path         relative(const path& p, const path& base=current_path());
        path         relative(const path& p,
                         const path& base, system::error_code& ec);

        bool         remove(const path& p);
        bool         remove(const path& p, system::error_code& ec);

        uintmax_t    remove_all(const path& p);
        uintmax_t    remove_all(const path& p, system::error_code& ec);

        void         rename(const path& from, const path& to);
        void         rename(const path& from, const path& to,
                       system::error_code& ec);

        void         resize_file(const path& p, uintmax_t size);
        void         resize_file(const path& p, uintmax_t size,
                       system::error_code& ec);

        space_info   space(const path& p);
        space_info   space(const path& p, system::error_code& ec);

        file_status  status(const path& p);
        file_status  status(const path& p, system::error_code& ec) noexcept;

        bool         status_known(file_status s) noexcept;

        file_status  symlink_status(const path& p);
        file_status  symlink_status(const path& p,
                       system::error_code& ec) noexcept;

        path         system_complete(const path& p);
        path         system_complete(const path& p, system::error_code& ec);

        path         temp_directory_path();
        path         temp_directory_path(system::error_code& ec);

        path         unique_path(const path& model="%%%%-%%%%-%%%%-%%%%");
        path         unique_path(const path& model, system::error_code& ec);

        path         weakly_canonical(const path& p);
        path         weakly_canonical(const path& p, system::error_code& ec);

      }  // namespace filesystem
    }  // namespace boost

## Error reporting [fs.err.report]

Filesystem library functions often provide two overloads, one that throws an exception to report file system errors, and another that sets an `error_code`.

> [Note: This supports two common use cases:

> - Uses where file system errors are truly exceptional and indicate a serious failure. Throwing an exception is the most appropriate response. This is the preferred default for most everyday programming.

> - Uses where file system system errors are routine and do not necessarily represent failure. Returning an error code is the most appropriate response. This allows application specific error handling, including simply ignoring the error.

> —end note]

Functions not having an argument of type `system::error_code&` report errors as follows, unless otherwise specified:

- When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type `filesystem_error` is thrown.

- Failure to allocate storage is reported by throwing an exception as described in the C++ standard, 17.6.4.10 [res.on.exception.handling].

- Destructors throw nothing.

Functions having an argument of type `system::error_code&` report errors as follows, unless otherwise specified:

- If a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, the `system::error_code&` argument is set as appropriate appropriate for the specific error. Otherwise, `clear()` is called on the `system::error_code&` argument.

- Failure to allocate storage is reported by throwing an exception as described in the C++ standard, 17.6.4.10 [res.on.exception.handling].

## Class `path` [class.path]

An object of class `path` represents a path, and contains a pathname Such an object is concerned only with the lexical and syntactic aspects of a path. The path does not necessarily exist in external storage, and the pathname is not necessarily valid for the current operating system or for a particular file system.

    namespace boost
    {
      namespace filesystem
      {
          class path
          {
          public:
            typedef see below                                    value_type;
            typedef std::basic_string<value_type>                string_type;
            typedef std::codecvt<wchar_t, char, std::mbstate_t>  codecvt_type;
            constexpr value_type                                 preferred_separator;

            // constructors and destructor
            path();
            path(const path& p);
            path(path&& p) noexcept;

            template <class Source>
              path(Source const& source, const codecvt_type& cvt=codecvt());

            template <class InputIterator>
              path(InputIterator begin, InputIterator end,
                const codecvt_type& cvt=codecvt());

           ~path();

            // assignments
            path& operator=(const path& p);
            path& operator=(path&& p) noexcept;

            template <class Source>
              path& operator=(Source const& source);

            template <class Source>
              path& assign(Source const& source, const codecvt_type& cvt)

            template <class InputIterator>
              path& assign(InputIterator begin, InputIterator end,
                const codecvt_type& cvt=codecvt());

            // appends
            path& operator/=(const path& p);

            template <class Source>
              path& operator/=(Source const& source);

            template <class Source>
              path& append(Source const& source, const codecvt_type& cvt);

            template <class InputIterator>
              path& append(InputIterator begin, InputIterator end,
                const codecvt_type& cvt=codecvt());

            // concatenation
            path& operator+=(const path& x);
            path& operator+=(const string_type& x);
            path& operator+=(const value_type* x);
            path& operator+=(value_type x);
            template <class Source>
              path& operator+=(Source const& x);
            template <class CharT>
              path& operator+=(CharT x);
            template <class Source>
              path& concat(Source const& x, const codecvt_type& cvt);
            template <class InputIterator>
              path& concat(InputIterator begin, InputIterator end);
            template <class InputIterator>
              path& concat(InputIterator begin, InputIterator end,
                const codecvt_type& cvt);

            // modifiers
            void  clear();
            path& make_preferred();
            path& remove_filename();
            path& replace_extension(const path& new_extension = path());
            void  swap(path& rhs);

            // lexical operations
            path lexically_normal() const;
            path lexically_relative(const path& base) const;

            // native format observers
            const string_type&  native() const noexcept;  // native format, encoding
            const value_type*   c_str() const noexcept;   // native().c_str()
            string_type::size_type size() const noexcept; // native().size()

            template <class String>
              String  string(const codecvt_type& cvt=codecvt()) const;
            string    string(const codecvt_type& cvt=codecvt()) const;
            wstring   wstring(const codecvt_type& cvt=codecvt()) const;
            u16string u16string() const;
            u32string u32string() const;

            // generic format observers
            template <class String>
              String  generic_string() const;

            string    generic_string(const codecvt_type& cvt=codecvt()) const;
            wstring   generic_wstring(const codecvt_type& cvt=codecvt()) const;
            u16string generic_u16string()
            u32string generic_u32string()

            // compare
            int   compare(const path& p) const noexcept;
            int   compare(const std::string& s) const;
            int   compare(const value_type* s) const;

            // decomposition
            path  root_name() const;
            path  root_directory() const;
            path  root_path() const;
            path  relative_path() const;
            path  parent_path() const;
            path  filename() const;
            path  stem() const;
            path  extension() const;

            // query
            bool empty() const;
            bool has_root_name() const;
            bool has_root_directory() const;
            bool has_root_path() const;
            bool has_relative_path() const;
            bool has_parent_path() const;
            bool has_filename() const;
            bool has_stem() const;
            bool has_extension() const;
            bool is_absolute() const;
            bool is_relative() const;

            // iterators
            class iterator;
            typedef iterator const_iterator;
            class reverse_iterator;
            typedef reverse_iterator const_reverse_iterator;

            iterator begin() const;
            iterator end() const;
            reverse_iterator rbegin() const;
            reverse_iterator rend() const;

            // imbued locale
            static std::locale imbue(const std::locale& loc);
            static const codecvt_type & codecvt();

          private:
            string_type pathname;  // exposition only
          };

      }  // namespace filesystem
    }  // namespace boost

`value_type` is a `typedef` for the character type used by the operating system to represent pathnames.

### `path` Usage concerns [path.usage]

#### Multithreading concerns

Filesystem library functions are not protected against data races. [Modifying an object of a Filesystem library type that is shared between threads risks undefined behavior unless objects of that type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. —end note] [Note: Thus the Filesystem library behaves as if it were part of the standard library, and C++ standard 17.6.4.10 Shared objects and the library [res.on.objects] would thus apply. —end note]

#### Windows concerns

Visual C++ at least through version 2012 does not employ C++11-style static initialization locks, so the initialization of `path::codecvt()` can race, either with itself or `path::imbue::()` if they are called from a different thread. A workaround is to call:

> `path::codecvt(); // ensure VC++ does not race during initialization.`

in the main thread before launching any additional threads. [Note: The obvious fix of the Filesystem implementation doing the locking doesn't work because of unrelated problems with the Microsoft compiler; for static linking the runtime tries to do the initialization before main() starts, but doesn't permit operating system lock calls at that time. —end note]

#### POSIX concerns

Filesystem library initialization may throw an exception on POSIX systems (e.g. Linux, but not Mac OS X) that use environmental variables to determine the encoding of paths. This happens when `std::locale("")` throws because an environmental variable such as LANG is set to an invalid value, so it can affect any use of `std::locale("")`, not just the Filesystem library. Filesystem uses lazy initialization so the exception is only thrown if a valid `std::locale("")` is actually needed, and also so that the exception is thrown after `main()` starts.

Rather than waiting until a call to some Filesystem library function unexpectedly triggers the exception when it calls `path::codecvt()`, a program that needs be highly robust against environmental variable problems may want to preemptively call `std::locale("")` within a try block, catch the exception, and diagnose or repair the invalid environmental variable.

### `path` Conversions [path.cvt]

#### `path` argument conversions [path.arg.cvt]

##### `path` argument format conversions [path.arg.fmt.cvt]

Member function arguments that take character sequences representing paths may use the generic pathname format or the native pathname format. Iff such arguments are in the generic format and the generic format is not acceptable to the operating system as a native path, conversion to native format shall be performed during the processing of the argument. See [fs.os.examples].

> [Note: Depending on the operating system, there may be no unambiguous way for an implementation to always be able to distinguish between native format and generic format arguments. This is by design as it simplifies use for operating systems that do not require disambiguation. Should an implementation encounter an operating system where disambiguation is required, an implementation can defined an extension to distinguish between the formats. —end note]

If the native format requires paths for regular files to be formatted differently from paths for directories, the path shall be treated as a directory path if last element is a separator, otherwise it shall be treated as a regular file path.

##### `path` argument encoding conversions [path.arg.encoding.cvt]

For member function arguments that take character sequences representing paths, if the value type of the argument is not `value_type and` one of the value types is `char` and the other is `wchar_t`, conversion to `value_type` shall be performed by the `path::codecvt()` facet. ([path.imbued.locale]).

#### `path` Conversions to generic format [fs.cvt.to.generic]

Generic format observer functions shall return strings formatted according to the generic pathname format using preferred-separator. See [fs.os.examples].

### `path` Requirements [path.req]

Template parameters named `InputIterator` are required meet the requirements for a C++ standard library `RandomIterator` compliant iterator. The iterator's value type is required to be `char`, `wchar_t`, `char16_t`, or `char32_t`.

Template parameters named `Source` are required to be one of:

- A container with a value type of `char`, `wchar_t`, `char16_t`, or `char32_t`.

- An iterator for a null terminated byte-string. The value type is required to be `char`, `wchar_t`, `char16_t`, or `char32_t`.

- A C-array. The value type is required to be `char`, `wchar_t`, `char16_t`, or `char32_t`.

- A `boost::filesystem::directory_entry`.

### `path` constructors [path.construct]

    template <class Source>
      path(Source const& source, const codecvt_type& cvt=codecvt());

    template <class InputIterator>
      path(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

> Effects: Stores the contents [`begin`,`end`) or `source` in `pathname`, converting format and encoding if required ([path.arg.convert]).

### `path` assignments [path.assign]

    template <class Source>
      path& operator=(Source const& source);
      template <class Source>
      path& assign(Source const& source, const codecvt_type& cvt);
      template <class InputIterator>
      path& assign(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

> Effects: Stores the contents [`begin`,`end`) or `source` in `pathname`, converting format and encoding if required ([path.arg.convert]).

> Returns: `*this`

### `path` appends [path.append]

The append operations use `operator/=` to denote their semantic effect of appending preferred-separator when needed.

    path& operator/=(const path& p);

> Effects:

> > Appends `path::preferred_separator` to `pathname`, converting format and encoding if required ([path.arg.convert]), unless:

> > - an added separator would be redundant, or

> > - would change an relative path to an absolute path, or

> > - `p.empty()`, or

> > - `*p.native().cbegin()` is a directory separator.

> > Then appends `p.native()` to `pathname`.

> Returns: `*this`

    template <class Source>
      path& operator/=(Source const & source);
      template <class Source>
      path& append(Source const & source, const codecvt_type& cvt);
      template <class InputIterator>
      path& append(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

> Effects:

> > Appends `path::preferred_separator` to `pathname`, converting format and encoding if required ([path.arg.convert]), unless:

> > - an added separator would be redundant, or

> > - would change an relative path to an absolute path, or

> > - `p.empty()`, or

> > - `*p.native().cbegin()` is a separator.

> > Appends the contents [`begin`,`end`) or `source` to `pathname`, converting format and encoding if required ([path.arg.convert]).

> Returns: `*this`

### `path` concatenation [path.concat]

    path& operator+=(const path& x);
    path& operator+=(const string_type& x);
    path& operator+=(const value_type* x);
    path& operator+=(value_type x);
    template <class Source>
      path& operator+=(Source const& x);
    template <class CharT>
      path& operator+=(CharT x);
    template <class InputIterator>
      path& concat(InputIterator begin, InputIterator end);
    template <class InputIterator>
      path& concat(InputIterator begin, InputIterator end, const codecvt_type& cvt);

> Postcondition: `native() == prior_native + effective-argument`, where `prior_native` is `native()` prior to the call to `operator+=`, and `effective-argument` is:

> - `x.native()` if `x` is present and is `const path&`, otherwise

> - `s`, where `s` is `std::basic_string<typename std::iterator_traits<InputIterator>::value_type>
>   s(begin, end)`, if `begin` and `end` arguments are present, otherwise

> - `x`.

> If the value type of `effective-argument` would not be `path::value_type`, the actual argument or argument range is first converted so that `effective-argument` has value type `path::value_type`.

> Returns: `*this`

### `path` modifiers [path.modifiers]

    void clear();

> Postcondition: `this->empty()` is true.

    path& make_preferred();

> Effects: directory-separators are converted to prefered-separators. See [fs.os.examples].

> Returns: `*this`

    path& remove_filename();

> Returns: As if, `*this = parent_path();`

> [Note: This function is needed to efficiently implement `directory_iterator`. It is exposed to allow additional uses. The actual implementation may be much more efficient than `*this = parent_path()` —end note]

    path& replace_extension(const path& new_extension = path());

> Effects:

> - Any existing `extension()` is removed from the stored path, then

> - iff `new_extension` is not empty and does not begin with a dot character, a dot character is appended to the stored path, then

> - `new_extension` is appended to the stored path.

> Returns: `*this`

    void swap(path& rhs) noexcept;

> Effects: Swaps the contents of the two paths.

> Complexity: constant time.

### `path` lexical operations [path.lex.ops]

    path lexically_normal() const;

> Overview: Returns `*this` with redundant current directory (dot), parent directory (dot-dot), and directory-separator elements removed.

> Returns: `*this` in normal form.

> Remarks: Uses `operator/=` to compose the returned path.

> [Example:

> `assert(path("foo/./bar/..").lexically_normal() == "foo");
> assert(path("foo/.///bar/../").lexically_normal() == "foo/.");`

> The above assertions will succeed. On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect `path` equality. —end example]

    path lexically_relative(const path& base) const;

> Overview: Returns `*this` made relative to `base`. Treats empty or identical paths as corner cases, not errors. Does not resolve symlinks. Does not first normalize `*this` or `base`.

> Remarks: Uses `std::mismatch(begin(), end(), base.begin(), base.end())`, to determine the first mismatched element of `*this` and `base`. Uses `operator==` to determine if elements match.

> Returns:

> - `path()` if the first mismatched element of `*this` is equal to `begin()` or the first mismatched element of `base` is equal to `base.begin()`, or
> - `path(".")` if the first mismatched element of `*this` is equal to `end()` and the first mismatched element of `base` is equal to `base.end()`, or
> - An object of class `path` composed via application of `operator/= path("..")` for each element in the half-open range [first mismatched element of `base`, `base.end()`), and then application of `operator/=` for each element in the half-open range [first mismatched element of `*this`, `end()`).

> [Example:

> `assert(path("/a/d").lexically_relative("/a/b/c") == "../../d");
> assert(path("/a/b/c").lexically_relative("/a/d") == "../b/c");
> assert(path("a/b/c").lexically_relative("a") == "b/c");
> assert(path("a/b/c").lexically_relative("a/b/c/x/y") == "../..");
> assert(path("a/b/c").lexically_relative("a/b/c") == ".");
> assert(path("a/b").lexically_relative("c/d") == "");`

> The above assertions will succeed. On Windows, the returned path's directory-separators will be backslashes rather than forward slashes, but that does not affect `path` equality. —end example]

> [Note: If symlink following semantics are desired, use the operational function `relative` —end note]

> [Note: If normalization is needed to ensure consistent matching of elements, apply `lexically_normal()` to `*this`, `base`, or both. —end note]

> ### `path` native format observers [path.native.obs]

> The string returned by all native format observers is in the native pathname format.

>     const string_type&  native() const noexcept;

> > Returns: `pathname`.

>     const value_type* c_str() const noexcept;

> > Returns: `pathname.c_str()`.

>     string_type::size_type size() const noexcept;

> > Returns: `pathname.size()`.

>     template <class String>
>     String string(const codecvt_type& cvt=codecvt()) const;

> > Returns: `pathname`.

> > Remarks: If `string_type` is a different type than `String`, conversion is performed by `cvt`.

>     string string(const codecvt_type& cvt=codecvt()) const;
>     wstring wstring(const codecvt_type& cvt=codecvt()) const;
>     u16string u16string() const;
>     u32wstring u32wstring() const;

> > Returns: `pathname`.

> > Remarks: If `string_type` is a different type than function's return type, conversion is performed by `cvt`.

> ### `path` generic format observers [path.generic.obs]

> The string returned by all generic format observers is in the generic pathname format.

>     template <class String>
>     String generic_string(const codecvt_type& cvt=codecvt()) const;

> > Returns: `pathname`.

> > Remarks: If `string_type` is a different type than `String`, conversion is performed by `cvt`.

>     string generic_string(const codecvt_type& cvt=codecvt()) const;
>     wstring generic_wstring(const codecvt_type& cvt=codecvt()) const;
>     u16string generic_u16string() const;
>     u32wstring generic_u32wstring() const;

> > Returns: `pathname`.

> > Remarks: If `string_type` is a different type than function's return type, conversion is performed by `cvt`.

> ### `path` compare [path.compare]

>     int compare(const path& p) const noexcept;

> > Returns: A value less than 0 if the elements of `*this` are lexicographically less than the elements of `p`, otherwise a value greater than 0 if the elements of `*this` are lexicographically greater than the elements of `p`, otherwise 0.

> > Remark: The elements are determined as if by iteration over the half-open range [`begin()`, `end()`) for `*this` and `p`.

>     int compare(const std::string& s) const

> > Returns: `compare(path(s))`.

>     int compare(const value_type* s) const

> > Returns: `compare(path(s))`.

> ### `path` decomposition [path.decompose]

> See the Path decomposition table for examples for values returned by decomposition functions. The Tutorial may also be helpful.

>     path root_name() const;

> > Returns: root-name, if `pathname` includes root-name, otherwise `path()`.

>     path root_directory() const;

> > Returns: root-directory, if `pathname` includes root-directory, otherwise `path()`.

> > If root-directory is composed of slash name, slash is excluded from the returned string.

>     path root_path() const;

> > Returns: `root_name() / root_directory()`

>     path relative_path() const;

> > Returns: A `path` composed from `pathname`, if `!empty()`, beginning with the first filename after root-path. Otherwise, `path()`.

>     path parent_path() const;

> > Returns: `(empty() || begin() == —end()) ? path() : pp`, where `pp` is constructed as if by starting with an empty `path` and successively applying `operator/=` for each element in the range `begin()`, `—end()`.

> > [Example:

> > >     std::cout << path("/foo/bar.txt").parent_path(); // outputs "/foo"
> > >     std::cout << path("/foo/bar").parent_path();     // outputs "/foo"
> > >     std::cout << path("/foo/bar/").parent_path();    // outputs "/foo/bar"
> > >     std::cout << path("/").parent_path();            // outputs ""
> > >     std::cout << path(".").parent_path();            // outputs ""
> > >     std::cout << path("..").parent_path();           // outputs ""

> > See the last bullet item in the forward traversal order list for why the `"/foo/bar/"` example doesn't output `"/foo"`.

> > —end example]

>     path filename() const;

> > Returns: `empty() ? path() : *—end()`

> > [Example:

> > >     std::cout << path("/foo/bar.txt").filename(); // outputs "bar.txt"
> > >     std::cout << path("/foo/bar").filename();     // outputs "bar"
> > >     std::cout << path("/foo/bar/").filename();    // outputs "."
> > >     std::cout << path("/").filename();            // outputs "/"
> > >     std::cout << path(".").filename();            // outputs "."
> > >     std::cout << path("..").filename();           // outputs ".."

> > See the last bullet item in the forward traversal order list for why the `"/foo/bar/"` example doesn't output `"bar"`.

> > —end example]

>     path stem(const path& p) const;

> > Returns: if `p.filename()` contains a dot but does not consist solely of one or to two dots, returns the substring of `p.filename()` starting at its beginning and ending at the last dot (the dot is not included). Otherwise, returns `p.filename()`.

> > [Example:

> > >     std::cout << path("/foo/bar.txt").stem(); // outputs "bar"
> > >     path p = "foo.bar.baz.tar";
> > >     for (; !p.extension().empty(); p = p.stem())
> > >       std::cout << p.extension() << '\n';
> > >       // outputs: .tar
> > >       //          .baz
> > >       //          .bar

> > —end example]

>     path extension(const path& p) const;

> > Returns: if `p.filename()` contains a dot but does not consist solely of one or to two dots, returns the substring of `p.filename()` starting at the rightmost dot and ending at the path's end. Otherwise, returns an empty `path` object.

> > Remarks: Implementations are permitted but not required to define additional behavior for file systems which append additional elements to extensions, such as alternate data streams or partitioned dataset names.

> > [Example:

> > >     std::cout << path("/foo/bar.txt").extension(); // outputs ".txt"

> > —end example]

> > [Note: The dot is included in the return value so that it is possible to distinguish between no extension and an empty extension. See http://permalink.gmane.org/gmane.comp.lib.boost.devel/199744 for more extensive rationale. —end note]

> ### `path` query [path.query]

>     bool empty() const;

> > Returns: `m_pathname.empty()`.

>     bool has_root_path() const;

> > Returns: `!root_path().empty()`

>     bool has_root_name() const;

> > Returns: `!root_name().empty()`

>     bool has_root_directory() const;

> > Returns: `!root_directory().empty()`

>     bool has_relative_path() const;

> > Returns: `!relative_path().empty()`

>     bool has_parent_path() const;

> > Returns: `!parent_path().empty()`

>     bool has_filename() const;

> > Returns: `!filename().empty()`

>     bool has_stem() const;

> > Returns: `!stem().empty()`

>     bool has_extension() const;

> > Returns: `!extension().empty()`

>     bool is_absolute() const;

> > Returns: `true` if the elements of `root_path()` uniquely identify a directory, else `false`.

>     bool is_relative() const;

> > Returns: `!is_absolute()`.

> ### `path` iterators [path.itr]

> Path iterators `iterator`, `const_iterator`, `reverse_iterator`, and `const_reverse_iterator` iterate over the elements of the stored pathname.

> Path iterators are constant iterators satisfying the requirements of a bidirectional iterator (C++ Std, 24.1.4 Bidirectional iterators [lib.bidirectional.iterators]). The `value_type` of an iterator is `path`.

> > [Note: Path iterators store their value objects internally and when dereferenced return references to those internal objects. They cannot be used with iterator adaptors such as `std::reverse_iterator` that assume references obtained by dereferencing an iterator point to objects that out-live the iterator itself. —end note]

> Calling any non-const member function of a `path` object invalidates all iterators referring to elements of that object.

> The forward traversal order is as follows:

> - The root-name element, if present.
> - The root-directory element, if present, in the generic format. [note: the generic format is required to ensure lexicographical comparison works correctly. —end note]
> - Each successive filename element, if present.
> - Dot, if one or more trailing non-root slash characters are present.|

> > [Note: Treating the last element during iteration as dot when there is a trailing directory separator enables lexical (i.e. syntactic) distinction between paths to directories versus paths to regular files. Such a distinction is usually irrelevant on POSIX and Windows based operating systems, but may be a requirement on other operating systems. —end note]

> The backward traversal order is the reverse of forward traversal.

>     iterator begin() const;

> > Returns: An iterator for the first element in forward traversal order. If no elements are present, the end iterator.

>     iterator end() const;

> > Returns: The end iterator.

>     reverse_iterator rbegin() const;

> > Returns: An iterator for the first element in backward traversal order. If no elements are present, the end iterator.

>     reverse_iterator rend() const;

> > Returns: The end iterator.

> ### `path` imbued locale [path.imbued.locale]

> `path` operations sometimes require encoding conversions between `pathname` and some other string object where one of the value types is `char` and the other is `wchar_t`. Such conversions shall be performed by the `path::codecvt()` facet.

> > [Example: ... —end example]

>     static std::locale imbue(const std::locale& loc);

> > Effects: Stores a copy of `loc` as the imbued `path` locale.

> > Returns: The previous imbued `path` locale.

> > Remarks: The initial value of the imbued `path` locale is operating system dependent. It shall be a locale with a `codecvt` facet for a `char` string encoding appropriate for the operating system. See ([fs.os.examples]).

>     static const codecvt_type& codecvt();

> > Returns: The `codecvt` facet for the imbued`path` locale .

> ### `path` deprecated functions

> Several member functions from previous versions of `class path` have been deprecated, either because they have been renamed or because the functionality is no longer desirable or has become obsolete.

> Deprecated functions available by default; will be suppressed if `BOOST_FILESYSTEM_NO_DEPRECATED` is defined:

> >     path&  remove_leaf()           { return remove_filename(); }
> >     path   leaf() const            { return filename(); }
> >     path   branch_path() const     { return parent_path(); }
> >     bool   has_leaf() const        { return !m_path.empty(); }
> >     bool   has_branch_path() const { return !parent_path().empty(); }

> Deprecated functions not available by default; will be supplied if `BOOST_FILESYSTEM_DEPRECATED` is defined:

> >     const std::string  file_string() const               { return native_string(); }
> >     const std::string  directory_string() const          { return native_string(); }
> >     const std::string  native_file_string() const        { return native_string(); }
> >     const std::string  native_directory_string() const   { return native_string(); }
> >     const string_type  external_file_string() const      { return native(); }
> >     const string_type  external_directory_string() const { return native(); }

> ### `path` non-member functions [path.non-member]

>     bool lexicographical_compare(path::iterator first1, path::iterator last1,
>                                  path::iterator first2, path::iterator last2);

> > Returns: `true` if the sequence of `native()` strings for the elements defined by the half-open range `[first1,last1)` is lexicographically less than the sequence of `native()` strings for the elements defined by the half-open range `[first2,last2)`. Returns `false` otherwise.

> > Remarks: If two sequences have the same number of elements and their corresponding elements are equivalent, then neither sequence is lexicographically less than the other. If one sequence is a prefix of the other, then the shorter sequence is lexicographically less than the longer sequence. Otherwise, the lexicographical comparison of the sequences yields the same result as the comparison of the first corresponding pair of elements that are not equivalent.

> > [Note: A `path` aware `lexicographical_compare` algorithm is provided for historical reasons. —end note]

>     path lexically_normal(const path& p);

> > Overview: Returns `p` with redundant current directory (dot), parent directory (dot-dot), and directory-separator elements removed.

> > Returns: `p` in normal form.

> > Remarks: Uses `operator/=` to compose the returned path.

> > [Example:

> > `assert(lexically_normal("foo/./bar/..") == "foo");
> > assert(lexically_normal("foo/.///bar/../") == "foo/.");`

> > All of the above assertions will succeed. On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect `path` equality. —end example]

>     path lexically_relative(const path& p, const path& base);

> > Overview: Returns `p` made relative to `base`. Treats empty or identical paths as corner cases, not errors. Does not resolve symlinks. Does not first normalize `p` or `base`.

> > Remarks: Uses `std::mismatch(p.begin(), p.end(), base.begin(), base.end())`, to determine the first mismatched element of `p` and `base`. Uses `operator==` to determine if elements match.

> > Returns:

> > - `path()` if the first mismatched element of `p` is equal to `p.begin()` or the first mismatched element of `base` is equal to `base.begin()`, or

> > - `path(".")` if the first mismatched element of `p` is equal to `p.end()` and the first mismatched element of `base` is equal to `base.end()`, or

> > - An object of class `path` composed via application of `operator/= path("..")` for each element in the half-open range [first mismatched element of `base`, `base.end()`), and then application of `operator/=` for each element in the half-open range [first mismatched element of `p`, `p.end()`).

> > [Example:

> > `assert(lexically_relative("/a/d", "/a/b/c") == "../../d");
> > assert(lexically_relative("/a/b/c", "/a/d") == "../b/c");
> > assert(lexically_relative("a/b/c", "a") == "b/c");
> > assert(lexically_relative("a/b/c", "a/b/c/x/y") == "../..");
> > assert(lexically_relative("a/b/c", "a/b/c") == ".");
> > assert(lexically_relative("a/b", "c/d") == "");`

> > All of the above assertions will succeed. On Windows, the returned path's directory-separators will be backslashes rather than forward slashes, but that does not affect `path` equality. —end example]

> > [Note: If symlink following semantics are desired, use the operational function `relative` —end note]

> > [Note: If normalization is needed to ensure consistent matching of elements, wrap `p`, `base`, or both in calls `lexically_normal()`. —end note]

>     void swap( path& lhs, path& rhs )

> > Effects: `lhs.swap(rhs)`.

>     std::size_t hash_value (const path& p);

> > Returns: A hash value for the path `p`. If for two paths, `p1 == p2` then `hash_value(p1) == hash_value(p2)`.

> > This allows paths to be used with Boost.Hash.

>     bool operator< (const path& lhs, const path& rhs);

> > Returns: `return lhs.compare(rhs.begin) < 0`.

>     bool operator<=(const path& lhs, const path& rhs);

> > Returns: `!(rhs < lhs)`.

>     bool operator> (const path& lhs, const path& rhs);

> > Returns: `rhs < lhs`.

>     bool operator>=(const path& lhs, const path& rhs);

> > Returns: `!(lhs < rhs)`.

>     bool operator==(const path& lhs, const path& rhs);

> > Returns: `!(lhs < rhs) && !(rhs < lhs)`.

> > [Note: Path equality and path equivalence have different semantics.

> > Equality is determined by the `path` non-member `operator==`, which considers the two path's lexical representations only. Thus `path("foo") == "bar"` is never `true`.

> > Equivalence is determined by the `equivalent()` non-member function, which determines if two paths resolve to the same file system entity. Thus `equivalent("foo", "bar")` will be `true` when both paths resolve to the same file.

> > Programmers wishing to determine if two paths are "the same" must decide if "the same" means "the same representation" or "resolve to the same actual file", and choose the appropriate function accordingly. —end note]

>     bool operator!=(const path& lhs, const path& rhs);

> > Returns: `!(lhs == rhs)`.

>     path operator/ (const path& lhs, const path& rhs);

> > Returns: `path(lhs) /= rhs`.

> ### `path` inserter and extractor [path.io]

> The inserter and extractor delimit the string with double-quotes (`"`) so that paths with embedded spaces will round trip correctly. Ampersand (`&`) is as an escape character, so the path can itself contain double quotes.

>     template <class Char, class Traits>
>     std::basic_ostream<Char, Traits>& operator<<(std::basic_ostream<Char, Traits>& os,
>                                                  const path& p)

> > Effects: Insert characters into `os`:

> > - A double-quote.

> > - Each character in `p.string<std::basic_string<Char>>()`. If the character to be inserted is equal to the escape character or a double-quote, as determined by `operator==`, first insert the escape character.

> > - A double-quote.

> > Returns: `os`

>     template <class Char, class Traits>
>     inline std::basic_istream<Char, Traits>& operator>>(std::basic_istream<Char, Traits>& is,
>                                                         path& p)

> > Effects: `std::basic_string<Char> str;
> > is >> boost::io::quoted(str, static_cast<Char>('&'));
> > p = str;`

> > Effects: Extract characters from os:

> > - If the first character that would be extracted is equal to double-quote, as determined by `operator==`, then:
> >   - Discard the initial double-quote.
> >   - Save the value and then turn off the `skipws` flag.
> >   - `p.clear()`
> >   - Until an unescaped double-quote character is reached or `is.not_good()`, extract characters from `os` and append them to `p`, except that if an escape character is reached, ignore it and append the next character to `p`.
> >   - Discard the final double-quote character.
> >   - Restore the `skipws` flag to its original value.
> > - Otherwise, `os >> p`.

> > Returns: `is`

> ## Class `filesystem_error` [class.filesystem_error]

>     namespace boost
>     {
>       namespace filesystem
>       {
>           class filesystem_error : public system_error
>           {
>           public:
>             filesystem_error();
>             filesystem_error(const filesystem_error&);
>             filesystem_error(const std::string& what_arg,
>               system::error_code ec);
>             filesystem_error(const std::string& what_arg,
>               const path& p1, system::error_code ec);
>             filesystem_error(const std::string& what_arg,
>               const path& p1, const path& p2, system::error_code ec);

>             filesystem_error& filesystem_error(const filesystem_error&);
>            ~filesystem_error();

>             filesystem_error& operator=(const filesystem_error&);

>             const path& path1() const;
>             const path& path2() const;

>             const char * what() const;
>           };
>       }  // namespace filesystem
>     }  // namespace boost

> The class template `filesystem_error` defines the type of objects thrown as exceptions to report file system errors from functions described in this reference documentation.

> ### `filesystem_error` members [filesystem_error.members]

>     filesystem_error(const std::string& what_arg, error_code ec);

> > Postcondition:

> > Expression | Value
> > `runtime_error::what()` | `what_arg.c_str()`
> > `code()` | `ec`
> > `path1().empty()` | `true`
> > `path2().empty()` | `true`

>     filesystem_error(const std::string& what_arg, const path& p1, error_code ec);

> > Postcondition:

> > Expression | Value
> > `runtime_error::what()` | `what_arg.c_str()`
> > `code()` | `ec`
> > `path1()` | Reference to stored copy of `p1`
> > `path2().empty()` | `true`

>     filesystem_error(const std::string& what_arg, const path& p1, const path& p2, error_code ec);

> > Postcondition:

> > Expression | Value
> > `runtime_error::what()` | `w` `hat_arg.c_str()`
> > `code()` | `ec`
> > `path1()` | Reference to stored copy of `p1`
> > `path2()` | Reference to stored copy of `p2`

>     const path& path1() const;

> > Returns: Reference to copy of `p1` stored by the constructor, or, if none, an empty path.

>     const path& path2() const;

> > Returns: Reference to copy of `p2` stored by the constructor, or, if none, an empty path.

>     const char* what() const;

> > Returns: A string containing `runtime_error::what()`. The exact format is unspecified. Implementations are encouraged but not required to include `path1.native_string()`if not empty, `path2.native_string()`if not empty, and `system_error::what()` strings in the returned string.

> ## Enum file_type [enum.file_type]

> This enum specifies constants uses to identify file types.

> Constant Name | Meaning
> `status_error` | An error occurred while trying to obtain the status of the file. The file simply not being found is not considered a status error.
> `file_not_found` | The file could not be found
> `regular_file` | Regular file
> `directory_file` | Directory file
> `symlink_file` | Symbolic link file
> `block_file` | Block special file
> `character_file` | Character special file
> `fifo_file` | FIFO or pipe file
> `socket_file` | Socket file
> `type_unknown` | The file exists, but it is of a system specific type not covered by any of the above cases.

> ## Enum perms [enum.perms]

> This `enum` specifies bitmask constants uses to identify file permissions. ISO/IEC 9945 (POSIX) specifies actual values, and those values have been adopted here because they are very familiar and ingrained for many POSIX users.

> > Windows: All permissions except write are currently ignored. There is only a single write permission; setting write permission for owner, group, or others sets write permission for all, and removing write permission for owner, group, or others removes write permission for all.

> Name | Value (octal) | ISO/IEC 9945 macro | Definition or notes
> `no_perms` | `0` |  | There are no permissions set for the file. Note: `file_not_found` is `no_perms` rather than `perms_not_known`
> `owner_read` | `0400` | `S_IRUSR` | Read permission, owner
> `owner_write` | `0200` | `S_IWUSR` | Write permission, owner
> `owner_exe` | `0100` | `S_IXUSR` | Execute/search permission, owner
> `owner_all` | `0700` | `S_IRWXU` | Read, write, execute/search by owner; `owner_read | owner_write | owner_exe`
> `group_read` | `040` | `S_IRGRP` | Read permission, group
> `group_write` | `020` | `S_IWGRP` | Write permission, group
> `group_exe` | `010` | `S_IXGRP` | Execute/search permission, group
> `group_all` | `070` | `S_IRWXG` | Read, write, execute/search by group; `group_read | group_write | group_exe`
> `others_read` | `04` | `S_IROTH` | Read permission, others
> `others_write` | `02` | `S_IWOTH` | Write permission, others
> `others_exe` | `01` | `S_IXOTH` | Execute/search permission, others
> `others_all` | `07` | `S_IRWXO` | Read, write, execute/search by others; `others_read | others_write | others_exe`
> `all_all` | `0777` |  | `owner_all | group_all | others_all`
> `set_uid_on_exe` | `04000` | `S_ISUID` | Set-user-ID on execution
> `set_gid_on_exe` | `02000` | `S_ISGID` | Set-group-ID on execution
> `sticky_bit` | `01000` | `S_ISVTX` | Operating system dependent. Inherently non-portable, even between ISO/IEC 9945 operating systems.
> `perms_mask` | `07777` |  | `all_all | set_uid_on_exe | set_gid_on_exe | sticky_bit`
> `perms_not_known` | `0xFFFF` |  | The permissions are not known, such as when a `file_status` object is created without specifying the permissions
> `add_perms` | `0x1000` |  | `permissions()` adds the argument permission bits to the file's current bits
> `remove_perms` | `0x2000` |  | `permissions()` removes the argument permission bits from the file's current bits
> `symlink_perms` | `0x4000` |  | On ISO/IEC 9945 `permissions()` resolves symlinks unless `symlink_perms` is specified. Meaningless on Windows as `permissions()` never resolves symlinks. Meaningless on Mac OS X and some other BSD systems as `permissions()` always resolves symlinks. Get over it.

> ## Class file_status [class.file_status]

>     namespace boost
>     {
>       namespace filesystem
>       {
>           class file_status
>           {
>           public:

>             // constructors
>             file_status() noexcept;
>             explicit file_status(file_type ft, perms prms = perms_not_known) noexcept;

>             // compiler generated
>             file_status(const file_status&) noexcept;
>             file_status& operator=(const file_status&) noexcept;
>            ~file_status() noexcept;

>             // observers
>             file_type  type() const noexcept;
>             perms      permissions() const noexcept;

>             // modifiers
>             void       type(file_type ft) noexcept;
>             void       permissions(perms prms) noexcept;
>           };
>       }  // namespace filesystem
>     }  // namespace boost

> An object of type `file_status` stores information about the type and permissions of a file.

> ### `file_status` constructors [file_status.cons]

>     explicit file_status() noexcept;

> > Postconditions: `type() == status_error`, `permissions() == perms_not_known`.

>     explicit file_status(file_type ft, perms prms = perms_not_known) noexcept;

> > Postconditions: `type() == ft`, `permissions() == prms`.

> ### `file_status` observers [file_status.obs]

>     file_type type() const noexcept;

> > Returns: The value of `type()` specified by the postconditions of the most recent call to a constructor, operator=, or `type(file_type)` function.

>     perms permissions() const noexcept;

> > Returns: The value of `permissions()` specified by the postconditions of the most recent call to a constructor, operator=, or `permissions(perms)` function.

> ### `file_status` modifiers [file_status.mods]

>     void type(file_type ft) noexcept;

> > Postconditions: `type() == ft`.

>     void permissions(perms prms) noexcept;

> > Postconditions: `permissions() == prms`.

> ## Class `directory_entry` [class.directory_entry]

>     namespace boost
>     {
>       namespace filesystem
>       {
>           class directory_entry
>           {
>           public:

>             // constructors and destructor
>             directory_entry();
>             directory_entry(const directory_entry&);
>             explicit directory_entry(const path& p, file_status st=file_status(),
>               file_status symlink_st=file_status());
>            ~directory_entry();

>             // modifiers
>             directory_entry& operator=(const directory_entry&);
>             void assign(const path& p, file_status st=file_status(),
>               file_status symlink_st=file_status());
>             void replace_filename(const path& p, file_status st=file_status(),
>               file_status symlink_st=file_status());

>             // observers
>             const path&  path() const;
>             file_status  status() const;
>             file_status  status(system::error_code& ec) const;
>             file_status  symlink_status() const;
>             file_status  symlink_status(system::error_code& ec) const;

>             bool operator< (const directory_entry& rhs);
>             bool operator==(const directory_entry& rhs);
>             bool operator!=(const directory_entry& rhs);
>             bool operator< (const directory_entry& rhs);
>             bool operator<=(const directory_entry& rhs);
>             bool operator> (const directory_entry& rhs);
>             bool operator>=(const directory_entry& rhs);
>           private:
>             path                 m_path;           // for exposition only
>             mutable file_status  m_status;         // for exposition only; stat()-like
>             mutable file_status  m_symlink_status; // for exposition only; lstat()-like
>           };

>       }  // namespace filesystem
>     }  // namespace boost

> A `directory_entry` object stores a `path object`, a `file_status` object for non-symbolic link status, and a `file_status` object for symbolic link status. The `file_status` objects act as value caches.

> > [Note: Because `status()`on a pathname may be a relatively expensive operation, some operating systems provide status information as a byproduct of directory iteration. Caching such status information can result is significant time savings. Cached and non-cached results may differ in the presence of file system races. —end note]

> > Actual cold-boot timing of iteration over a directory with 15,047 entries was six seconds for non-cached status queries versus one second for cached status queries. Windows XP, 3.0 GHz processor, with a moderately fast hard-drive. Similar speedups are expected on Linux and BSD-derived systems that provide status as a by-product of directory iteration.

> ### `directory_entry` constructors [directory_entry.cons]

>     directory_entry();

> > Postcondition:

> > Expression | Value
> > `path().empty()` | `true`
> > `status()` | `file_status()`
> > `symlink_status()` | `file_status()`

>     explicit directory_entry(const path& p, file_status st=file_status(), file_status symlink_st=file_status());

> > Postcondition:

> > Expression | Value
> > `path()` | `p`
> > `status()` | `st`
> > `symlink_status()` | `symlink_st`

> ### `directory_entry` modifiers [directory_entry.mods]

>     void assign(const path& p, file_status st=file_status(), file_status symlink_st=file_status());

> > Postcondition:

> > Expression | Value
> > `path()` | `p`
> > `status()` | `st`
> > `symlink_status()` | `symlink_st`

>     void replace_filename(const path& p, file_status st=file_status(), file_status symlink_st=file_status());

> > Postcondition:

> > Expression | Value
> > `path()` | `path().branch() / s`
> > `status()` | `st`
> > `symlink_status()` | `symlink_st`

> ### `directory_entry` observers [directory_entry.obs]

>     const path& path() const;

> > Returns: `m_path`

>     file_status status() const;
>     file_status status(system::error_code& ec) const;

> > Effects: As if,

> > >     if ( !status_known( m_status ) )
> > >     {
> > >       if ( status_known(m_symlink_status) && !is_symlink(m_symlink_status) )
> > >         { m_status = m_symlink_status; }
> > >       else { m_status = status(m_path[, ec]); }
> > >     }

> > Returns: `m_status`

> > Throws: As specified in Error reporting.

>     file_status  symlink_status() const;
>     file_status  symlink_status(system::error_code& ec) const;

> > Effects: As if,

> > >     if ( !status_known( m_symlink_status ) )
> > >     {
> > >       m_symlink_status = symlink_status(m_path[, ec]);
> > >     }

> > Returns: `m_symlink_status`

> > Throws: As specified in Error reporting.

>     bool operator==(const directory_entry& rhs);

> > Returns: `m_path == rhs.m_path`.

>     bool operator!=(const directory_entry& rhs);

> > Returns: `m_path != rhs.m_path`.

>     bool operator< (const directory_entry& rhs);

> > Returns: `m_path < rhs.m_path`.

>     bool operator<=(const directory_entry& rhs);

> > Returns: `m_path <= rhs.m_path`.

>     bool operator> (const directory_entry& rhs);

> > Returns: `m_path > rhs.m_path`.

>     bool operator>=(const directory_entry& rhs);

> > Returns: `m_path >= rhs.m_path`.

> ## Class `directory_iterator` [class.directory_iterator]

> Objects of type `directory_iterator` provide standard library compliant iteration over the contents of a directory. Also see class `recursive_directory_iterator`.

>     namespace boost
>     {
>       namespace filesystem
>       {
>           class directory_iterator
>           {
>           public:
>             // member functions

>             directory_iterator() noexcept;  // creates the "end" iterator
>             directory_iterator(const directory_iterator&);
>             explicit directory_iterator(const path& p);
>             directory_iterator(const path& p, system::error_code& ec);
>            ~directory_iterator();

>             directory_iterator& operator=(const directory_iterator&);

>             directory_iterator& operator++();
>             directory_iterator& increment(system::error_code& ec);

>             // other members as required by
>             //  C++ Std, 24.1.1 Input iterators [input.iterators]
>           };

>       }  // namespace filesystem
>     }  // namespace boost

> `directory_iterator` satisfies the requirements of an input iterator (C++ Std, 24.2.1, Input iterators [input.iterators]).

> A `directory_iterator` reads successive elements from the directory for which it was constructed, as if by calling ISO/IEC 9945 `readdir_r()`. After a `directory_iterator` is constructed, and every time `operator++` is called, it reads a directory element and stores information about it in a object of type `directory_entry`. `operator++` is not equality preserving; that is, `i == j` does not imply that `++i == ++j`.

> > [Note: The practical consequence of not preserving equality is that directory iterators can only be used for single-pass algorithms. —end note]

> If the end of the directory elements is reached, the iterator shall become equal to the end iterator value. The constructor `directory_iterator()` with no arguments always constructs an end iterator object, which shall be the only valid iterator for the end condition. The result of `operator*` on an end iterator is not defined. For any other iterator value a `const directory_entry&` is returned. The result of `operator->` on an end iterator is undefined behavior. For any other iterator value a `const directory_entry*` is returned.

> Two end iterators are always equal. An end iterator shall not be equal to a non-end iterator.

> > The above wording is based on the Standard Library's istream_iterator wording.

> The result of calling the `path()` member of the `directory_entry` object obtained by dereferencing a `directory_iterator` is a reference to a `path` object composed of the directory argument from which the iterator was constructed with filename of the directory entry appended as if by `operator/=`.

> Directory iteration shall not yield directory entries for the current (dot) and parent (dot dot) directories.

> The order of directory entries obtained by dereferencing successive increments of a `directory_iterator` is unspecified.

> > [Note: Programs performing directory iteration may wish to test if the path obtained by dereferencing a directory iterator actually exists. It could be a symbolic link to a non-existent file. Programs recursively walking directory trees for purposes of removing and renaming entries may wish to avoid following symbolic links.

> > If a file is removed from or added to a directory after the construction of a `directory_iterator` for the directory, it is unspecified whether or not subsequent incrementing of the iterator will ever result in an iterator whose value is the removed or added directory entry. See ISO/IEC 9945 `readdir_r()`. —end note]

> ### `directory_iterator` members [directory_iterator.members]

> `directory_iterator() noexcept;`

> > Effects: Constructs the end iterator.

>     explicit directory_iterator(const path& p);
>     directory_iterator(const path& p, system::error_code& ec);

> > Effects: Constructs a iterator representing the first entry in the directory `p` resolves to, if any; otherwise, the end iterator.

> > Throws: As specified in Error reporting.

> > [Note: To iterate over the current directory, use `directory_iterator(".")` rather than `directory_iterator("")`. —end note]

>     directory_iterator& operator++();
>     directory_iterator& increment(system::error_code& ec);

> > Effects: As specified by the C++ Standard, 24.1.1 Input iterators [input.iterators]

> > Returns: `*this`.

> > Throws: As specified in Error reporting.

> ### `directory_iterator` non-member functions

>     const directory_iterator& begin(const directory_iterator& iter);

> > Returns: `iter`.

>     directory_iterator end(const directory_iterator&);

> > Returns: `directory_iterator()`.

> ## Class `recursive_directory_iterator` [class.rec.dir.itr]

> Objects of type `recursive_directory_iterator` provide standard library compliant iteration over the contents of a directory, including recursion into its sub-directories.

>     namespace boost
>     {
>       namespace filesystem
>       {
>           class recursive_directory_iterator :
>             public iterator<input_iterator_tag, directory_entry>
>           {
>           public:

>             // constructors and destructor
>             recursive_directory_iterator() noexcept;
>             recursive_directory_iterator(const recursive_directory_iterator&);
>             explicit recursive_directory_iterator(const path& p,
>               symlink_option opt = symlink_option::none);
>             recursive_directory_iterator(const path& p,
>               symlink_option opt, system::error_code& ec);
>             recursive_directory_iterator(const path& p, system::error_code& ec);
>            ~recursive_directory_iterator();

>             // observers
>             int level() const noexcept;
>             bool no_push_pending() const noexcept;

>             // modifiers
>             recursive_directory_iterator& operator=(const recursive_directory_iterator&);

>             recursive_directory_iterator& operator++();
>             recursive_directory_iterator& increment(system::error_code& ec);

>             void pop();
>             void no_push(bool value=true);

>             // other members as required by
>             //  C++ Std, Input iterators [input.iterators]

>           private:
>             // actual data members will probably be stored in a shared object,
>             // or some similar mechanism, to achieve the required input iterator
>             // copy semantics
>             int            m_level;    // for exposition only
>             bool           m_no_push;  // for exposition only
>             symlink_option m_options;  // for exposition only
>           };

>       }  // namespace filesystem
>     }  // namespace boost

> The behavior of a `recursive_directory_iterator` is the same as a `directory_iterator` unless otherwise specified.

> - Incrementing a `recursive_directory_iterator` pointing to a directory causes that directory itself to be iterated over, as specified by the `operator++` and `increment` functions.
> - When a `recursive_directory_iterator` reaches the end of the directory currently being iterated over, or when `pop()` is called, `m_level` is decremented, and iteration of the parent directory continues.

>     recursive_directory_iterator() noexcept;

> > Effects: Constructs the end iterator.

>     explicit recursive_directory_iterator(const path& p, symlink_option opt = symlink_option::none);
>     recursive_directory_iterator(const path& p, symlink_option opt, system::error_code& ec);
>     recursive_directory_iterator(const path& p, system::error_code& ec);

> > Effects: Constructs a iterator representing the first entry in the directory `p` resolves to, if any; otherwise, the end iterator.

> > Postcondition: Unless the end iterator was constructed, `level() == 0 && no_push_pending() == false && m_options == opt`. For the signature without a `symlink_option` argument, `opt` is assumed to be `symlink_option::none`.

> > Throws: As specified in Error reporting.

> > [Note: To iterate over the current directory, use `recursive_directory_iterator(".")` rather than `recursive_directory_iterator("")`. —end note]

> > [Note: By default, `recursive_directory_iterator` does not follow directory symlinks. To follow directory symlinks, specify `opt` as `symlink_option::recurse` —end note]

>     int level() const noexcept;

> > Requires: `*this != recursive_directory_iterator()`.

> > Returns: `m_level`.

>     bool no_push_pending() const noexcept;

> > Requires: `*this != recursive_directory_iterator()`.

> > Returns: `m_no_push`.

>     recursive_directory_iterator& operator++();
>     recursive_directory_iterator& increment(system::error_code& ec);

> > Effects: As specified by the C++ Standard, 24.1.1 Input iterators [input.iterators], except:

> > - if `!no_push_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status()) || (m_options & symlink_option::recurse) != 0)` then `m_level` is incremented and directory `(*this)->path()` is recursively iterated into.

> > - if there are no more directory entries at this level then `m_level` is decremented and iteration of the parent directory resumes.

> > Postcondition: `no_push_pending() == false`.

> > Returns: `*this`.

> > Throws: As specified in Error reporting.

>     void pop();

> > Requires: `*this != recursive_directory_iterator()`.

> > Effects: If `level() == 0`, set `*this` to `recursive_directory_iterator()`. Otherwise, `--m_level`, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.

>     void no_push(bool value=true);

> > Requires: `*this != recursive_directory_iterator()`.

> > Postcondition: `no_push_pending() == value`.

> > [Note: `no_push()` is used to prevent unwanted recursion into a directory. —end note]

> ### `recursive_directory_iterator` non-member functions

>     const recursive_directory_iterator& begin(const recursive_directory_iterator& iter);

> > Returns: `iter`.

>     recursive_directory_iterator end(const recursive_directory_iterator&);

> > Returns: `recursive_directory_iterator()`.

> ## Operational functions [fs.op.funcs]

> Operational functions query or modify files, including directories, in external storage.

> Operational functions access a file by resolving an object of class `path` to a particular file in a file hierarchy. The path is resolved as if by the ISO/IEC 9945 Pathname Resolution mechanism.

> [Note: Because hardware failures, network failures, file system races, and many other kinds of errors occur frequently in file system operations, users should be aware that any filesystem operational function, no matter how apparently innocuous, may encounter an error. See Error reporting. —end note]

>     path absolute(const path& p, const path& base=current_path());

> > Returns: A absolute path composed according to the following table

> > | `p. has_root_directory()` | `!p.has_root_directory()`
> > `p.has_root_name()` | `return p` | `return p.root_name() / absolute(base) .root_directory() / absolute(base) .relative_path() / p.relative_path()`
> > `!p.has_root_name()` | `return absolute(base) .root_name() / p` | `return absolute(base) / p`

> > [Note: For the returned path, `rp,` `rp.is_absolute()` is true. —end note]

> > Throws: If `base.is_absolute()` is true, throws only if memory allocation fails.

>     path canonical(const path& p, const path& base = current_path());
>     path canonical(const path& p, system::error_code& ec);
>     path canonical(const path& p, const path& base, system::error_code& ec);

> > Overview: Converts `p`, which must exist, to an absolute path that has no symbolic link, dot, or dot-dot elements.

> > Returns: A canonical path that refers to the same file system object as `absolute(p,base)`. For the overload without a `base` argument, `base` is `current_path()`.

> > Throws: As specified in Error reporting.

> > Remarks: `!exists(p)` is an error.

> > [Note: Canonical pathnames allow security checking of a path (eg. does this path live in /home/goodguy or /home/badguy?) —end note]

>     void copy(const path& from, const path& to);
>     void copy(const path& from, const path& to, system::error_code& ec);

> > Effects: As if

> > >     file_status s(symlink_status(from[, ec]));
> > >     if(is_symlink(s))
> > >       copy_symlink(from, to[, ec]);
> > >     else if(is_directory(s))
> > >       copy_directory(from, to[, ec]);
> > >     else if(is_regular_file(s))
> > >       copy_file(from, to, copy_option::fail_if_exists[, ec]);
> > >     else
> > >      Report error as specified in Error reporting.

> > Throws: As specified in Error reporting.

>     void copy_directory(const path& from, const path& to);
>     void copy_directory(const path& from, const path& to, system::error_code& ec);

> > This function is poorly named; it should probably be an overload of `create_directory` with an additional argument.

> > Effects: Creates directory `to`, with attributes copied from directory `from`. The set of attributes copied is operating system dependent.

> > > [Note: For ISO 9945/POSIX based operating systems the attributes are those copied by native API `stat(from.c_str(), &from_stat)` followed by `mkdir(to.c_str(),from_stat.st_mode)`. For Windows based operating systems the attributes are those copied by native API `CreateDirectoryExW(from.c_str(), to.c_str(), 0)`. —end note]

> > Throws: As specified in Error reporting.

>     void copy_file(const path& from, const path& to);
>     void copy_file(const path& from, const path& to, system::error_code& ec);

> > Effects: `copy_file(from, to, copy_option::fail_if_exists`[`, ec`]`)`.

> > Throws: As specified in Error reporting.

>     void copy_file(const path& from, const path& to, copy_option option);
>     void copy_file(const path& from, const path& to, copy_option option, system::error_code& ec);

> > Effects: If `option == copy_option::``fail_if_exists && exists(to)`, an error is reported. Otherwise, the contents and attributes of the file `from` resolves to are copied to the file `to` resolves to.

> > Throws: As specified in Error reporting.

>     void copy_symlink(const path& existing_symlink, const path& new_symlink);
>     void copy_symlink(const path& existing_symlink, const path& new_symlink, system::error_code& ec);

> > Effects: `create_symlink(read_symlink(existing_symlink`[`, ec`]`), new_symlink`[`, ec`]`)`.

> > Throws: As specified in Error reporting.

>     bool create_directories(const path& p);
>     bool create_directories(const path& p, system::error_code& ec);

> > Effects: Establishes the postcondition by calling `create_directory()` for any element of `p` that does not exist.

> > Postcondition: `is_directory(p)`

> > Returns: `true` if a new directory was created, otherwise `false`.

> > Throws: As specified in Error reporting.

> > Complexity: O(n+1) where n is the number of elements of `p` that do not exist.

>     bool create_directory(const path& p);
>     bool create_directory(const path& p, system::error_code& ec);

> > Effects: Establishes the postcondition by attempting to create the directory `p` resolves to, as if by ISO/IEC 9945 `mkdir()` with a second argument of S_IRWXU|S_IRWXG|S_IRWXO. Creation failure because `p` resolves to an existing directory shall not be treated as an error.

> > Postcondition: `is_directory(p)`

> > Returns: `true` if a new directory was created, otherwise `false`.

> > Throws: As specified in Error reporting.

>     void create_directory_symlink(const path& to, const path& new_symlink);
>     void create_directory_symlink(const path& to, const path& new_symlink, system::error_code& ec);

> > Effects: Establishes the postcondition, as if by ISO/IEC 9945 `symlink()`.

> > Postcondition: `new_symlink` resolves to a symbolic link file that contains an unspecified representation of `to`.

> > Throws: As specified in Error reporting.

> > [Note: Some operating systems, such as Windows, require symlink creation to identify that the link is to a directory. Portable code should use `create_directory_symlink()` to create directory symlinks rather than `create_symlink()` —end note]

> > [Note: Some operating systems do not support symbolic links at all or support them only for regular files. Some file systems do not support symbolic links regardless of the operating system - the FAT file system used on memory cards and flash drives, for example. —end note]

>     void create_hard_link(const path& to, const path& new_hard_link);
>     void create_hard_link(const path& to, const path& new_hard_link, system::error_code& ec);

> > Effects: Establishes the postcondition, as if by ISO/IEC 9945 `link()`.

> > Postcondition:

> > - `exists(to) && exists(``new_hard_link``) && equivalent(to,` `new_hard_link``)`
> > - The contents of the file or directory `to` resolves to are unchanged.

> > Throws: As specified in Error reporting.

> > [Note: Some operating systems do not support hard links at all or support them only for regular files. Some file systems do not support hard links regardless of the operating system - the FAT file system used on memory cards and flash drives, for example. Some file systems limit the number of links per file. —end note]

>     void create_symlink(const path& to, const path& new_symlink);
>     void create_symlink(const path& to, const path& new_symlink, system::error_code& ec);

> > Effects: Establishes the postcondition, as if by ISO/IEC 9945 `symlink()`.

> > Postcondition: `new_symlink` resolves to a symbolic link file that contains an unspecified representation of `to`.

> > Throws: As specified in Error reporting.

> > [Note: Some operating systems do not support symbolic links at all or support them only for regular files. Some file systems do not support symbolic links regardless of the operating system - the FAT system used on memory cards and flash drives, for example. —end note]

>     path current_path();
>     path current_path(system::error_code& ec);

> > Returns: The current working directory path, as if by ISO/IEC 9945 `getcwd()`. `is_absolute()` is true for the returned path.

> > Throws: As specified in Error reporting.

> > [Note: The `current_path()` name was chosen to emphasize that the return is a path, not just a single directory name.

> > The current path as returned by many operating systems is a dangerous global variable. It may be changed unexpectedly by a third-party or system library functions, or by another thread. —end note]

>     void current_path(const path& p);
>     void current_path(const path& p, system::error_code& ec);

> > Effects: Establishes the postcondition, as if by ISO/IEC 9945 `chdir()`.

> > Postcondition: `equivalent(p, current_path())`.

> > Throws: As specified in Error reporting.

> > [Note: The current path for many operating systems is a dangerous global state. It may be changed unexpectedly by a third-party or system library functions, or by another thread. —end note]

>     bool exists(file_status s) noexcept;

> > Returns: `status_known(s) && s.type() != file_not_found`

>     bool exists(const path& p);
>     bool exists(const path& p, system::error_code& ec) noexcept;

> > Returns: `exists(status(p))` or `exists(status(p, ec))`, respectively. If ec != 0 and an error

> > Throws: As specified in Error reporting.

>     bool equivalent(const path& p1, const path& p2);
>     bool equivalent(const path& p1, const path& p2, system::error_code& ec);

> > Effects: Determines `file_status s1` and `s2`, as if by `status(p1)` and `status(p2)`, respectively.

> > Returns: `true`, if `sf1 == sf2` and `p1` and `p2` resolve to the same file system entity, else `false`.

> > > Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the ISO/IEC 9945 `stat` structure`,` obtained as if by `stat()` for the two paths, having equal `st_dev` values and equal `st_ino` values.

> > > [Note: ISO/IEC 9945 requires that "st_dev must be unique within a Local Area Network". Conservative ISO/IEC 9945 implementations may also wish to check for equal `st_size` and `st_mtime` values. Windows implementations may use `GetFileInformationByHandle()` as a surrogate for `stat()`, and consider "same" to be equal values for `dwVolumeSerialNumber`, `nFileIndexHigh`, `nFileIndexLow`, `nFileSizeHigh`, `nFileSizeLow`, `ftLastWriteTime.dwLowDateTime`, and `ftLastWriteTime.dwHighDateTime`. —end note]

> > Throws: `filesystem_error` if `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))`, otherwise as specified in Error reporting.

>     uintmax_t file_size(const path& p);
>     uintmax_t file_size(const path& p, system::error_code& ec);

> > Returns: If `exists(p) && is_regular_file(p)`, the size in bytes of the file `p` resolves to, determined as if by the value of the ISO/IEC 9945 `stat` structure member `st_size` obtained as if by ISO/IEC 9945 `stat()`. Otherwise, `static_cast<uintmax_t>(-1)`.

> > Throws: As specified in Error reporting.

>     uintmax_t hard_link_count(const path& p);
>     uintmax_t hard_link_count(const path& p, system::error_code& ec);

> > Returns: The number of hard links for `p`.

> > Throws: As specified in Error reporting.

>     const path& initial_path();
>     const path& initial_path(system::error_code& ec);

> > Returns: `current_path()` as of the first call to `initial_path()`.

> > [Note: `initial_path()` is not thread safe, and may return an undesirable result if called subsequent to a change to the current directory. These problems can be avoided by calling `initial_path()` immediately on entry to main(). —end note]

> > Throws: For the first call, as specified in Error reporting. Subsequent calls throw nothing.

>     bool is_directory(file_status s) noexcept;

> > Returns: `s.type() == directory_file`

>     bool is_directory(const path& p);
>     bool is_directory(const path& p, system::error_code& ec) noexcept;

> > Returns: `is_directory(status(p))` or `is_directory(status(p, ec))`, respectively.

> > Throws: `filesystem_error`; overload with `error_code&` throws nothing.

>     bool is_empty(const path& p);
>     bool is_empty(const path& p, system::error_code& ec);

> > Effects: Determines `file_status s`, as if by `status(p, ec)`.

> > Returns: `is_directory(s)
> > ? directory_iterator(p) == directory_iterator()
> > : file_size(p) == 0;`

>     bool is_regular_file(file_status s) noexcept;

> > Returns: `s.type() == regular_file`

>     bool is_regular_file(const path& p);

> > Returns: `is_regular_file(status(p))`.

> > Throws: `filesystem_error` if `status(p)` would throw `filesystem_error.`

>     bool is_regular_file(const path& p, system::error_code& ec) noexcept;

> > Effects: Sets `ec` as if by `status(p, ec)`. [Note: `status_error`, `file_not_found` and `type_unknown` cases set `ec` to error values. To distinguish between cases, call the `status` function directly. —end note]

> > Returns: `is_regular_file(status(p, ec))`.

>     bool is_other(file_status s) noexcept;

> > Returns: `return exists(s) && !is_regular_file(s) && !is_directory(s) && !is_symlink(s)`

>     bool is_other(const path& p);
>     bool is_other(const path& p, system::error_code& ec) noexcept;

> > Returns: `is_other(status(p))` or `is_other(status(p, ec))`, respectively.

> > Throws: `filesystem_error`; overload with `error_code&` throws nothing.

>     bool is_symlink(file_status s) noexcept;

> > Returns: `s.type() == symlink_file`

>     bool is_symlink(const path& p);
>     bool is_symlink(const path& p, system::error_code& ec) noexcept;

> > Returns: `is_symlink(symlink_status(p))` or `is_symlink(symlink_status(p, ec))`, respectively.

> > Throws: `filesystem_error`; overload with `error_code&` throws nothing.

>     std::time_t last_write_time(const path& p);
>     std::time_t last_write_time(const path& p, system::error_code& ec);

> > Returns: The time of last data modification of `p`, determined as if by the value of the ISO/IEC 9945 `stat` structure member `st_mtime` obtained as if by ISO/IEC 9945 `stat()`.

> > Throws: As specified in Error reporting.

>     void last_write_time(const path& p, const std::time_t new_time);
>     void last_write_time(const path& p, const std::time_t new_time, system::error_code& ec);

> > Effects: Sets the time of last data modification of the file resolved to by `p` to `new_time`, as if by ISO/IEC 9945 `stat()` followed by ISO/IEC 9945 `utime()`.

> > Throws: As specified in Error reporting.

> > [Note: A postcondition of `last_write_time(p) == new_time` is not specified since it might not hold for file systems with coarse time granularity. —end note]

>     void permissions(const path& p, perms prms);
>     void permissions(const path& p, perms prms, system::error_code& ec);

> > Requires: `!((prms & add_perms) && (prms & remove_perms))`.

> > Effects: Applies the effective permissions bits from `prms` to the file `p` resolves to, as if by ISO/IEC 9945 `fchmodat()`. The effective permission bits are determined as specified by the following table.

> > bits present in `prms` | Effective bits applied
> > Neither `add_perms` nor `remove_perms` | `prms & perms_mask`
> > `add_perms` | `status(p).permissions() | (prms & perms_mask)`
> > `remove_perms` | `status(p)``.permissions() & ~(prms & perms_mask)`

> > [Note: Conceptually permissions are viewed as bits, but the actual implementation may use some other mechanism. -- end note]

> > Throws: As specified in Error reporting.

>     path read_symlink(const path& p);
>     path read_symlink(const path& p, system::error_code& ec);

> > Returns: If `p` resolves to a symbolic link, a `path` object containing the contents of that symbolic link. Otherwise an empty `path` object.

> > Throws: As specified in Error reporting. [Note: It is an error if `p` does not resolve to a symbolic link. —end note]

>     path relative(const path& p, system::error_code& ec);

> > Returns: `relative(p, current_path(), ec)`.

> > Throws: As specified in Error reporting.

>     path relative(const path& p, const path& base=current_path());
>     path relative(const path& p, const path& base, system::error_code& ec);

> > Overview: Returns `p` made relative to `base`. Treats empty or identical paths as corner cases, not errors. Resolves symlinks and normalizes both `p` and `base` before other processing.

> > Returns: `weakly_canonical(p).lexically_relative(weakly_canonical(base))`. The second form returns `path()` if an error occurs.

> > Throws: As specified in Error reporting.

>     bool remove(const path& p);
>     bool remove(const path& p, system::error_code& ec);

> > Effects: If `exists(symlink_status(p,ec))`, it is removed as if by ISO/IEC 9945 `remove()`.

> > > [Note: A symbolic link is itself removed, rather than the file it resolves to being removed. —end note]

> > Postcondition: `!exists(symlink_status(p))`.

> > Returns: `false` if p did not exist in the first place, otherwise `true`.

> > Throws: As specified in Error reporting.

>     uintmax_t remove_all(const path& p);
>     uintmax_t remove_all(const path& p, system::error_code& ec);

> > Effects: Recursively deletes the contents of p if it exists, then deletes file `p` itself, as if by ISO/IEC 9945 `remove()`.

> > > [Note: A symbolic link is itself removed, rather than the file it resolves to being removed. —end note]

> > Postcondition: `!exists(p)`

> > Returns: The number of files removed.

> > Throws: As specified in Error reporting.

>     void rename(const path& old_p, const path& new_p);
>     void rename(const path& old_p, const path& new_p, system::error_code& ec);

> > Effects: Renames `old_p` to `new_p`, as if by ISO/IEC 9945 `rename()`.

> > > [Note: If `old_p` and `new_p` resolve to the same existing file, no action is taken. Otherwise, if `new_p` resolves to an existing non-directory file, it is removed, while if `new_p` resolves to an existing directory, it is removed if empty on ISO/IEC 9945 but is an error on Windows. A symbolic link is itself renamed, rather than the file it resolves to being renamed. —end note]

> > Throws: As specified in Error reporting.

>     void resize_file(const path& p, uintmax_t new_size);
>     void resize_file(const path& p, uintmax_t new_size, system::error_code& ec);

> > Postcondition: `file_size() == new_size`.

> > Throws: As specified in Error reporting.

> > Remarks: Achieves its postconditions as if by ISO/IEC 9945 `truncate()`.

>     space_info space(const path& p);
>     space_info space(const path& p, system::error_code& ec);

> > Returns: An object of type `space_info`. The value of the `space_info` object is determined as if by using ISO/IEC 9945 `statvfs()` to obtain a ISO/IEC 9945 struct `statvfs`, and then multiplying its `f_blocks`, `f_bfree`, and `f_bavail` members by its `f_frsize` member, and assigning the results to the `capacity`, `free`, and `available` members respectively. Any members for which the value cannot be determined shall be set to -1.

> > Throws: As specified in Error reporting.

>     file_status status(const path& p);

> > Effects: As if:

> > >     system::error_code ec;
> > >     file_status result = status(p, ec);
> > >     if (result == status_error)
> > >       throw filesystem_error(implementation-supplied-message, p, ec);
> > >     return result;

> > Returns: See above.

> > Throws: `filesystem_error`. [Note: `result` values of `file_status(file_not_found)`and `file_status(type_unknown)` are not considered failures and do not cause an exception to be thrown. —end note]

>     file_status status(const path& p, system::error_code& ec) noexcept;

> > Effects:

> > > If possible, determines the attributes of the file `p` resolves to, as if by ISO/IEC 9945 `stat()`.

> > > If, during attribute determination, the underlying file system API reports an error, sets `ec` to indicate the specific error reported. Otherwise, `ec.clear()`.

> > > > [Note: This allows users to inspect the specifics of underlying API errors even when the value returned by `status()` is not `file_status(status_error)`. —end note]

> > Returns:

> > > If `ec != error_code()`:

> > > - If the specific error indicates that `p` cannot be resolved because some element of the path does not exist, return `file_status(file_not_found)`. [Note: ISO/IEC 9945 errors that indicate this are ENOENT or ENOTDIR. Windows equivalents include ERROR_FILE_NOT_FOUND, ERROR_PATH_NOT_FOUND, ERROR_INVALID_NAME, ERROR_INVALID_PARAMETER, ERROR_BAD_PATHNAME, and ERROR_BAD_NETPATH. -- end note]
> > > - Otherwise, if the specific error indicates that `p` can be resolved but the attributes cannot be determined, return `file_status(type_unknown)`. [Note: For example, Windows ERROR_SHARING_VIOLATION errors. For ISO/IEC 9945, the case never arises. —end note]
> > > - Otherwise, return `file_status(status_error)`.

> > > > [Note: These semantics distinguish between `p` being known not to exist, `p` existing but not being able to determine its attributes, and there being an error that prevents even knowing if `p` exists. These distinctions are important to some use cases. —end note]

> > > Otherwise,

> > > - If the attributes indicate a regular file, as if by ISO/IEC 9945 S_ISREG(), return `file_status(regular_file)`. [Note: `regular_file` implies appropriate `<fstream>` operations would succeed, assuming no hardware, permission, access, or file system race errors. Lack of `regular_file` does not necessarily imply `<fstream>` operations would fail on a directory. —end note]
> > > - Otherwise, if the attributes indicate a directory, as if by ISO/IEC 9945 S_ISDIR(), return `file_status(directory_file)`. [Note: `directory_file` implies `directory_iterator(p)`would succeed. —end note]
> > > - Otherwise, if the attributes indicate a block special file, as if by ISO/IEC 9945 S_ISBLK(), return `file_status(block_file)`.
> > > - Otherwise, if the attributes indicate a character special file, as if by ISO/IEC 9945 S_ISCHR(), return `file_status(character_file)`.
> > > - Otherwise, if the attributes indicate a fifo or pipe file, as if by ISO/IEC 9945 S_ISFIFO(), return `file_status(fifo_file)`.
> > > - Otherwise, if the attributes indicate a socket, as if by ISO/IEC 9945 S_ISSOCK(), return `file_status(socket_file)`.
> > > - Otherwise, return `file_status(type_unknown)`.

> > Remarks: If a symbolic link is encountered during pathname resolution, pathname resolution continues using the contents of the symbolic link.

>     bool status_known(file_status s) noexcept;

> > Returns: `s.type() != status_error`

>     file_status symlink_status(const path& p);
>     file_status symlink_status(const path& p, system::error_code& ec) noexcept;

> > Effects: Same as status(), above, except that the attributes of `p` are determined as if by ISO/IEC 9945 `lstat()`.

> > Returns: Same as status(), above, except that if the attributes indicate a symbolic link, as if by ISO/IEC 9945 S_ISLNK(), return `file_status(symlink_file)`.

> > Remarks: Pathname resolution terminates if `p` names a symbolic link.

> > Throws: `filesystem_error`; overload with `error_code&` throws nothing.

>     path system_complete(const path& p);
>     path system_complete(const path& p, system::error_code& ec);

> > Effects: Composes an absolute path from `p`, using the same rules used by the operating system to resolve a path passed as the filename argument to standard library open functions.

> > Returns: The composed path.

> > Postcondition: For the returned path, `rp,` `rp.is_absolute()` is true.

> > Throws: As specified in Error reporting.

> > [Note: For ISO/IEC 9945, `system_complete(p)` has the same semantics as `complete(p, current_path())`.

> > For Windows, `system_complete(p)` has the same semantics as `complete(ph, current_path())` if `p.is_absolute() || !p.has_root_name()` or `p` and `base` have the same `root_name()`. Otherwise it acts like `complete(p, kinky)`, where `kinky` is the current directory for the `p.root_name()` drive. This will be the current directory of that drive the last time it was set, and thus may be residue left over from a prior program run by the command processor! Although these semantics are often useful, they are also very error-prone.

> > See complete() note for usage suggestions. —end note]

>     path temp_directory_path();
>     path temp_directory_path(system::error_code& ec);

> > Returns: A directory path suitable for temporary files under the conventions of the operating system. The specifics of how this path is determined are implementation defined. An error shall be reported if`!exists(p) || !is_directory(p)`, where `p` is the path to be returned.

> > ISO/IEC 9945: The path supplied by the first environment variable found in the list TMPDIR, TMP, TEMP, TEMPDIR. If none of these are found, `"/tmp"`, or, if macro `__ANDROID__` is defined, `"/data/local/tmp"`.

> > Windows: The path reported by the Windows `GetTempPath` API function.

> > Throws: As specified in Error reporting.

> > [Note: The `temp_directory_path()` name was chosen to emphasize that the return is a path, not just a single directory name. —end note]

>     path unique_path(const path& model="%%%%-%%%%-%%%%-%%%%");
>     path unique_path(const path& model, system::error_code& ec);

> > The `unique_path` function generates a path name suitable for creating temporary files, including directories. The name is based on a model that uses the percent sign character to specify replacement by a random hexadecimal digit. [Note: The more bits of randomness in the generated path name, the less likelihood of prior existence or being guessed. Each replacement hexadecimal digit in the model adds four bits of randomness. The default model thus provides 64 bits of randomness. This is sufficient for most applications. —end note]

> > Returns: A path identical to `model`, except that each occurrence of a percent sign character is replaced by a random hexadecimal digit character in the range 0-9, a-f.

> > Throws: As specified in Error reporting.

> > Remarks: Implementations are encouraged to obtain the required randomness via a cryptographically secure pseudo-random number generator, such as one provided by the operating system. [Note: Such generators may block until sufficient entropy develops. —end note]

>     path weakly_canonical(const path& p);
>     path weakly_canonical(const path& p, system::error_code& ec);

> > Overview: Returns `p` with symlinks resolved and the result normalized.

> > Returns: A path composed of the result of calling the `canonical` function on a path composed of the leading elements of `p` that exist, if any, followed by the elements of `p` that do not exist, if any.

> > Postcondition: The returned path is in normal form.

> > Remarks: Uses `operator/=` to compose the returned path. Uses the `status` function to determine existence.

> > Remarks: Implementations are encouraged to avoid unnecessary normalization such as when `canonical` has already been called on the entirety of `p`.

> > Throws: As specified in Error reporting.

> ### File streams - <boost/filesystem/fstream.hpp>

> Replacements are provided for the file stream classes from the C++ standard library's `<fstream>` header. These replacement classes publicly inherit from the standard library classes. In the Boost.Filesystem version, constructors and open functions take `const path&` arguments instead of `const char*` arguments. There are no other differences in syntax or semantics.

>     namespace boost
>     {
>       namespace filesystem
>       {
>         template < class charT, class traits = std::char_traits<charT> >
>         class basic_filebuf : public std::basic_filebuf<charT,traits>
>         {
>         public:
>           basic_filebuf<charT,traits>*
>             open(const path& p, std::ios_base::openmode mode);
>         };

>         template < class charT, class traits = std::char_traits<charT> >
>         class basic_ifstream : public std::basic_ifstream<charT,traits>
>         {
>         public:
>           explicit basic_ifstream(const path& p, std::ios_base::openmode mode=std::ios_base::in)
>           void open(const path& p, std::ios_base::openmode mode=std::ios_base::in);
>         };

>         template < class charT, class traits = std::char_traits<charT> >
>         class basic_ofstream : public std::basic_ofstream<charT,traits>
>         {
>         public:
>           explicit basic_ofstream(const path& p, std::ios_base::openmode mode=std::ios_base::out);
>           void open(const path& p, std::ios_base::openmode mode=std::ios_base::out);
>         };

>         template < class charT, class traits = std::char_traits<charT> >
>         class basic_fstream : public std::basic_fstream<charT,traits>
>         {
>         public:
>           explicit basic_fstream(const path& p,
>             std::ios_base::openmode mode=std::ios_base::in | std::ios_base::out);
>           void open(const path& p,
>             std::ios_base::openmode mode=std::ios_base::in | std::ios_base::out);
>         };

>         typedef basic_filebuf<char> filebuf;
>         typedef basic_ifstream<char> ifstream;
>         typedef basic_ofstream<char> ofstream;
>         typedef basic_fstream<char> fstream;

>         typedef basic_filebuf<wchar_t> wfilebuf;
>         typedef basic_ifstream<wchar_t> wifstream;
>         typedef basic_fstream<wchar_t> wfstream;
>         typedef basic_ofstream<wchar_t> wofstream;

>       }  // namespace filesystem
>     }  // namespace boost

> ## Path decomposition table

> The table is generated by a program compiled with the Boost implementation.

> Shaded entries indicate cases where ISO/IEC 9945 (POSIX) and Windows implementations yield different results. The top value is the ISO/IEC 9945 result and the bottom value is the Windows result.

> Constructor argument | Iteration over Elements | `string()` | `generic_ string()` | `root_ path()` | `root_ name()` | `root_ directory()` | `relative_ path()` | `parent_ path()` | `filename()`
> empty | empty | empty | empty | empty | empty | empty | empty | empty | empty
> `.` | `.` | `.` | `.` | empty | empty | empty | `.` | empty | `.`
> `..` | `..` | `..` | `..` | empty | empty | empty | `..` | empty | `..`
> `foo` | `foo` | `foo` | `foo` | empty | empty | empty | `foo` | empty | `foo`
> `/` | `/` | `/` | `/` | `/` | empty | `/` | empty | empty | `/`
> `/foo` | `/,foo` | `/foo` | `/foo` | `/` | empty | `/` | `foo` | `/` | `foo`
> `foo/` | `foo,.` | `foo/` | `foo/` | empty | empty | empty | `foo/` | `foo` | `.`
> `/foo/` | `/,foo,.` | `/foo/` | `/foo/` | `/` | empty | `/` | `foo/` | `/foo` | `.`
> `foo/bar` | `foo,bar` | `foo/bar` | `foo/bar` | empty | empty | empty | `foo/bar` | `foo` | `bar`
> `/foo/bar` | `/,foo,bar` | `/foo/bar` | `/foo/bar` | `/` | empty | `/` | `foo/bar` | `/foo` | `bar`
> `//net` | `//net` | `//net` | `//net` | `//net` | `//net` | empty | empty | empty | `//net`
> `//net/foo` | `//net,/,foo` | `//net/foo` | `//net/foo` | `//net/` | `//net` | `/` | `foo` | `//net/` | `foo`
> `///foo///` | `/,foo,.` | `///foo///` | `///foo///` | `/` | empty | `/` | `foo///` | `///foo` | `.`
> `///foo///bar` | `/,foo,bar` | `///foo///bar` | `///foo///bar` | `/` | empty | `/` | `foo///bar` | `///foo` | `bar`
> `/.` | `/,.` | `/.` | `/.` | `/` | empty | `/` | `.` | `/` | `.`
> `./` | `.,.` | `./` | `./` | empty | empty | empty | `./` | `.` | `.`
> `/..` | `/,..` | `/..` | `/..` | `/` | empty | `/` | `..` | `/` | `..`
> `../` | `..,.` | `../` | `../` | empty | empty | empty | `../` | `..` | `.`
> `foo/.` | `foo,.` | `foo/.` | `foo/.` | empty | empty | empty | `foo/.` | `foo` | `.`
> `foo/..` | `foo,..` | `foo/..` | `foo/..` | empty | empty | empty | `foo/..` | `foo` | `..`
> `foo/./` | `foo,.,.` | `foo/./` | `foo/./` | empty | empty | empty | `foo/./` | `foo/.` | `.`
> `foo/./bar` | `foo,.,bar` | `foo/./bar` | `foo/./bar` | empty | empty | empty | `foo/./bar` | `foo/.` | `bar`
> `foo/..` | `foo,..` | `foo/..` | `foo/..` | empty | empty | empty | `foo/..` | `foo` | `..`
> `foo/../` | `foo,..,.` | `foo/../` | `foo/../` | empty | empty | empty | `foo/../` | `foo/..` | `.`
> `foo/../bar` | `foo,..,bar` | `foo/../bar` | `foo/../bar` | empty | empty | empty | `foo/../bar` | `foo/..` | `bar`
> `c:` | `c:` | `c:` | `c:` | empty `c:` | empty `c:` | empty | `c:` empty | empty | `c:`
> `c:/` | `c:,.` `c:,/` | `c:/` | `c:/` | empty `c:/` | empty `c:` | empty `/` | `c:/` empty | `c:` | `.` `/`
> `c:foo` | `c:foo` `c:,foo` | `c:foo` | `c:foo` | empty `c:` | empty `c:` | empty | `c:foo` `foo` | empty `c:` | `c:foo` `foo`
> `c:/foo` | `c:,foo` `c:,/,foo` | `c:/foo` | `c:/foo` | empty `c:/` | empty `c:` | empty `/` | `c:/foo` `foo` | `c:` `c:/` | `foo`
> `c:foo/` | `c:foo,.` `c:,foo,.` | `c:foo/` | `c:foo/` | empty `c:` | empty `c:` | empty | `c:foo/` `foo/` | `c:foo` | `.`
> `c:/foo/` | `c:,foo,.` `c:,/,foo,.` | `c:/foo/` | `c:/foo/` | empty `c:/` | empty `c:` | empty `/` | `c:/foo/` `foo/` | `c:/foo` | `.`
> `c:/foo/bar` | `c:,foo,bar` `c:,/,foo,bar` | `c:/foo/bar` | `c:/foo/bar` | empty `c:/` | empty `c:` | empty `/` | `c:/foo/bar` `foo/bar` | `c:/foo` | `bar`
> `prn:` | `prn:` | `prn:` | `prn:` | empty `prn:` | empty `prn:` | empty | `prn:` empty | empty | `prn:`
> `c:\` | `c:\` `c:,/` | `c:\` | `c:\` `c:/` | empty `c:\` | empty `c:` | empty `\` | `c:\` empty | empty `c:` | `c:\` `\`
> `c:foo` | `c:foo` `c:,foo` | `c:foo` | `c:foo` | empty `c:` | empty `c:` | empty | `c:foo` `foo` | empty `c:` | `c:foo` `foo`
> `c:\foo` | `c:\foo` `c:,/,foo` | `c:\foo` | `c:\foo` `c:/foo` | empty `c:\` | empty `c:` | empty `\` | `c:\foo` `foo` | empty `c:\` | `c:\foo` `foo`
> `c:foo\` | `c:foo\` `c:,foo,.` | `c:foo\` | `c:foo\` `c:foo/` | empty `c:` | empty `c:` | empty | `c:foo\` `foo\` | empty `c:foo` | `c:foo\` `.`
> `c:\foo\` | `c:\foo\` `c:,/,foo,.` | `c:\foo\` | `c:\foo\` `c:/foo/` | empty `c:\` | empty `c:` | empty `\` | `c:\foo\` `foo\` | empty `c:\foo` | `c:\foo\` `.`
> `c:\foo/` | `c:\foo,.` `c:,/,foo,.` | `c:\foo/` | `c:\foo/` `c:/foo/` | empty `c:\` | empty `c:` | empty `\` | `c:\foo/` `foo/` | `c:\foo` | `.`
> `c:/foo\bar` | `c:,foo\bar` `c:,/,foo,bar` | `c:/foo\bar` | `c:/foo\bar` `c:/foo/bar` | empty `c:/` | empty `c:` | empty `/` | `c:/foo\bar` `foo\bar` | `c:` `c:/foo` | `foo\bar` `bar`

> ## Warning: Long paths on Windows and the extended-length \\?\ prefix

> The Microsoft Windows "Maximum Path Length Limitation" specifies:

> > In the Windows API (with some exceptions ...), the maximum length for a path is MAX_PATH, which is defined as 260 characters.

> > The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters. ... To specify an extended-length path, use the "\\?\" prefix. For example, "\\?\D:\very long path". [C++ string literals require backslashes be doubled, of course.]

> Because most Boost.Filesystem operational functions just pass the contents of a class path object to the Windows API, they do work with the extended-length prefixes. But some won't work, because to the limitations imposed by Windows. Read the following cautions carefully!

> ### Cautions for paths with extended-length prefixes

> - Individual components of a path are still are limited to whatever is supported for the particular filesystem, commonly 255 characters.
> - Only backslashes only are acceptable as directory separators. Slashes are not treated as separators.
> - All paths must be absolute - relative paths are not allowed.
> - Once an absolute path grows beyond 260 characters, it is essentially poisoned and all operations must use extended-length prefixes. So even a simple operation like `create_directory("a")` will fail if the absolute path of the resulting directory would exceed 260 characters.
> - Certain Boost.Filesystem functions that decompose their argument path and then work on individual relative directories or files will not work properly with extended-length prefix paths.

> ## Acknowledgements

> This Filesystem Library is dedicated to my wife, Sonda, who provided the support necessary to see both a trial implementation and the proposal itself through to completion. She gave me the strength to continue after a difficult year of cancer treatment in the middle of it all.

> Many people contributed technical comments, ideas, and suggestions to the Boost Filesystem Library. See http://www.boost.org/libs/filesystem/doc/index.htm#Acknowledgements.

> Dietmar Kuehl contributed the original Boost Filesystem Library directory_iterator design. Peter Dimov, Walter Landry, Rob Stewart, and Thomas Witt were particularly helpful in refining the library.

> The create_directories, extension, basename, and replace_extension functions were developed by Vladimir Prus. The temp_directory_path function was contributed by Jeff Flinn. David Svoboda suggested the canonical function and provided psuedo-code.

> Howard Hinnant and John Maddock reviewed a draft of the version 2 proposal, and identified a number of mistakes or weaknesses, resulting in a more polished final document.

> Peter Dimov suggested a single class path, with member templates to adapt to multiple string types. His idea became the basis for the version 3 path design.

> ## References

> [ISO/IEC 9945] | ISO/IEC 9945:2003, IEEE Std 1003.1-2001, and The Open Group Base Specifications, Issue 6. Also known as The Single Unix® Specification, Version 3. Available from each of the organizations involved in its creation. For example, read online or download from www.unix.org/single_unix_specification/. The ISO JTC1/SC22/WG15 - POSIX homepage is www.open-std.org/jtc1/sc22/WG15/
> [Abrahams] | Dave Abrahams, Error and Exception Handling, www.boost.org/more/error_handling.html

> © Copyright Beman Dawes, 2002, 2006, 2007, 2009, 2010, 2011

> Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt

> Revised 25 October 2015