Filesystem Library Update for TR2 (Preliminary)

This paper proposes that the filesystem library component of C++ Standard Library Technical Report 2 be based on Version 3 of the Boost Filesystem Library (see www.boost.org/libs/filesystem). Preliminary wording is provided. A TODO list identifies remaining work to be done.

J16/06-0045 = WG21/N1975, Filesystem Library Proposal for TR2 (Revision 3), was adopted by the committee in April, 2006, at the Berlin meeting. Shortly afterward the Library Working Group set aside work on TR2 to concentrate on C++0x. In the meantime, work on the Boost version of the Filesystem Library has continued, and Version 3 of the library has been released. Changes include:

• A single class path handles all aspects of internationalization, replacing the previous template and its path and wpath instantiations. Character types char, wchar_t, char16_t, and char32_t are supported. This is a major simplification of the path abstraction, particularly for functions that take path arguments. This change was based on a suggestion by Peter Dimov.
   
• Several operational functions have been added, including much better symlink support, the ability to resize a file, and the ability to generate a unique path.
   
• Support for error reporting via error_code is now uniform throughout the operations functions.|
   
• Several functions have been renamed, based on feedback from users.

Motivation and Scope

The motivation and scope for a filesystem library were described in N1975, and are not repeated here. A minor scope reduction is that an addition to the current C++ runtime library is no longer needed.

Boost Filesystem Version 3 introduced a single path type that interoperates well with both basic_string and user defined string types. Thus the following Design alternatives paragraph is no long applicable:

Single path type which can at runtime accept narrow or wide character pathnames. Although certainly interesting, and possibly superior, such a design would not interoperate well with the current Standard Library's compile-time typed basic_string. A new runtime polymorphic string class would be the best place to experiment with this concept, not a path class.

TODO

• Apply C++0X features. Boost.Filesystem needs to implement these to verify their application is correct.
• Boost.Filesystem needs to implement char16_t and char32_t support to verify the specification for these is correct.
• Class file_status is overdesigned. It holds just a single file_type variable, and that's unlikely to change because of performance considerations. It would be simpler to eliminate class file_status, and just use file_type in its stead. Perhaps rename to file_status. This hasn't been done in the Boost because it would break existing code, albeit noisily.
• Replace path inserter and extractor Effects with prose, since the current wording relies on boost::io::quoted.
• Replace uses of boost::iterator_facade.
• Source is not specified as actually implemented. Expose path_traits?
• Effects for copy and copy_directory need to be reviewed, revised, tested.

Table of Contents

Proposed wording

Gray-shaded italic text is commentary on the proposal. It is not to be added to the TR.

The following wording is a lighted edited version of the Boost Filesystem reference documentation. Although further editing is required, the features described are relatively stable and major feature changes are not anticipated.

Add the following to the Technical Report's front matter:

The following standard contains provisions which, through reference in this text, constitute provisions of this Technical Report. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this Technical Report are encouraged to investigate the possibility of applying the most recent editions of the standard indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.

• ISO/IEC 9945:2003, Portable Operating System Interface (POSIX¹), part 1 (Base Definitions) and part 2 (System Interfaces), both as corrected by their respective 2004 Correction 1 documents.

  [Note: ISO/IEC 9945:2003 is also IEEE Std 1003.1-2001, and The Open Group Base Specifications, Issue 6, and is also known as The Single Unix² Specification, Version 3. It is available from each of those organizations, and may be read online or downloaded from www.unix.org/single_unix_specification/ -- end note]

ISO/IEC 9945:2003, with the indicated corrections, is hereinafter called POSIX.

Footnote 1: POSIX® is a registered trademark of The IEEE.

Footnote 2: UNIX® is a registered trademark of The Open Group.

Add the following to the Technical Report as a new Clause:

Filesystem Library

This Clause describes components that perform operations on file systems and their components, such as paths, regular files, and directories.

Operating systems such as MAC OS, UNIX, and Windows are mentioned in this Clause for purposes of illustration or to give guidance to implementers. No slight to other operating systems is implied or intended.

Footnote 3: MAC OS® is a registered trademark of Apple Inc.

Footnote 4: Windows® is a registered trademark of Microsoft Corporation.

Conformance

Some library behavior is specified in this Clause by reference to POSIX. How such behavior is actually implemented is unspecified.

[Note: This constitutes an "as if" rule for implementation of operating system dependent behavior. Presumably implementations will call native operating system API's, but this is not required. --end note]

Implementations are encouraged, but not required, to support such behavior as it is defined by POSIX. Implementations shall document behavior that differs from the POSIX defined behavior. Implementations that do not support exact POSIX behavior are encouraged to provide behavior as close to POSIX behavior as is reasonable given the limitations of actual operating systems and file systems. A diagnostic is required if an implementation cannot provide reasonable behavior.

The behavior of functions described in this Clause may not be achieved in the presence of race conditions. No diagnostic is required.

If the possibility of race conditions would make it unreliable for a program to test for a precondition before calling a function described in this clause, the precondition is not specified in the function's Requires element.

Definitions

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

File system
A collection of files and certain of their attributes.

Filename
The name of a file. Slash and 0x00 characters are not permitted. Implementations may define additional characters or specific names that are not permitted. Filenames .  and ..  have special meaning. Implementations may define additional filenames that have special meaning.

[Note: Most operating systems prohibit the ASCII control characters (0x00-0x1F) in filenames.

Windows prohibits the characters 0x00-0x1F, ", *, :, <, >, ?, \, /, and | --end note]

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

Absolute path
 A path that uniquely identifies a file. The format is implementation defined.

[Note: For POSIX-like implementations, including Unix variants, Linux, and Mac OS X, only paths that begin with a slash are absolute paths.

For Windows-like implementations, including Cygwin and MinGW, only paths that begin with a drive specifier followed by a slash, or begin with two slashes, are absolute paths. --end note]

Relative path
A path that uniquely identifies a file only when considered relative to some other path. [Note: Paths "." and ".." are considered to be relative paths. --end note]

Pathname
A character string that represents a path. Pathnames are formatted according to the generic pathname format or the native pathname format.

Generic pathname format

pathname:
            root-name_opt root-directory_opt relative-path_opt

root-name:
            implementation-defined

[Note: Most POSIX and Windows based operating system define a name beginning with two slashes (or backslashes, for Windows) as a root-name identifying network locations. Windows defines a single letter followed by a ":" as a root-name identifying a disc drive. --end note]

root-directory:
            directory-separator

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

filename:
            name
            "."
            ".."

directory-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 current directory. Specific filenames may have special meaning for a particular operating system.

Native pathname format
An implementation defined format. [Note: For POSIX-like operating systems, the native format is the same as the generic format. For Windows, the native format is similar to the generic format, but the directory-separator characters can be either slashes or backslashes. --end note]

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
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
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]

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

Dot, Dot Dot: Synonyms for the filenames "." and "..", respectively. The dot filename names the current directory. The dot dot filename names the parent directory.

Header <filesystem> synopsis

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class path;
      class filesystem_error;
      class directory_entry;
      class directory_iterator;
      class recursive_directory_iterator;
      class file_status;

      void swap(path& lhs, path& rhs);
      bool lexicographical_compare(path::iterator first1, path::iterator last1,
                                   path::iterator first2, path::iterator last2);

      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);

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

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

      struct space_info  // returned by space function
      {
        uintmax_t capacity;
        uintmax_t free; 
        uintmax_t available; // free space available to a 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());

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

      bool         status_known(file_status s);

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

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

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

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

    } // namespace filesystem
  }} // namespaces std and tr2

Error reporting

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 recommended approach for much 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. Code littered with try - catch blocks is an indication that error reporting by error_code may be more appropriate.

--end note]

Functions not having an argument of type 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 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 error_code& argument is set as appropriate appropriate for the specific error. Otherwise, clear() is called on the 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

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 may not actually exist in external storage, and may contain pathnames which are not even valid for the current operating system.

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class path
      {
      public:
        typedef see below                          value_type;  // char for POSIX, wchar_t for Windows
        typedef basic_string<value_type>           string_type;
        typedef codecvt<wchar_t, char, mbstate_t>  codecvt_type;

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

        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);

        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());

        // modifiers
        void  clear();
        path& make_absolute(const path& base);
        path& make_preferred();  // POSIX: no effect. Windows: convert slashes to backslashes
        path& remove_filename();
        path& replace_extension(const path& new_extension = path());
        void  swap(path& rhs);

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

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

        const string        string(const codecvt_type& cvt=codecvt()) const;   // native format
        const wstring       wstring(const codecvt_type& cvt=codecvt()) const;  // ditto
        const u16string     u16string() const;                                 // ditto
        const u32string     u32string() const;                                 // ditto

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

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

        // 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;

        iterator begin() const;
        iterator end() const;
        
        // encoding conversion
        static locale imbue( const locale& loc );
        static const codecvt_type & codecvt();

      private:
        string_type pathname;  // exposition only
      };

    } // namespace filesystem
  }} // namespaces std and tr2

value_type is an implementation-defined typedef for the character type used by the implementation to represent pathnames.

If the value type of  [begin,end) or source arguments for member functions is not value_type, and no cvt argument is supplied, conversion to value_type occurs using an imbued locale. This imbued locale is initialized with a codecvt facet appropriate for the operating system.

For Apple OS X implementations, path::value_type is char. The default imbued locale provides a UTF-8 codecvt facet. [Rationale: "All BSD system functions expect their string parameters to be in UTF-8 encoding and nothing else." See Apple docs. -- end rationale]

For Windows-like implementations, including Cygwin and MinGW, path::value_type is wchar_t. The default imbued locale provides a codecvt facet that invokes Windows MultiByteToWideChar or WideCharToMultiByte API's with a codepage of CP_THREAD_ACP if Windows AreFileApisANSI()is true, otherwise codepage CP_OEMCP. [Rationale: this is the current behavior of C and C++ programs that perform file operations using narrow character string to identify paths. Changing this in the Filesystem library would be too surprising, particularly where user input is involved. -- end rationale]

For all other implementations, including Linux, path::value_type is char. The default imbued locale is locale(""). [Rationale: ISO C specifies this as "the locale-specific native environment", while POSIX says it "Specifies an implementation-defined native environment." -- end rationale

For member functions described as returning const string, const wstring, const u16string, or const u32string, implementations are permitted to return const string&, const wstring&, const u16string&, or const u32string&, respectively.

[Note: This allows implementations to avoid unnecessary copies when no conversion is required. Return-by-value is specified as const to ensure programs won't break if moved to a return-by-reference implementation. -- end note]

InputIterator is required to meet the RandomIterator requirements. The iterator's value type is required to be char, wchar_t, char16_t, or char32_t.

Source is 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 directory_entry.

The specifications for path functions require that arguments in the generic pathname format be converted to native pathname format as they are stored in pathname. If the native format requires regular file paths and directory paths to be formatted differently, the implementation shall determine which format to use according to whether or not the last element of the generic format string is a separator. [Example: On OpenVMS, a path constructed from "/cats/jane" would considered a regular file path, and have a native format of "[CATS]JANE", while a path constructed from "/cats/jane/" would be considered a directory path, and have a native format of "[CATS.JANE]". --end example] [Note: POSIX and Windows use the same native format for both regular file and directory pathnames, so this paragraph does not apply to them. --end note]

path constructors

path();

Postcondition: empty().

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. If the contents are in the generic format and the generic format is unacceptable to the operating system's API, they are converted to the native format. [Note: For POSIX and Windows implementations, the generic format is already acceptable as a native format, so no generic to native conversion is performed. --end note]

Remarks: If the value type of  [begin,end) or source is not value_type, conversion is performed by cvt.

path assignments

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. If the contents are in the generic format, they are converted to the native format. [Note: For POSIX and Windows based implementations, the generic format is already acceptable as a native format, so no generic to native conversion is performed. --end note]

Returns: *this

Remarks: If the value type of  [begin,end) or source is not value_type, conversion is performed by cvt.

path appends

The append operations use operator/= to denote their semantic effect of appending the platform's preferred directory separator when needed. The preferred directory separator is implementation-defined.

[Note: For POSIX-like implementations, including Unix variants, Linux, and Mac OS X, the preferred directory separator is a single forward slash.

For Windows-like implementations, including Cygwin and MinGW, the preferred directory separator is a single backslash.--end note]

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

Effects:

Appends the preferred directory separator to the contained pathname, 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.

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 a native directory separator to the contained pathname, unless:

• an added separator would be redundant, or

• would change an relative path to an absoute path, or

• p.empty(), or

• *p.native().cbegin() is a separator.

Appends the contents [begin,end) or source to pathname. If the contents are in the generic format, they are converted to the native format. [Note: For POSIX and Windows based implementations, the generic format is already acceptable as a native format, so no generic to native conversion is performed. --end note]

Remarks: If the value type of  [begin,end) or source is not value_type, conversion is performed by cvt.

Returns: *this

path modifiers

void clear();

Postcondition: this->empty() is true.

path& make_preferred();

Effects: The contained pathname is converted to the preferred native format. [Note: On Windows, the effect is to replace slashes with backslashes. On POSIX, there is no effect. -- end note]

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());

Postcondition: extension() == replacement, where replacement is new_extension if new_extension.empty() || new_extension[0] == the dot character, otherwise replacement is the dot character followed by new_extension.

Returns: *this

void swap(path& rhs);

Effects: Swaps the contents of the two paths.

Throws: nothing.

Complexity: constant time.

path native format observers

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

const string_type&  native() const;

Returns: pathname.

Throws: nothing.

const value_type* c_str() const;

Returns: pathname.c_str().

Throws: nothing.

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.

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

Returns: pathname.

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

If string_type is the same type as the function's return type, the function is permitted to return by const& rather than const value. [Note: For POSIX, this occurs for string(), for Windows, wstring(). --end note]

path generic format observers

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

[Note: For POSIX, no conversion occurs, since the native format and generic format are the same. For Windows, backslashes are converted to slashes --end note]

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.

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

If string_type is of the same type as the function's return type, and the generic format is the same as the native format, the function is permitted to return by const& rather than const value. [Note: For POSIX, this occurs for string(). It never occurs for Windows, because backslashes must be converted to slashes. --end note]

path decomposition

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().

path filename() const;

Returns: empty() ? path() : *--end()

[Example:

cout << path("/foo/bar.txt").filename(); // outputs "bar.txt" (without the quotes)

--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:

cout << path("/foo/bar.txt").stem(); // outputs "bar" (without the quotes)

path p = "foo.bar.baz.tar";
for (; !p.extension().empty(); p = p.stem())
  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:

cout << path("/foo/bar.txt").extension(); // outputs ".txt" (without the quotes)

--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.  -- end note]

 See http://permalink.gmane.org/gmane.comp.lib.boost.devel/199744 for more extensive rationale.

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.

[Note: On POSIX, path("/foo").is_absolute() returns true. On Windows, path("/foo").is_absolute() returns false. --end note]

bool is_relative() const;

Returns: !is_absolute().

path iterators

A path::iterator is a constant iterator satisfying all the requirements of a bidirectional iterator (C++ Std, 24.1.4 Bidirectional iterators [lib.bidirectional.iterators]). Its value_type is path.

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.

• Each successive filename element, if present.

• Dot, if one or more trailing non-root slash characters are present.

The backward traversal order is the reverse of forward traversal.

iterator begin() const;

Returns: An iterator for the first present element in the traversal list above. If no elements are present, the end iterator.

iterator end() const;

Returns: The end iterator.

path encoding conversion

static locale imbue(const locale& loc);

Effects: Stores loc as the default locale for all objects of type path.

Returns: The previous default locale for all objects of type path.

static const codecvt_type& codecvt();

Returns: The codecvt facet for the default locale for all objects of type path.

path non-member functions

void swap( path& lhs, path& rhs )

Effects: lhs.swap(rhs).

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 range [first1,last1) is lexicographically less than the sequence of native() strings for the elements defined by the 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.

  for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
    if (first1->native() < first2->native()) return true;
    if (first2->native() < first1->native()) return false;
  }
  return first1 == last1 && first2 != last2;

[Note: A path aware lexicographical_compare is provided to avoid infinite recursion in lexicographical_compare due to the path iterator's value type itself being path. --end note]

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

Returns: return lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end()).

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: Actual implementations may use an equivalent, but more efficient, algorithm. --end note]

[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

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

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

Effects:  os << boost::io::quoted(p.string<basic_string<Char>>(), static_cast<Char>('&'));

Returns: os

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

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

Returns: is

Class filesystem_error

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class basic_filesystem_error : public system_error
      {
      public:
        filesystem_error();
        filesystem_error(const filesystem_error&);
        filesystem_error(const string& what_arg, error_code ec);
        filesystem_error(const string& what_arg, const path& p1, error_code ec);
        filesystem_error(const string& what_arg, const path& p1, const path& p2, 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
  }} // namespaces std and tr2

The class template basic_filesystem_error defines the type of objects thrown as exceptions to report file system errors from functions described in this clause.

filesystem_error members

filesystem_error(const 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 string& what_arg, const path_type& 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 string& what_arg, const path_type& p1, const path_type& p2, error_code ec);

Postcondition:

Expression	Value
runtime_error::what()	what_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.

Class directory_entry

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class directory_entry
      {
      public:

        // constructors and destructor
        directory_entry();
        directory_entry(const directory_entry&);
        explicit directory_entry(const path_type& 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_type& 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(error_code& ec) const;
        file_status  symlink_status() const;
        file_status  symlink_status(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_type            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
  }} // namespaces std and tr2

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 very 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 race conditions. -- 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();

Postcondition:

Expression	Value
path().empty()	true
status()	file_status()
symlink_status()	file_status()

explicit directory_entry(const path_type& 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

void assign(const path_type& 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

const path& path() const;

Returns: m_path

file_status status() const;
file_status status(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(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

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

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class directory_iterator
        : public boost::iterator_facade< directory_iterator,
                                         directory_entry,
                                         boost::single_pass_traversal_tag >
      {
      public:
        // member functions

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

        directory_iterator& operator=(const directory_iterator&);

        directory_iterator& operator++();
        directory_iterator& increment(error_code& ec);

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

    } // namespace filesystem
  }} // namespaces std and tr2

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 POSIX 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 becomes equal to the end iterator value. The constructor directory_iterator() with no arguments always constructs an end iterator object, which is the only legitimate iterator to be used 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 not defined. For any other iterator value a const directory_entry* is returned.

Two end iterators are always equal. An end iterator is not 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 POSIX readdir_r(). --end note]

directory_iterator members

directory_iterator();

Effects: Constructs the end iterator.

Throws: Nothing.

explicit directory_iterator(const path& p);
directory_iterator(const path& p, 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(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.

Class recursive_directory_iterator

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

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class recursive_directory_iterator :
        public iterator<input_iterator_tag, directory_entry>
      {
      public:

        // constructors and destructor
        recursive_directory_iterator();
        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, error_code& ec);
        recursive_directory_iterator(const path& p, error_code& ec);
       ~recursive_directory_iterator();

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

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

        recursive_directory_iterator& operator++();
        recursive_directory_iterator& increment(error_code& ec);

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

        // other members as required by
        //  C++ Std, 24.1.2 Input iterators [input.iterators]

      private:
        // actual data members may be stored in a shared pimpl 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
  }} // namespaces std and tr2

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 ovee, 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();

Effects: Constructs the end iterator.

Throws: Nothing.

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

Requires: *this != recursive_directory_iterator().

Returns: m_level.

Throws: Nothing.

bool no_push_pending() const;

Requires: *this != recursive_directory_iterator().

Returns: m_no_push.

Throws: Nothing.

recursive_directory_iterator& operator++();
recursive_directory_iterator& increment(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.

Throws: Nothing.

void no_push(bool value=true);

Requires: *this != recursive_directory_iterator().

Postcondition: no_push_pending() == value.

Throws: Nothing.

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

Class file_status

  namespace std { namespace tr2 {
  {
    namespace filesystem
    {
      class file_status
      {
      public:
        file_status();
        file_status(const file_status&);
        explicit file_status(file_type v=status_error);
       ~file_status();

        file_type type() const;
        void type(file_type v);
      };
    } // namespace filesystem
  } // namespace std { namespace tr2 {

An object of type file_status stores information about the status of a file. The internal form of the stored information is unspecified.

[Note: The class may be extended in the future to store additional status information. --end note]

Members

explicit file_status(file_type v=status_error);

Effects: Stores v.

Throws: Nothing.

file_type type() const;

Returns: The stored file_type.

void type(file_type v);

Effects: Stores v, replacing the previously stored value.

Operational functions

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 POSIX Pathname Resolution mechanism.

[Note: Because hardware failures, network failures, race conditions, 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]

Operational function specifications

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

Returns: A path composed according to 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

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

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

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

Effects:

Throws: As specified in Error reporting.

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

Requires: p.empty() ||
forall px: px == p || is_parent(px, p): is_directory(px) || !exists( px )

Postcondition: is_directory(p)

Returns: The value of !exists(p) prior to the establishment of the postcondition.

Throws: As specified in Error reporting.

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

Effects: Attempts to create the directory p resolves to, as if by POSIX mkdir() with a second argument of S_IRWXU|S_IRWXG|S_IRWXO.

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, error_code& ec);

Effects: Establishes the postcondition, as if by POSIX 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. Windows prior to Vista, for example, did not support symbolic links. Some file systems do not support symbolic links regardless of the operating system - the FAT system used on floppy discs, memory cards and flash drives, for example. Thus symbolic links should only be used if these situations are not concerns, or if workarounds are provided. -- 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, error_code& ec);

Effects: Establishes the postcondition, as if by POSIX 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 system used on floppy discs, memory cards and flash drives, for example. Some file systems limit the number of links per file. Thus hard links should only be used if these situations are not concerns, or if workarounds are provided. -- end note]

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

Effects: Establishes the postcondition, as if by POSIX 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. Windows prior to Vista, for example, did not support symbolic links. Some file systems do not support symbolic links regardless of the operating system - the FAT system used on floppy discs, memory cards and flash drives, for example. Thus symbolic links should only be used if these situations are not concerns, or if workarounds are provided. -- end note]

path current_path();
path current_path(error_code& ec);

Returns: The current working directory path, as if by POSIX 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, error_code& ec);

Effects: Establishes the postcondition, as if by POSIX 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);

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

Throws: Nothing.

bool exists(const path& p);
bool exists(const path& p, error_code& ec);

Returns: exists(status(p)) or exists(status(p, ec)), respectively.

Throws: filesystem_error; overload with error_code& throws nothing.

bool equivalent(const path& p1, const path& p2);
bool equivalent(const path& p1, const path& p2, 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 POSIX stat structure, obtained as if by stat() for the two paths, having equal st_dev values and equal st_ino values.

[Note: POSIX requires that "st_dev must be unique within a Local Area Network". Conservative POSIX 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, error_code& ec);

Remarks:

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 POSIX stat structure member st_size obtained as if by POSIX 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, 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(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);

Returns: s.type() == directory_file

Throws: Nothing.

bool is_directory(const path& p);
bool is_directory(const path& p, error_code& ec);

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, 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);

Returns: s.type() == regular_file

Throws: Nothing.

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, error_code& ec);

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)).

Throws: Nothing.

bool is_other(file_status s);

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

Throws: Nothing.

bool is_other(const path& p);
bool is_other(const path& p, error_code& ec);

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);

Returns: s.type() == symlink_file

Throws: Nothing.

bool is_symlink(const path& p);
bool is_symlink(const path& p, error_code& ec);

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

Throws: filesystem_error; overload with error_code& throws nothing.

time_t last_write_time(const path& p);
time_t last_write_time(const path& p, error_code& ec);

Returns: The time of last data modification of p, determined as if by the value of the POSIX stat structure member st_mtime  obtained as if by POSIX stat().

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

Effects: Sets the time of last data modification of the file resolved to by p to new_time, as if by POSIX stat() followed by POSIX 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]

path read_symlink(const path& p);
path read_symlink(const path& p, 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]

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

Effects:  If exists(symlink_status(p,ec)), it is removed as if by POSIX 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, error_code& ec);

Effects:  Recursively deletes the contents of p if it exists, then deletes file p itself, as if by POSIX 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, error_code& ec);

Effects: Renames old_p to new_p, as if by POSIX 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 POSIX 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, error_code& ec);

Postcondition: file_size() == new_size.

Throws: As specified in Error reporting.

Remarks: Achieves its postconditions as if by POSIX truncate().

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

Returns: An object of type space_info. The value of the space_info object is determined as if by using POSIX statvfs() to obtain a POSIX 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:

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, error_code& ec);

Effects:

If possible, determines the attributes of the file p resolves to, as if by POSIX 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: POSIX 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 POSIX, 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 POSIX S_ISREG(), return file_status(regular_file). [Note: regular_file implies appropriate <fstream> operations would succeed, assuming no hardware, permission, access, or race condition 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 POSIX 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 POSIX S_ISBLK(), return file_status(block_file).
   

• Otherwise, if the attributes indicate a character special file, as if by POSIX S_ISCHR(), return file_status(character_file).
   

• Otherwise, if the attributes indicate a fifo or pipe file, as if by POSIX S_ISFIFO(), return file_status(fifo_file).
   

• Otherwise, if the attributes indicate a socket, as if by POSIX S_ISSOCK(), return file_status(socket_file).
   

• Otherwise, return file_status(type_unknown).

Throws: Nothing.

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);

Returns: s.type() != status_error

Throws: Nothing.

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

Effects:  Same as status(), above, except that the attributes of p are determined as if by POSIX lstat().

Returns: Same as status(), above, except that if the attributes indicate a symbolic link, as if by POSIX S_ISLNK(), return file_status(symlink_file).

Throws: Nothing.

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

path system_complete(const path& p);
path system_complete(const path& p, 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.

[Note: For POSIX, 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(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.

POSIX: The path supplied by the first environment variable found in the list TMPDIR, TMP, TEMP, TEMPDIR. If none of these are found, "/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, 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]

Additions to Header <fstream>

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. Constructors and open functions take const path& arguments. There are no other differences in syntax or semantics.

namespace std { namespace tr2 {
  namespace filesystem
  {
    template < class charT, class traits = 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 = char_traits<charT> >
    class basic_ifstream : public std::basic_ifstream<charT,traits>
    {
    public:
      explicit basic_ifstream(const path& p, std::ios_base::openmode mode=ios_base::in)
      void open(const path& p, std::ios_base::openmode mode=std::ios_base::in);
    };

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

    template < class charT, class traits = 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
}} // namespaces std and tr2

End of the proposed wording

Path decomposition table

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

Shaded entries indicate cases where POSIX and Windows implementations yield different results. The top value is the POSIX result and the bottom value is the Windows result.

Path Decomposition Table

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

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.

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.

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.