Document number:	N3505
Date:	2013-01-12
Project:	Programming Language C++
Reply-to:	Beman Dawes <bdawes at acm dot org>

Filesystem Library Proposal (Revision 4)

Introduction

This paper proposes a filesystem library component suitable for a C++ Standard Library Technical Specification. The proposed library is based on the Boost Filesystem Library, version 3.

The proposal is a pure addition; no existing standard library headers are modified.  No existing code is broken by this proposal, assuming the usual namespace discipline caveats. No special compiler or runtime support is required, although C++11 features are used where appropriate. The entire library is in a namespace within namespace std.

Library background

The Boost version of the library has been in widespread use for ten years. The Dinkumware version of the library, based on N1975 (equivalent to version 2 of the Boost library), ships with Microsoft Visual C++ 2012.

The motivation and scope for a filesystem library were described in N1975, and are not repeated here.

Differences from Boost Filesystem

In the Boost library, facilities in class path to handle narrow (i.e. char) character strings with encodings other than the operating system's native encoding depend on codecvt arguments. In the proposal, these facilities depend on locale arguments and additional UTF-8 facilities have been added. The Filesystem Study Group believes this approach is more reliable and useful, particularly as UTF-8 use continues to spread.

Otherwise, the proposal is very similar to the Boost library. Indeed, most of the proposed wording is generated from the same source as the Boost library's reference documentation.

Design decisions for paths

The Filesystem library specification for paths carefully to duplicates the encoding related behavior of the traditional C++ and C library I/O functions that take narrow character path arguments. (E.G. on POSIX pass the arguments unchanged to system API functions, on Windows let system API functions handle the narrow to wide conversion.) Users who have been satisfied with that behavior, and have not been plagued by portability or other path name encoding problems, presumably will continue to supply paths to file system related functions the same way they do currently and will continue to be satisfied. But users who have been frustrated with portability or other path name encoding problems may use C++11 char16_t and char32_t arguments to and returns from class path, the class path constructors that take locales, and the UTF-8 specific functions to address encoding problems.

Revision history

N3505, Filesystem Library Proposal (Revision 4). Changes include:

• Editorial: Update POSIX references to ISO/IEC/IEEE 9945:2009.
• Editorial: Remove the table and shading formatting from examples and notes that mention trademarked names. The SG reviewed the use of trademarked names in Portland, and is satisfied they are useful and that ISO requirements have been met.
• Editorial: Remove the TODO list. Items have either been completed or moved elsewhere.
• Editorial: Corrections reported by Jens Maurer.
• Editorial: Corrections reported by Daniel Krügler.
• Editorial: Cleanup of operating system references.
• Editorial: Change document HTML encoding to charset=utf-8.
• Editorial: Add "native encoding" definition for both narrow and wide encoding, and then replace uses of "native encoding" with "native narrow encoding" or "native wide encode" in response to request for clarification from Jean-Marc Bourguet, and a suggestion to add definition from Pete Becker.
• Fix some class path editorial issues, remove "= default" from special member functions, and supply semantics for these functions. (Daniel Krügler, Alberto Ganesh Barbati)
• Remove all codecvt arguments, as agreed by the SG in Portland.
• Add two class path constructors and two path factory functions, with the UTF-8 and locale functionality agreed on by the SG in Portland. The functionality for the constructors and factory functions were reversed compared to the Portland discussion. This eliminated the need for an encoding enum, shortened the UTF-8 use case from path(my_string, encoding::utf8) to u8path(my_string), and shortened the locale argument use case from make_path(my_string, my_locale) to path(my_string, my_locale). Thanks to Daniel Krügler for help with the exact semantics.
• Add class path observer functions u8string() and generic_u8string(), as agreed by the SG in Portland.
• Add operator string_type() const; to class path. Microsoft and Dinkumware are already shipping this in their implementations. It provides a reasonable level of interoperation with standard library file steam constructors and open functions without modifying existing standard library classes or headers.

N3399=12-0089, Filesystem Library Proposal (Revision 3). Changes include:

• Numerous corrections and improvements to the standardese of the proposed wording, based on comments and suggestions from the Filesystem Study Group, particularly Daniel Krügler and Robert Stewart.
• Add path::operator+= and concat functions to tack on things like suffixes or numbers. Suggested by Ed Smith-Rowland and others.
• Add section tags and section numbers, per LWG discussion in Kona.
• Minimize use of trademarked names. Enclose uses in green highlighted boxes to facilitate discussion. Apply footnote required by ISO/IEC directives.
• Introduce a definition for "operating system dependent", and then use it to replace "implementation defined behavior" wherever the operating system's rules determine behavior.
• Replace uses of  const string return types with non-const string return types, per LWG discussion in Kona.
• Remove permission for implementations to return const string& in certain cases, per LWG discussion in Kona.
• Remove path inserter and extractor dependency on Boost quoted manip (Issue 7).
• Replace use of time_t with file_time_type, a typedef for std::chrono::system_clock::time_point, per LWG discussion in Kona.
• More consistent semantics and descriptions for return values from operational functions with error_code& arguments that encounter an error.
• recursive_directory_iterator specifications are now given in terms of behavior only. Private "for exposition only" members have been removed.
• recursive_directory_iterator functions no_push_pending() and no_push() have been renamed recursion_pending() and disable_recursion_pending(). Option constant name recurse has been renamed follow_directory_symlinks.  These are responses to a request from Ed Smith-Rowland to improve clarity by reducing negative logic.
• A recursive_directory_iterator option skip_permission_denied has been added to skip directories with permission denied, responding to several user requests.
• symlink_option has been renamed directory_options, since it has become more general and now covers both follow_directory_symlinks and skip_permission_denied options. 
• Add missing semantics for copy_directory.
• Change template parameters for path::string and path::generic_string to the same as std::basic_string in response to comments from the FSG.
• Remove initial_path() operational function. The functionality as currently specified is trivial for a user to duplicate using current_path(). The original spec for initial_path() required support from the runtime library. That doesn't appear to be practical, and without it the motivation for the function is too weak for standardization.
• Remove required changes to <fstream> header. The LWG does not wish to require modifications existing headers. Also, the Dinkumware/Microsoft version of the library has come up with a clever way to make class path work with the existing <fstream> classes constructors and open functions.
• Remove directory_entry as a Source in [path.req]. It was Boost specific and is inappropriate for the standard library.
• Change name of recursive_directory_iterator::level() to depth().
• Change enum file_type to enum class file_type, simplify constant names, supply explicit values for constant names; per FSG discussions.
• Change name of copy_directory function to create_directory to more clearly reflect semantics per FSG discussions.
• Major upgrade to copy_options in response to FSG discussions..
• Major functionality upgrades to copy()and file_copy() per FSG discussions.
• Add path member function replace_filename(), suggested by Robert Stewart and others.
• Add C++11 features to both the proposal and the Boost implementation and test suite:
  • R-value reference move semantics.
  • noexcept.
  • constexpr.
  • Scoped enums.
  • Range-based for statement support.
  • New character types.
  • Replace explicit codecvt manipulation with a reference to <cuchar> conversion facilities.

N3365=12-0055, Filesystem Library Proposal (Revision 2). Changes include:

• Namespaces changed to std::tbd::filesystem per discussion in Kona.
• Removed references to Boost within the proposed wording.
• Minor fixes to the proposed wording.
• Added Issue 7: How should class path inserter and extractor be specified?
• Marked up all issues to reflect Kona LWG discussions.

N3335=12-0025, Filesystem Library for C++11/TR2 (Revision 1). Changes include:

• Regenerated the proposed wording from the Boost Filesystem library reference documentation, using an automated process. This process reduces the likelihood of inadvertent discrepancies between descriptions.
• An Issues list was added, seeded with issues raised by the LWG review of N3239 at the Bloomington meeting, and private communications from LWG members.
• Namespace changed to files as an experiment. Made this issue number 1 so the LWG can pass judgment.
• New functions were added, suggested by David Svoboda, to generate canonical paths and manage permissions.
• More C++11 functionality was applied. This process is still incomplete, however.
• Added proposed changes to header <fstream>. The previous paper had inadvertently supplied the wrong wording.
• Continued the general cleanup of wording.

N3239 = 11-0009, Filesystem Library Update for TR2 (Preliminary), reflected changes made to the Boost library version 3 since the previously accepted committee paper:

• 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.

N1975 = 06-0045, 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.

Proposed Wording Table of Contents

1 Filesystem Library
1.1 Definitions
   1.1.1 absolute path
   1.1.2 canonical path
   1.1.3 directory
   1.1.4 file
   1.1.5 file system
   1.1.6 file system race
   1.1.7 filename
   1.1.8 hard link
   1.1.9 link
   1.1.10 native encoding
   1.1.11 native pathname format
   1.1.12 NTCTS
   1.1.13 operating system dependent behavior
   1.1.14 parent directory
   1.1.15 path
   1.1.16 pathname
   1.1.17 pathname resolution
   1.1.18 relative path
   1.1.19 symbolic link
1.2 Conformance
   1.2.1 ISO/IEC/IEEE 9945 conformance
   1.2.2 Operating system dependent behavior conformance
1.3 Requirements
1.4 Header <filesystem> synopsis
1.5 Error reporting
1.6 Class path
   1.6.1 Generic pathname format grammar
   1.6.2 path conversions
      1.6.2.1 path argument format conversions
      1.6.2.2 path type and encoding conversions
   1.6.3 path Requirements
   1.6.4 path constructors
   1.6.5 path assignments
   1.6.6 path appends
   1.6.7 path concatenation
   1.6.8 path modifiers
   1.6.9 path native format observers
   1.6.10 path generic format observers
   1.6.11 path compare
   1.6.12 path decomposition
   1.6.13 path query
   1.6.14 path iterators
   1.6.15 path non-member functions
   1.6.16 path inserter and extractor
   1.6.17 path factory functions
1.7 Class filesystem_error
   1.7.1 filesystem_error members
1.8 Enum class file_type
1.9 Enum class copy_options
1.10 Enum class perms
1.11 Enum class permissions_options
1.12 Class file_status
   1.12.1 file_status constructors
   1.12.2 file_status observers
   1.12.3 file_status modifiers
1.13 Class directory_entry
   1.13.1 directory_entry constructors
   1.13.2 directory_entry modifiers
   1.13.3 directory_entry observers
1.14 Class directory_iterator
   1.14.1 directory_iterator members
   1.14.2 directory_iterator non-member functions
1.15 Class recursive_directory_iterator
   1.15.1 recursive_directory_iterator members
   1.15.2 recursive_directory_iterator non-member functions
1.16 Operational functions

Proposed Wording

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

Trademarked names are used in non-normative examples and notes to illustrate operating system dependent behavior.  Such use is in accordance with ISO/IEC Directives, Part2, section 6.6.3, Use of trade names and trademarks.

Start of proposed wording:

1 Filesystem Library [filesystem]

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

ISO/IEC/IEEE 9945 contains provisions which, through referenced in this text, constitute provisions of this technical specification. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this technical specification 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/IEEE 9945:2009, Portable Operating System Interface (POSIX®) Base Specifications, Issue 7.

This technical specification mentions commercially available operating systems for purposes of exposition. ^[footnote]

This footnote is required by ISO/IEC Directives, Part2, section 6.6.3 Use of trade names and trademarks:

^[footnote] POSIX® is a registered trademark of The IEEE. MAC OS® is a registered trademark of Apple Inc. Windows® is a registered trademark of Microsoft Corporation. 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.

End of footnote.

1.1 Definitions [fs.definitions]

The following definitions shall apply throughout this technical specification:

1.1.1 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.

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

An absolute path that has no elements that are symbolic links, and no dot or dot-dot elements ([path.generic]).

1.1.3 directory [fs.def.directory]

A file within a file system that acts as a container of directory entries that contain information about other files, possibly including other directory files.

1.1.4 file [fs.def.file]

An object within a file system that holds user or system data. Files 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.

1.1.5 file system [fs.def.filesystem]

A collection of files and certain of their attributes.

1.1.6 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.

1.1.7 filename [fs.def.filename]

The name of a file. Filenames dot  and dot-dot  have special meaning. The following characteristics of filenames are operating system dependent:

• The permitted characters. [Example: Some operating systems prohibit the ASCII control characters (0x00-0x1F) in filenames. --end example].

• Filenames that are not permitted.

• 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.

1.1.8 hard link [fs.def.hardlink]

A link ([fs.def.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]

1.1.9 link [fs.def.link]

A directory entry object that associates a filename with a file. A link is either a hard link ([fs.def.hardlink]) or a symbolic link ([fs.def.symlink]).

1.1.10 native encoding [fs.def.native.encode]

For narrow character strings, the operating system dependent current encoding for path names. For wide character strings, the implementation defined execution wide-character set encoding (C++ standard [lex.charset]).

1.1.11 native pathname format [fs.def.native]

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

1.1.12 NTCTS [fs.def.ntcts]

Acronym for "null-terminated character-type sequence". Describes a sequence of values of a given encoded character type terminated by that type's null character. If the encoded character type is charT, the null character can be constructed by charT(). This is an expansion of the definition in chapter 17; that definition is very hard to parse. A request for clarification has been sent to the C++ standard project editor.

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

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

1.1.14 parent directory [fs.def.parent]

When discussing a given directory, the directory that both contains a directory entry for the given directory and is represented by the pathname dot-dot in the given directory.

When discussing other types of files, a directory containing a directory entry for the file under discussion.

This concept does not apply to dot and dot-dot.

1.1.15 path [fs.def.path]

A sequence of elements that identify the location of a file within a filesystem. The elements are the root-name_opt, root-directory_opt, and an optional sequence of filenames.

1.1.16 pathname [fs.def.pathname]

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

1.1.17 pathname resolution [fs.def.pathres]

Pathname resolution is the operating system dependent mechanism for resolving a pathname to a particular file in a file hierarchy. There may be multiple pathnames that resolve to the same file.  [Example: The ISO/IEC/IEEE 9945 POSIX standard specifies the mechanism in section 4.11, Pathname resolution. --end example]

1.1.18 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]

1.1.19 symbolic link [fs.def.symlink]

A link ([fs.def.link]) 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: Symbolic links are often called symlinks. 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]

1.2 Conformance [fs.conformance]

1.2.1 ISO/IEC/IEEE 9945 conformance [fs.conform.9945]

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

[Note: This constitutes an "as if" rule allowing implementations to call native operating system or other API's. --end note]

Implementations are encouraged to provide such behavior as it is defined by ISO/IEC/IEEE 9945. Implementations shall document any behavior that differs from the behavior defined by ISO/IEC/IEEE 9945. Implementations that do not support exact ISO/IEC/IEEE 9945 behavior are encouraged to provide behavior as close to ISO/IEC/IEEE 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, so implementations are not required to support those features on the FAT file system. -- end example]

The behavior of functions described in this technical specification may differ from their specification in the presence of file system races ([fs.def.race]). 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 herein, Requires is not specified for the function.

[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]

1.2.2 Operating system dependent behavior conformance [fs.conform.os]

Some behavior is specified as being operating system dependent ([fs.def.osdep]). The operating 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 underlying operating system.

1.3 Requirements [fs.req]

Throughout this technical specification, char, wchar_t, char16_t, and char32_t are collectively called encoded character types.

Template parameters named charT shall be one of the encoded character types.

Template parameters named InputIterator shall meet the C++ standard library input iterator requirements ([input.iterators]) and shall have a value type that is one of the encoded character types.

[Note: Use of an encoded character type implies an associated encoding. Since signed char and unsigned char have no implied encoding, they are not included as permitted types. --end note]

To permit UDT types would require providing a way to specify type and encoding conversion mechanisms. That might be worth considering in the future if some form of a String Interoperability proposal is accepted, but as it stands now would involve exposing numerous implementation details so is not proposed here.

1.4 Header <filesystem> synopsis [filesystem.synopsis]

namespace std { namespace tbd { namespace filesystem {

    class path;
    
    void swap(path& lhs, path& rhs) noexcept;
    size_t hash_value(const path& p) noexcept;

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

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

    template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const path& p);

    template <class charT, class traits>
    basic_istream<charT, traits>&
    operator>>(basic_istream<charT, traits>& is, path& p);

    template <class Source>  
      path u8path(Source const& source);
    template <class InputIterator>
      path u8path(InputIterator begin, InputIterator end);

    class filesystem_error;      
    class directory_entry;

    class directory_iterator;

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

    class recursive_directory_iterator;

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

    enum class file_type;  // [enum.file_type]

    enum class perms;  // [enum.perms]

    class file_status;

    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_options;  // [enum.copy_options]

    enum class directory_options
    {
      none,
      follow_directory_symlink,
      skip_permission_denied
    };

    typedef chrono::time_point<unspecified-trivial-clock>  file_time_type;

    // 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, error_code& ec);
    path         canonical(const path& p, const path& base, error_code& ec);

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

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

    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) noexcept;

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

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

    void         create_directory(const path& p, const path& attributes);
    void         create_directory(const path& p, const path& attributes, error_code& ec) noexcept;

    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) noexcept;

    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) noexcept;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    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) noexcept;

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

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

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

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

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

    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);

} } }  // namespaces std::tbd::filesystem

unspecified-trivial-clock is an unspecified type provided by the implementation that satisfies the TrivialClock requirements ([time.clock.req]) and that is capable of representing and measuring file time values.

1.5 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 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 shall be thrown. For functions with a single path argument, that argument shall be passed to the filesystem_error constructor with a single path argument.  For functions with two path arguments, the first of these arguments shall be passed to the filesystem_error constructor as the path1 argument, and the second shall be passed as the path2 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].
   
• 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.

1.6 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 std { namespace tbd { namespace filesystem {

      class path
      {
      public:
        typedef see below                      value_type;
        typedef basic_string<value_type>       string_type;
        static constexpr value_type            preferred_separator = see below;

        // constructors and destructor
        path();
        path(const path& p);
        path(path&& p) noexcept;
        template <class Source>
          path(Source const& source);
        template <class InputIterator>
          path(InputIterator begin, InputIterator end);
        template <class Source>  
          path(Source const& source, const locale& loc);
        template <class InputIterator>
          path(InputIterator begin, InputIterator end, const locale& loc);
       ~path() = default;

        // 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)
        template <class InputIterator>
          path& assign(InputIterator begin, InputIterator end);

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

        // 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);
        template <class InputIterator>
          path& concat(InputIterator begin, InputIterator end);
        
        // modifiers
        void  clear() noexcept;
        path& make_preferred();
        path& remove_filename();
        path& replace_filename(const path& replacement);
        path& replace_extension(const path& replacement = path());
        void  swap(path& rhs) noexcept;

        // native format observers
        const string_type&  native() const noexcept;
        const value_type*   c_str() const noexcept;
        operator string_type() const;

        template <class charT, class traits = char_traits<charT>,
                  class Allocator = allocator<charT> >
        basic_string<charT, traits, Allocator>
          string(const Allocator& a = Allocator()) const;
        string    string() const;
        wstring   wstring() const;
        string    u8string() const;
        u16string u16string() const;
        u32string u32string() const;

        // generic format observers
        template <class charT, class traits = char_traits<charT>,
                  class Allocator = allocator<charT> >
        basic_string<charT, traits, Allocator>
          generic_string(const Allocator& a = Allocator()) const;
        string    generic_string() const;
        wstring   generic_wstring() const;
        string    generic_u8string() const;
        u16string generic_u16string() const;
        u32string generic_u32string() const;

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

      private:
        string_type pathname;  // exposition only
      };

} } }  // namespaces std::tbd::filesystem

value_type is a typedef for the operating system dependent encoded character type used to represent pathnames.

The value of preferred_separator is the operating system dependent preferred-separator character ([path.generic]).

[Example: For POSIX based operating systems, value_type is char and preferred_separator is the slash character (/).  For Windows based operating systems, value_type is wchar_t and preferred_separator is the backslash character (\).  --end example]

1.6.1 Generic pathname format grammar [path.generic]

pathname:
            root-name root-directory_opt relative-path_opt
               root-directory relative-path_opt
               relative-path

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
            dot
            dot-dot

name:
            A sequence of characters other than directory-separator characters.

[Note: Operating systems often place restrictions on the characters that may be used in a filename. For wide portability, users may wish to limit filename characters to the POSIX Portable Filename Character Set:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 . _ - --end note]

dot:
            The filename consisting solely of a single period character (.).

dot-dot:
            The filename consisting solely of two period characters (..).

directory-separator:
            slash
      slash directory-separator
            preferred-separator
            preferred-separator directory-separator

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

slash:
           The slash character (/).

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

The filename dot is treated as a reference to the current directory. The filename dot-dot is treated as a reference to the parent directory. Specific filenames may have special meanings for a particular operating system.

1.6.2 path conversions [path.cvt]

1.6.2.1 path argument format conversions [path.fmt.cvt]

[Note: The format conversions described in this section are not applied on POSIX or Windows based operating systems because on these systems:

• The generic format is acceptable as a native path.
• There is no need to distinguish between native format and generic format arguments.
• Paths for regular files and paths for directories share the same syntax.

 -- end note]

Functions arguments that take character sequences representing paths may use the generic pathname format grammar ([path.generic]) or the native pathname format. If and only if 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.

[Note: Some operating systems may have no unambiguous way 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. An implementation for an operating system where disambiguation is required is permitted as 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 directory-separator, otherwise it shall be treated as a regular file path.

1.6.2.2 path type and encoding conversions [path.type.cvt]

For member function arguments that take character sequences representing paths and for member functions returning strings, value type and encoding conversion is performed if the value type of the argument or return differs from path::value_type. Encoding and method of conversion for the argument or return value to be converted to is determined by its value type:

• char: Encoding is the native narrow encoding ([fs.def.native.encode]). Conversion, if any, is operating system dependent.

  [Note: For POSIX based operating systems path::value_type is char so no conversion from char value type arguments or to char value type returns is performed.

  For Windows based operating systems, the native narrow encoding is determined by calling a Windows API function. -- end note]

  On Windows, the encoding is determined by testing AreFileApisANSI(); if true, the CP_ACP codepage applies, otherwise the CP_OEMCP codepage applies.

  [Note: This results in behavior identical to other C and C++ standard library functions that perform file operations using narrow character strings to identify paths. Changing this behavior would be surprising and error prone. --end note]
   

• wchar_t: Encoding is the native wide encoding ([fs.def.native.encode]). Conversion method is unspecified.

  [Note: For Windows based operating systems path::value_type is wchar_t so no conversion from wchar_t value type arguments or to wchar_t value type returns is performed. -- end note]
   

• char16_t: Encoding is UTF-16. Conversion method is unspecified.
   

• char32_t: Encoding is UTF-32. Conversion method is unspecified.

1.6.3 path Requirements [path.req]

In addition to the [fs.req] requirements, function template parameters named Source shall be one of:

• basic_string<charT, traits, Allocator>. The type charT shall be an encoded character type ([fs.req]).  A function argument Source const& source shall have an effective range [source.begin(), source.end()).

• A type meeting the input iterator requirements that iterates over a NTCTS. The value type shall be an encoded character type. A function argument Source const& source shall have an effective range  [source, end) where end is the first iterator value with an element value equal to iterator_traits<Source>::value_type().

• A character array that after array-to-pointer decay results in a pointer to a NTCTS. The value type shall be an encoded character type. A function argument Source const& source shall have an effective range [source, end) where end is the first iterator value with an element value equal to iterator_traits<decay<Source>::type>::value_type().

[Note: See path conversions ([path.cvt]) for how these value types and their encodings convert to path::value_type and its encoding. -- end note]

Arguments of type Source shall not be null pointers.

1.6.4 path constructors [path.construct]

path();

Effects: Constructs an object of class path.

Postconditions: empty().

path(const path& p);
path(path&& p) noexcept;

Effects: Constructs an object of class path with pathname having the original value of p.pathname. In the second form, p is left in a valid but unspecified state.

Postconditions: empty().

template <class Source>
  path(Source const& source);
template <class InputIterator>
  path(InputIterator begin, InputIterator end);

Effects: Constructs an object of class path, storing the effective range of source ([path.req]) or the range [begin,end) in pathname, converting format and encoding if required ([path.arg.convert]).

template <class Source>
  path(Source const& source, const locale& loc);
template <class InputIterator>
  path(InputIterator begin, InputIterator end, const locale& loc);

Requires: The value type of Source and InputIterator is char.

Effects: Constructs an object of class path, storing the effective range of source or the range [begin,end) in pathname, after converting format if required and after converting the encoding as follows:

If value_type is wchar_t, converts to the native wide encoding ([fs.def.native.encode]) using the codecvt<wchar_t, char> facet of loc. Otherwise a conversion is performed using the codecvt<wchar_t, char> facet of loc, and then a second conversion to the current narrow encoding.

[Example:

A string is to be read from a database that is encoded in ISO/IEC 8859-1, and used to create a directory:

namespace fs = std::tbd::filesystem;
std::string latin1_string = read_latin1_data();
codecvt_8859_1<wchar_t> latin1_facet;
std::locale latin1_locale(std::locale(), latin1_facet);
fs::create_directory(fs::path(latin1_string, latin1_locale);

For POSIX based operating systems the path is constructed by first using latin1_facet to convert ISO/IEC 8859-1 encoded latin1_string to a wide character string in the native wide encoding ([fs.def.native.encode]). The resulting wide string is then converted to a narrow character pathname string in the current native narrow encoding. If the native wide encoding is UTF-16 or UTF-32, and the current native narrow encoding is UTF-8, all of the characters in the ISO/IEC 8859-1 character set will be converted to their Unicode representation, but for other native narrow encodings some characters may have no representation.

For Windows based operating systems the path is constructed by using latin1_facet to convert ISO/IEC 8859-1 encoded latin1_string to a UTF-16 encoded wide character pathname string. All of the characters in the ISO/IEC 8859-1 character set will be converted to their Unicode representation.

--end example]

1.6.5 path assignments [path.assign]

path& operator=(const path& p);

Effects: If *this and p are the same object, has no effect. Otherwise, modifies pathname to have the original value of p.pathname.

Returns: *this

path& operator=(path&& p) noexcept;

Effects: If *this and p are the same object, has no effect. Otherwise, modifies pathname to have the original value of p.pathname.  p is left in a valid but unspecified state. [Note: A valid implementation is swap(p). —end note]

Returns: *this

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

Effects: Stores the effective range of source ([path.req]) or the range [begin,end) in pathname, converting format and encoding if required ([path.arg.convert]).

Returns: *this

1.6.6 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 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);
template <class InputIterator>
  path& append(InputIterator begin, InputIterator end);

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 effective range of source ([path.req]) or the range [begin,end) to pathname, converting format and encoding if required ([path.arg.convert]).

Returns: *this

1.6.7 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 Source>
  path& concat(Source const& x);
template <class InputIterator>
  path& concat(InputIterator begin, InputIterator end);

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
• the effective range source ([path.req]), if source is present, otherwise,
• the range [begin,end), if begin and end 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 ([path.arg.convert]) so that effective-argument has value type path::value_type.

Returns: *this

1.6.8 path modifiers [path.modifiers]

void clear() noexcept;

Postcondition: this->empty() is true.

path& make_preferred();

Effects: Each directory-separator is converted to preferred-separator.

Returns: *this

[Example:

path p("foo/bar");
std::cout << p << '\n';
p.make_preferred();
std::cout << p << '\n';

On an operating system where preferred-separator is the same as directory-separator, the output is:

"foo/bar"
"foo/bar"

On an operating system where preferred-separator is a backslash, the output is:

"foo/bar"
"foo\bar"

-- end example]

path& remove_filename();

Postcondition: !has_filename().

Returns: *this.

[Example:

std::cout << path("/foo").remove_filename();  // outputs "/"
std::cout << path("/").remove_filename();     // outputs ""

--end example]

path& replace_filename(const path& replacement);

Effects:

remove_filename();
operator/=(replacement);

Returns: *this.

[Example:

std::cout << path("/foo").replace_filename("bar");  // outputs "/bar"
std::cout << path("/").replace_filename("bar");     // outputs "bar"

--end example]

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

Effects:

• Any existing extension() is removed from the stored path, then
• If replacement is not empty and does not begin with a dot character, a dot character is appended to the stored path, then
• replacement is appended to the stored path.

Returns: *this

void swap(path& rhs) noexcept;

Effects: Swaps the contents of the two paths.

Complexity: constant time.

See [c++std-filesystem-44] from Daniel Krügler for noexcept rationale.

1.6.9 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().

operator string_type() const;

Returns: pathname.

[Note: Conversion to string_type is provided so that an object of class path can be given as an argument to existing standard library file stream constructors and open functions. This provides basic interoperability without the need to modify existing standard library classes or headers. --end note]

Implementations of the standard library for systems where string_type is wstring, such as Windows, are encouraged to provide an extension to existing standard library file stream constructors and open functions that adds overloads that accept wstrings for file names. Microsoft and Dinkumware already provide such an extension.

template <class charT, class traits = char_traits<charT>,
          class Allocator = allocator<charT> >
basic_string<charT, traits, Allocator>
  string(const Allocator& a = Allocator()) const;

Returns: pathname.

Remarks: All memory allocation, including for the return value, shall be performed by a. Conversion, if any, is specified by [path.cvt]).

string string() const;
wstring wstring() const;
string u8string() const;
u16string u16string() const;
u32string u32string() const; 

Returns: pathname.

Remarks: Conversion, if any, is performed as specified by [path.cvt]. The encoding of the string returned by u8string() is always UTF-8.

1.6.10 path generic format observers [path.generic.obs]

Generic format observer functions return strings formatted according to the generic pathname format ([path.generic]). The forward slash ('/') character is used as the directory-separator character.

[Example: On an operating system that uses backslash as its preferred-separator,  path("foo\\bar").generic_string() returns "foo/bar". -- end example]

template <class charT, class traits = char_traits<charT>,
          class Allocator = allocator<charT> >
basic_string<charT, traits, Allocator>
  generic_string(const Allocator& a = Allocator()) const;

Returns: pathname, reformatted according to the generic pathname format ([path.generic]).

Remarks: All memory allocation, including for the return value, shall be performed by a. Conversion, if any, is specified by [path.cvt].

string generic_string() const;
wstring generic_wstring() const;
string generic_u8string() const;
u16string generic_u16string() const;
u32string generic_u32string() const; 

Returns: pathname, reformatted according to the generic pathname format ([path.generic]).

Remarks:  Conversion, if any, is specified by [path.cvt]. The encoding of the string returned by generic_u8string() is always UTF-8.

1.6.11 path compare [path.compare]

int compare(const path& p) const noexcept;

Returns: A value less than 0 if native() for the elements of *this are lexicographically less than native() for the elements of p, otherwise a value greater than 0 if native() for the elements of *this are lexicographically greater than native() for 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 string& s) const

Returns: compare(path(s)).

int compare(const value_type* s) const

Returns: compare(path(s)).

1.6.12 path decomposition [path.decompose]

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:

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

--end example]

path stem() const;

Returns: if filename() contains a character but does not consist solely of one or to two periods, returns the substring of filename() starting at its beginning and ending with the character before the last period. Otherwise, returns 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;

Returns: if filename() contains a period but does not consist solely of one or to two periods, returns the substring of filename() starting at the rightmost period and for the remainder of the path. Otherwise, returns an empty path object.

Remarks: Implementations are permitted 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 period is included in the return value so that it is possible to distinguish between no extension and an empty extension. Also note that for a path p, p.stem()+p.extension() == p. -- end note]

1.6.13 path query [path.query]

bool empty() const noexcept;

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 file system location, else false.

[Example: path("/").is_absolute() is true for POSIX based operating systems, and false for Windows based operating systems.  --end example]

bool is_relative() const;

Returns: !is_absolute().

1.6.14 path iterators [path.itr]

Path iterators iterate over the elements of the stored pathname.

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, 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.

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.

1.6.15 path non-member functions [path.non-member]

void swap(path& lhs, path& rhs) noexcept;

Effects: lhs.swap(rhs).

size_t hash_value (const path& p) noexcept;

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

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

Returns: return lhs.compare(rhs) < 0.

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

Returns: !(rhs < lhs).

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

Returns: rhs < lhs.

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

Returns: !(lhs < rhs).

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

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) noexcept;

Returns: !(lhs == rhs).

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

Returns: path(lhs) /= rhs.

1.6.16 path inserter and extractor [path.io]

template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const path& p);

Effects: os << p.string<charT, traits>().

[Note: Pathnames containing spaces require special handling by the user to avoid truncation when read by the extractor. --end note]

The Boost inserter outputs quoted strings, which are recognized as such by the extractor. There is a sentiment within the Filesystem Study Group that supplying a quoted and/or escaped string manipulator as a string extension would be a better solution than a path specific feature. See proposal N3431, Quoted Strings Library Proposal.

Returns: os

template <class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, path& p);

Effects: 

basic_string<charT, traits> tmp;
is >> tmp;
p = tmp;

Returns: is

1.6.17 path factory functions [path.factory]

template <class Source>  
  path u8path(Source const& source);
template <class InputIterator>
  path u8path(InputIterator begin, InputIterator end);

Requires: The source and [begin,end) sequences are UTF-8 encoded. The value type of Source and InputIterator is char.

Returns:

• If value_type is char and the current native narrow encoding ([fs.def.native.encode]) is UTF-8, path(source) or path(begin, end), else
• if value_type is wchar_t and the native wide encoding is UTF-16, or if value_type is char16_t or char32_t,  convert source or [begin,end) to a temporary, tmp, of type string_type and return path(tmp), else
• convert source or [begin,end) to a temporary, tmp, of type u32string and return path(tmp).

Remarks: Argument format conversion ([path.fmt.cvt]) applies to the arguments for these functions. How Unicode encoding conversions are performed is unspecified.

[Example:

A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:

namespace fs = std::tbd::filesystem;
std::string utf8_string = read_utf8_data();
fs::create_directory(fs::u8path(utf8_string));

For POSIX based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs.

For POSIX based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation.

For Windows based operating systems a conversion from UTF-8 to UTF-16 occurs.

--end example]

1.7 Class filesystem_error [class.filesystem_error]

namespace std { namespace tbd { namespace filesystem {

      class filesystem_error : public system_error
      {
      public:
        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);

        const path& path1() const noexcept;
        const path& path2() const noexcept;
        const char* what() const noexcept;
      };
} } }  // namespaces std::tbd::filesystem

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

1.7.1 filesystem_error members [filesystem_error.members]

Constructors are provided that store zero, one, or two paths associated with an error.

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& 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& p1, const path& 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 noexcept;

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

const path& path2() const noexcept;

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

const char* what() const noexcept;

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.

1.8 Enum class file_type [enum.file_type]

This enum class specifies constants used to identify file types.

1.9 Enum class copy_options [enum.copy_options]

This enumeration specifies bitmask constants used to control the semantics of copy operations.  The constants are specified in option groups. Constant none is shown in each option group for purposes of exposition; implementations shall provide only a single definition.  Calling a Filesystem library function with more than a single constant for an option group results in undefined behavior.

1.10 Enum class perms [enum.perms]

This enumeration specifies bitmask constants used to identify file permissions. ISO/IEC/IEEE 9945 (POSIX) specifies actual values, and those values have been adopted here because they are very familiar and ingrained for many 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. The Cygwin users guide has extensive discussion of the problems involved. Input from security experts is needed.

1.11 Enum class permissions_options [enum.perm-opts]

1.12 Class file_status [class.file_status]

namespace std { namespace tbd { namespace filesystem {

      class file_status
      {
      public:

        // constructors
        explicit file_status(file_type ft = file_type::none,
                             perms prms = perms_not_known) noexcept;
        file_status(const file_status&) noexcept = default;
        file_status(file_status&&) noexcept = default;
       ~file_status() = default;

        file_status& operator=(const file_status&) noexcept = default;
        file_status& operator=(file_status&&) noexcept = default;

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

        // modifiers
        void       type(file_type ft) noexcept;
        void       permissions(perms prms) noexcept;
      };
} } }  // namespaces std::tbd::filesystem

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

1.12.1 file_status constructors [file_status.cons]

explicit file_status() noexcept;

Postconditions: type() == file_type::none, permissions() == perms_not_known.

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

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

1.12.2 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.

1.12.3 file_status modifiers [file_status.mods]

void type(file_type ft) noexcept;

Postconditions: type() == ft.

void permissions(perms prms) noexcept;

Postconditions: permissions() == prms.

1.13 Class directory_entry [class.directory_entry]

namespace std { namespace tbd { namespace filesystem {

      class directory_entry
      {
      public:

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

        // modifiers
        directory_entry& operator=(const directory_entry&) = default;
        directory_entry& operator=(directory_entry&&) noexcept = default;
        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 noexcept;
        file_status  status() const;
        file_status  status(error_code& ec) const noexcept;
        file_status  symlink_status() const;
        file_status  symlink_status(error_code& ec) const noexcept;

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

} } }  // namespaces std::tbd::filesystem

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 in significant time savings. Cached and non-cached results may differ in the presence of file system races. -- end note]

1.13.1 directory_entry constructors [directory_entry.cons]

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

1.13.2 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

1.13.3 directory_entry observers [directory_entry.obs]

const path& path() const noexcept;

Returns: m_path

file_status status() const;
file_status status(error_code& ec) const noexcept;

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 noexcept;

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) const noexcept;

Returns: m_path == rhs.m_path.

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

Returns: m_path != rhs.m_path.

bool operator< (const directory_entry& rhs) const noexcept;

Returns: m_path < rhs.m_path.

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

Returns: m_path <= rhs.m_path.

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

Returns: m_path > rhs.m_path.

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

Returns: m_path >= rhs.m_path.

1.14 Class directory_iterator [class.directory_iterator]

An object of type directory_iterator provides an iterator for a sequence of directory_entry elements representing the files in a directory.  [Note: For iteration into sub-directories,  see class recursive_directory_iterator ([class.rec.dir.itr]). --end note]

namespace std { namespace tbd { namespace filesystem {

      class directory_iterator :
        public iterator<input_iterator_tag, directory_entry>
      {
      public:
        // member functions
        directory_iterator() noexcept;
        explicit directory_iterator(const path& p);
        directory_iterator(const path& p, error_code& ec) noexcept;
        directory_iterator(const directory_iterator&) = default;
        directory_iterator(directory_iterator&&) = default;
       ~directory_iterator() = default;

        directory_iterator& operator=(const directory_iterator&) = default;
        directory_iterator& operator=(directory_iterator&&) = default;

        const directory_entry& operator*() const;
        const directory_entry* operator->() const;
        directory_iterator&    operator++();
        directory_iterator&    increment(error_code& ec) noexcept;

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

} } }  // namespaces std::tbd::filesystem

directory_iterator satisfies the requirements of an input iterator ([input.iterators]).

If an iterator of type directory_iterator is advanced past the last directory element, that iterator shall become equal to the end iterator value. The directory_iterator default constructor shall create an iterator equal to the end iterator value, and this shall be the only valid iterator for the end condition.

The result of operator* on an end iterator is undefined behavior. 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 subsequently incrementing the iterator will ever result in an iterator referencing the removed or added directory entry. See ISO/IEC/IEEE 9945 readdir_r(). --end note]

1.14.1 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, error_code& ec) noexcept;

Effects: For the directory that p resolves to, constructs an iterator for the first element in a sequence of directory_entry elements representing the files in the directory, 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) noexcept;

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

Returns: *this.

Throws: As specified in Error reporting.

1.14.2 directory_iterator non-member functions [directory_iterator.nonmembers]

These functions enable use of directory_iterator with C++11 range-based for statements.

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

Returns: iter.

directory_iterator end(const directory_iterator&) noexcept;

Returns: directory_iterator().

1.15 Class recursive_directory_iterator [class.rec.dir.itr]

An object of type recursive_directory_iterator provides an iterator for a sequence of directory_entry elements representing the files in a directory and its sub-directories.

namespace std { namespace tbd { 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&) = default;
        explicit recursive_directory_iterator(const path& p,
          directory_options options = directory_options::none);
        recursive_directory_iterator(const path& p,
          directory_options options, error_code& ec) noexcept;
        recursive_directory_iterator(const path& p, error_code& ec) noexcept;
        recursive_directory_iterator(directory_iterator&&) = default;
       ~recursive_directory_iterator() = default;

        // observers
        directory_options  options() const;
        int                depth() const;
        bool               recursion_pending() const;

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

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

        void pop();
        void disable_recursion_pending();

        // ... other members as required for Input iterators [input.iterators]
      };

} } }  // namespaces std::tbd::filesystem

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

1.15.1 recursive_directory_iterator members [rec.dir.itr.members]

recursive_directory_iterator() noexcept;

Effects: Constructs the end iterator.

explicit recursive_directory_iterator(const path& p,
  directory_options options = directory_options::none);
recursive_directory_iterator(const path& p, directory_options options, 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: options() == options for the signatures with a directory_options argument, otherwise options() == directory_options::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 options as directory_options::follow_directory_symlink -- end note]

directory_options options() const;

Requires: *this != recursive_directory_iterator().

Returns: The value of the constructor options argument, if present, otherwise directory_options::none.

Throws: Nothing.

int depth() const;

Requires: *this != recursive_directory_iterator().

Returns: The current depth of the directory tree being traversed. [Note: The initial directory is depth 0, its immediate subdirectories are depth 1, and so forth. -- end note]

Throws: Nothing.

bool recursion_pending() const;

Requires: *this != recursive_directory_iterator().

Returns: true if disable_recursion_pending() has not been called subsequent to the prior construction or increment operation, otherwise false.

Throws: Nothing.

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

Requires: *this != recursive_directory_iterator().

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

• If there are no more entries at this depth, then if depth()!= 0 iteration over the parent directory resumes; otherwise *this = recursive_directory_iterator().

• Otherwise if recursion_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status()) || (options() & directory_options::follow_directory_symlink) != 0) then directory (*this)->path() is recursively iterated into.

Returns: *this.

Throws: As specified in Error reporting.

void pop();

Requires: *this != recursive_directory_iterator().

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

void disable_recursion_pending();

Requires: *this != recursive_directory_iterator().

Postcondition: recursion_pending() == false.

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

1.15.2 recursive_directory_iterator non-member functions [rec.dir.itr.nonmembers]

These functions enable use of recursive_directory_iterator with C++11 range-based for statements.

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

Returns: iter.

recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

Returns: recursive_directory_iterator().

1.16 Operational functions [fs.op.funcs]

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

[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: An 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, error_code& ec);
path canonical(const path& p, const path& base, error_code& ec);

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

Returns: A path that refers to the same file system object as absolute(p,base). For the overload without a base argument, base is current_path(). Signatures with argument ec return path() if an error occurs.

Throws:  As specified in Error reporting.

Remarks: !exists(p) is an error.

[Note: Canonical pathnames allow security checking of a path (e.g. 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, error_code& ec) noexcept;

Effects: copy(from, to, copy_options::none[, ec]).

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

Precondition: At most one constant from each option group ([enum.copy_options]) is present in options.

Effects:

Before the first use of f and t:

• If (options & copy_options::create_symlinks) || (options & copy_options::skip_symlinks), then auto f = symlink_status(from) and if needed auto t = symlink_status(to).
• Otherwise, auto f = status(from) and if needed auto t = status(to).

Report an error as specified in Error reporting if:

• !exists(f).
• equivalent(f, t).
• is_other(f) || is_other(t).
• is_directory(f) && is_regular_file(t).

If is_symlink_file(f), then:

• If options & copy_options::skip_symlinks, then return.
• Otherwise if !exists(t), then copy_symlink(from, to, options).
• Otherwise report an error as specified in Error reporting.

Otherwise if is_regular_file(f), then:

• If options & copy_options::structure_only, then return.
• Otherwise if options & copy_options::create_symlinks, then create a symbolic link to the source file.
• Otherwise if options & copy_options::create_hard_links, then create a hard link to the source file.
• Otherwise:
  • If is_directory(t), then copy_file(from, to/from.filename(), options).
  • Otherwise, copy_file(from, to, options).

Otherwise if is_directory(f) && ((options & copy_options::recursive) || !(options & copy_options::unspecified)) then:

• If  !exists(t), then create_directory(to, from).
• Then, iterate over the files in from, as if by for (directory_entry& x : directory_iterator(from)), and for each iteration copy(x.path(), to/x.path().filename(), options | copy_options::unspecified).

Otherwise no effects.

Throws: As specified in Error reporting.

Remarks: For the signature with argument ec, any Filesystem library functions called by the implementation shall have an error_code argument if applicable.

[Example: Given this directory structure:

/dir1
  file1
  file2
  dir2
    file3

Calling copy("/dir1", "/dir3") would result in:

/dir1
  file1
  file2
  dir2
    file3
/dir3
  file1
  file2

Alternatively, calling copy("/dir1", "/dir3", copy_options::recursive) would result in:

/dir1
  file1
  file2
  dir2
    file3  
/dir3
  file1
  file2
  dir2
    file3

-- end example]

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

Returns: copy_file(from, to, copy_options::none[, ec]).

Throws: As specified in Error reporting.

bool copy_file(const path& from, const path& to, copy_options options);
bool copy_file(const path& from, const path& to, copy_options options,
               error_code& ec) noexcept;

Precondition: At most one constant from each copy_options option group ([enum.copy_options]) is present in options.

Effects:

If  exists(to) && !(options & (copy_options::skip | copy_options::overwrite | copy_options::update)) report a file already exists error as specified in Error reporting.

If !exists(to) || (options & copy_options::overwrite) || ((options & copy_options::update) && last_write_time(from) > last_write_time(to)) || !(options & (copy_options::skip | copy_options::overwrite | copy_options::update)) copy the contents and attributes of the file from resolves to the file to resolves to.

Returns: true if the from file was copied, otherwise false. The signature with argument ec return false if an error occurs.

Throws: As specified in Error reporting.

Complexity: At most one direct or indirect invocation of status(to).

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) noexcept;

Effects: function(read_symlink(existing_symlink[, ec]), new_symlink[, ec]), where function is create_symlink or create_directory_symlink, as appropriate.

Throws: As specified in Error reporting.

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

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. The signature with argument ec returns false if an error occurs.

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, error_code& ec) noexcept;

Effects: Establishes the postcondition by attempting to create the directory p resolves to, as if by ISO/IEC/IEEE 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. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

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

Effects: Establishes the postcondition by attempting to create the directory p resolves to, with attributes copied from directory existing_p. The set of attributes copied is operating system dependent. Creation failure because p resolves to an existing directory shall not be treated as an error.

[Note: For POSIX based operating systems the attributes are those copied by native API stat(existing_p.c_str(), &attributes_stat) followed by mkdir(p.c_str(), attributes_stat.st_mode).  For Windows based operating systems the attributes are those copied by native API CreateDirectoryExW(existing_p.c_str(), p.c_str(), 0).  --end note]

Postcondition: is_directory(p)

Returns: true if a new directory was created, otherwise false. The signature with argument ec returns false if an error occurs.

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) noexcept;

Effects: Establishes the postcondition, as if by ISO/IEC/IEEE 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 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,
                                      error_code& ec) noexcept;

Effects: Establishes the postcondition, as if by ISO/IEC/IEEE 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,
                    error_code& ec) noexcept;

Effects: Establishes the postcondition, as if by ISO/IEC/IEEE 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(error_code& ec);

Returns: The absolute path of the current working directory, obtained as if by ISO/IEC/IEEE 9945 getcwd(). The signature with argument ec returns path() if an error occurs.

Throws: As specified in Error reporting.

Remarks: The current working directory is the directory, associated with the process, that is used as the starting location in pathname resolution for relative paths.

[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) noexcept;

Effects: Establishes the postcondition, as if by ISO/IEC/IEEE 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_type::not_found

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

Returns: exists(status(p)) or exists(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

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

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. The signature with argument ec returns false if an error occurs.

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/IEEE 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/IEEE 9945 requires that "st_dev must be unique within a Local Area Network". Conservative ISO/IEC/IEEE 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, error_code& ec) noexcept;

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/IEEE 9945 stat structure member st_size obtained as if by ISO/IEC/IEEE 9945 stat(). Otherwise, static_cast<uintmax_t>(-1). The signature with argument ec returns static_cast<uintmax_t>(-1) if an error occurs.

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) noexcept;

Returns: The number of hard links for p. The signature with argument ec returns static_cast<uintmax_t>(-1) if an error occurs.

Throws: As specified in Error reporting.

bool is_block_file(file_status s) noexcept;

Returns: s.type() == file_type::block

bool is_block_file(const path& p);
bool is_block_file(const path& p, error_code& ec) noexcept;

Returns: is_block_file(status(p)) or is_block_file(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

bool is_character_file(file_status s) noexcept;

Returns: s.type() == file_type::character

bool is_character_file(const path& p);
bool is_character_file(const path& p, error_code& ec) noexcept;

Returns: is_character_file(status(p)) or is_character_file(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

bool is_directory(file_status s) noexcept;

Returns: s.type() == file_type::directory

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

Returns: is_directory(status(p)) or is_directory(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

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

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

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

The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

bool is_fifo(file_status s) noexcept;

Returns: s.type() == file_type::fifo

bool is_fifo(const path& p);
bool is_fifo(const path& p, error_code& ec) noexcept;

Returns: is_fifo(status(p)) or is_fifo(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

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, error_code& ec) noexcept;

Returns: is_other(status(p)) or is_other(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

bool is_regular_file(file_status s) noexcept;

Returns: s.type() == file_type::regular.

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) noexcept;

Effects: Sets ec as if by status(p, ec). [Note: file_type::none, file_type::not_found and file_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)). Returns false if an error occurs.

bool is_socket(file_status s) noexcept;

Returns: s.type() == file_type::socket

bool is_socket(const path& p);
bool is_socket(const path& p, error_code& ec) noexcept;

Returns: is_socket(status(p)) or is_socket(status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

bool is_symlink(file_status s) noexcept;

Returns: s.type() == file_type::symlink

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

Returns: is_symlink(symlink_status(p)) or is_symlink(symlink_status(p, ec)), respectively. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

file_time_type last_write_time(const path& p);
file_time_type last_write_time(const path& p, error_code& ec) noexcept;

Returns: The time of last data modification of p, determined as if by the value of the ISO/IEC/IEEE 9945 stat structure member st_mtime  obtained as if by ISO/IEC/IEEE 9945 stat(). The signature with argument ec returns static_cast<file_time_type>(-1) if an error occurs.

Throws: As specified in Error reporting.

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

Effects: Sets the time of last data modification of the file resolved to by p to new_time, as if by ISO/IEC/IEEE 9945 stat() followed by ISO/IEC/IEEE 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, error_code& ec) noexcept;

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/IEEE 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, error_code& ec);

Returns:  If p resolves to a symbolic link, a path object containing the contents of that symbolic link. Otherwise path(). The signature with argument ec returns path() if an error occurs.

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) noexcept;

Effects:  If exists(symlink_status(p,ec)), it is removed as if by ISO/IEC/IEEE 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. The signature with argument ec returns false if an error occurs.

Throws: As specified in Error reporting.

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

Effects:  Recursively deletes the contents of p if it exists, then deletes file p itself, as if by ISO/IEC/IEEE 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. The signature with argument ec returns static_cast<uintmax_t>(-1) if an error occurs.

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) noexcept;

Effects: Renames old_p to new_p, as if by ISO/IEC/IEEE 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/IEEE 9945 compliant operating systems but is an error on some other operating systems. 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) noexcept;

Postcondition: file_size() == new_size.

Throws: As specified in Error reporting.

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

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

Returns: An object of type space_info. The value of the space_info object is determined as if by using ISO/IEC/IEEE 9945 statvfs() to obtain a ISO/IEC/IEEE 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 static_cast<uintmax_t>(-1). For the signature with argument ec, all members are set to static_cast<uintmax_t>(-1) if an error occurs.

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 == file_type::none)
  throw filesystem_error(implementation-supplied-message, p, ec);
return result;

Returns: See above.

Throws: filesystem_error. [Note: result values of file_status(file_type::not_found) and file_status(file_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) noexcept;

Effects:

If possible, determines the attributes of the file p resolves to, as if by ISO/IEC/IEEE 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(file_type::none).  --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_type::not_found). ISO/IEC/IEEE 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.
   
• Otherwise, if the specific error indicates that p can be resolved but the attributes cannot be determined, return file_status(file_type::unknown). For example, Windows ERROR_SHARING_VIOLATION errors. For ISO/IEC/IEEE 9945, the case never arises.
   
• Otherwise, return file_status(file_type::none).

[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/IEEE 9945 S_ISREG(), return file_status(file_type::regular). [Note: file_type::regular implies appropriate <fstream> operations would succeed, assuming no hardware, permission, access, or file system race errors. Lack of file_type::regular 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/IEEE 9945 S_ISDIR(), return file_status(file_type::directory). [Note: file_type::directory implies directory_iterator(p)would succeed. -- end note]
   
• Otherwise, if the attributes indicate a block special file, as if by ISO/IEC/IEEE 9945 S_ISBLK(), return file_status(file_type::block).
   
• Otherwise, if the attributes indicate a character special file, as if by ISO/IEC/IEEE 9945 S_ISCHR(), return file_status(file_type::character).
   
• Otherwise, if the attributes indicate a fifo or pipe file, as if by ISO/IEC/IEEE 9945 S_ISFIFO(), return file_status(file_type::fifo).
   
• Otherwise, if the attributes indicate a socket, as if by ISO/IEC/IEEE 9945 S_ISSOCK(), return file_status(file_type::socket).
   
• Otherwise, return file_status(file_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() != file_type::none

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

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

Returns: Same as status(), above, except that if the attributes indicate a symbolic link, as if by ISO/IEC/IEEE 9945 S_ISLNK(), return file_status(file_type::symlink). The signature with argument ec returns file_status(file_type::none) if an error occurs.

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

Throws: As specified in Error reporting.

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. The signature with argument ec returns path() if an error occurs.

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

Throws: As specified in Error reporting.

[Example: For POSIX based operating systems, system_complete(p) has the same semantics as complete(p, current_path()).

For Windows based operating systems, system_complete(p) has the same semantics as absolute(ph, current_path()) if p.is_absolute() || !p.has_root_name() or p and base have the same root_name(). Otherwise it acts like absolute(p, kinky), where kinky is the current directory for the p.root_name() drive. This will be the current directory for 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 useful, they are very error-prone. -- end example]

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

Returns: An unspecifed directory path suitable for temporary files. An error shall be reported if !exists(p) || !is_directory(p), where p is the path to be returned. The signature with argument ec returns path() if an error occurs.

Throws: As specified in Error reporting.

The temp_directory_path() name was chosen to emphasize that the return is a path, not just a single directory name.

[Example: For POSIX based operating systems, an implementation might return the path supplied by the first environment variable found in the list TMPDIR, TMP, TEMP, TEMPDIR, or if none of these are found, "/tmp".

For Windows based operating systems, an implementation might return the path reported by the Windows GetTempPath API function. -- end example]

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

The unique_path function generates a name suitable for 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 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. --end note]

Returns: A path identical to model, except that each occurrence of the percent sign character is replaced by a random hexadecimal digit character in the range 0-9, a-f. The signature with argument ec returns path() if an error occurs.

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]

[Example:

cout << unique_path("test-%%%%%%%%%%%.txt") << endl;

Typical output would be "test-0db7f2bf57a.txt". Because 11 hexadecimal output characters are specified, 44 bits of randomness are supplied.  -- end example]

End of Proposed wording.

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.

Members of the C++ standards committee Filesystem Study Group contributed numerous comments, suggestions, and bug-fixes to the post-C++11 proposal for standardizing the library. Daniel Krügler and Robert Steward reviewed the text in great detail, and made major contributions to the standardization effort both in terms of fixes and suggested improvements.

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. The Library Working Group requested the removal of explicit codecvt arguments, leading to the separation of string interoperability features into a separate library.

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