Doc. no. WG21/D1934=06-0004
Date: 2006-01-31
Project: Programming Language C++
Reply to: Beman Dawes <bdawes@acm.org>

# Draft

# Filesystem Library Proposal for TR2 (Revision 2)

## Table of Contents

Introduction Motivation and Scope Impact on the Standard Important Design Decisions Proposed Text for TR2 Introductory chapter Filesystem library chapter Definitions Requirements Requirements on programs Requirements on implementations Header <filesystem> synopsis Path traits Class template basic_path Pathname formats Pathname grammar Filename conversion Requirements | Class template basic_path (continued) basic_path constructors basic_path assignments basic_path modifiers basic_path operators basic_path observers basic_path iterators basic_path non-member functions basic_path inserter and extractor Class filesystem_error Class template basic_filesystem_error basic_filesystem_error constructors basic_filesystem_error observers Class template basic_directory_entry basic_directory_entry constructors basic_directory_entry modifiers basic_directory_entry observers basic_directory_entry comparisons | Filesystem library chapter (continued) Class template basic_directory_iterator basic_directory_iterator constructors Class template basic_recursive_directory_iterator Class file_status Non-member operational functions Status functions Predicate functions Attribute functions Other operations functions Convenience functions Additions to header <cerrno> Additions to header <fstream> Suggestions for `<fstream>``` implementations Path decomposition table Issues Acknowledgements References Revision History

## Introduction

This paper proposes addition of a filesystem library component to the C++ Standard Library Technical Report 2. The proposal is based on the Boost Filesystem Library (see www.boost.org/libs/filesystem).

The library provides portable facilities to query and manipulate paths, files, and directories. The Boost version of the library is widely used. It would be a pure addition to the C++ standard, leaving in place existing standard library functionality in the relatively few areas where there is overlap.

Users say they prefer the Boost Filesystem Library interface to native operating system or POSIX API's, even in code without portability requirements, because the design follows modern C++ practice.

The proposed text includes an example of a program using the library.

## Motivation and Scope

Why is this important?

The motivation for the library is the desire to perform safe, portable, script-like filesystem operations from within C++ programs. Because the C++ Standard Library currently contains no facilities for such filesystem tasks as directory iteration or directory creation, programmers currently must rely on operating system specific interfaces, making it difficult to write portable programs.

The intent is not to compete with Python, Perl, or shell scripting languages, but rather to provide file system operations where C++ is already the language of choice. The design encourages, but does not require, safe and portable usage.

What kinds of problems does it address, and what kinds of programmers is it intended to support?

The library addresses everyday needs, for both application programs and libraries. It is useful across every application domain that uses files. It is intended to be useful to all levels of programmers, from rank beginners to seasoned experts.

Is it based on existing practice?

Yes, very much so. The proposal is based on the Boost Filesystem Library, which has been in use since 2002 and by now is in very wide use. For example, current versions of Adobe Systems products such as Adobe Reader use the Boost Filesystem Library on the many platforms they support.

Note, however, that until recently all the Boost experience was with a narrow-character only version of the library. The internationalized version as described in this proposal is just starting to be used, and will not be fully released until Boost release 1.34.

The underlying mechanisms have been in use for decades on the world's most wide-spread operating systems, such as POSIX, Windows, and various mainframe operating systems. What this proposal brings to the table is an approach that is C++ Standard Library friendly and fully internationalized.

Is there a reference implementation?

Yes. The Boost Filesystem Library is freely and publicly available. The Boost library will track the TR2 proposed library as the proposal evolves.

## Impact on the Standard

What does it depend on, and what depends on it?

It depends on some standard library components, such as basic_string. No other proposals depend on it.

If a revision to the Code Conversion Proposal (See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1683.html) is accepted, it may be advantageous for the Filesystem Library to use that library rather than the current code conversion facilities proposed below.

Is it a pure extension, or does it require changes to standard components?

Most of the proposed library is a pure extension.

There are additions to header <cerrno>. Since the critical portions that might require change to C headers (always a sore point) are already mandated for POSIX compliance, and codify existing practice for many non-POSIX implementations such as for Windows, it is not expected that they will cause any problems.

There are additions to header <fstream>. These have been carefully specified to avoid breaking existing code in common operating environments such as POSIX, Windows, and OpenVMS. See Suggestions for `<fstream>` implementations for techniques to avoid breaking existing code in other environments, particularly on operating systems allowing slashes in filenames.

Can it be implemented using today's compilers, or does it require language features that will only be available as part of C++0x?

It can be (and has been) implemented with today's compilers.

There is one minor function that can best be implemented by an addition to current C++ runtime libraries, although an acceptable workaround is documented.

On operating systems with built-in support for wide-character file names, such as Windows, high-quality implementation of the header <fstream> additions require an addition to the C++ Standard Library implementation. The addition is relatively small and localized. There is a workaround that avoids modifying the standard library, but it is very much a hack and depends on a Windows feature (8.3 filename support) which some users disable, thereby disabling the workaround. The issue doesn't affect implementations on operating systems which only support narrow character file names.

## Important Design Decisions

#### Why did you choose the specific design that you did?

Many of the specific design decisions were driven by the desire to provide a modern C++ interface that works well with the C++ Standard Library. The intent is that Standard Library users can become comfortable with the Filesystem Library in very short order.

The proposed library encourages both syntactic and semantic portability, yet does not force implementors into heroic efforts on hopeless systems. This balances the benefits to users of both code and knowledge portability with the realities faced by implementors on some operating systems.

In some cases users always need portable semantics. In some cases users always need platform specific semantics. In some cases users need to be able to choose between portable and platform specific semantics. The design evolved over a period of years to identify and meet each of those needs.

Because of the desire to support simple "script-like" usage, use cases often drove design choices. For example, users can write `if (exists("foo"))` rather than the lengthier `if (exists(path("foo")))`.

Because filesystem operations often encounter unexpected runtime errors, the library reports runtime errors via C++ exceptions, and ensures enough information is provided for meaningful error messages, including internationalized error messages.

What alternatives did you consider, and what are the tradeoffs?

Additional observers and modifiers for file system attributes. Attribute functions which cannot supply portable semantics are not provided, avoiding the illusion of portability in cases where it cannot in fact exist.

A larger number of operational convenience functions. Convenience functions (functions which can be portably created by composition from basic functions) were not provided unless there was widespread agreement on usefulness and need.

Compile-time or run-time options for operational functions. Numerous trial implementations were abandoned because the added complexity out weighed the benefits, and because consensus could not be reached on the feature set.

Automatic path name checking. This feature, supplied by the Boost library for several years, allowed users to specify both default and per constructor path name checking, and thus allowed the desired degree of portability to be automatically enforce. This implicit name checking was abandoned because of user confusion and complaints.

Separate path types for regular file and directory pathnames. Pathname formats that use different syntax for regular pathnames versus directory pathnames are passing into extinction. Why prolong the agony at the cost of torturing those using modern systems? It is perhaps significant that one of the few web sites dedicated to preserving a dual pathname format operating system is named Deathrow (http://deathrow.vistech.net/).

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

What are the consequences of your choices, for users and implementors?

The design has evolved over a period of four years of actual experience by Boost users, and the most frequent causes of user complaints (such as enforced name-checking and several over-strict preconditions) were eliminated. The TR process will allow further refinement. The intent is to ensure user needs are met.

Because the Boost implementation is tested and used in a wide range of POSIX and Windows environments, many implementation concerns have already been addressed.

What decisions are left up to implementors?

Because implementations of the library are dependent on facilities of the underlying operating system, implementors are given unusual freedom to redefine semantics of the library. That being said, implementors are given strong normative encouragement to provide the TR described semantics whenever feasible.

If there are any similar libraries in use, how do their design decisions compare to yours?

There are a number of libraries which address the problem domain. Most of the C/C++ libraries have C, rather than C++ interfaces. For example, see the Apache Portable Runtime Project (http://apr.apache.org). The ACE toolkit (http://www.cs.wustl.edu/~schmidt/ACE.html) uses a C++ approach, but doesn't mesh well with the C++ Standard Library. For example, the ACE directory iterator differs greatly from Standard Library iterator requirements.

## Proposed Text for Technical Report 2

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

Italic text is editorial guidance. It is not to be added to the TR.

Add to the introductory section of the TR:

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

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

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

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

Some library behavior in this Technical Report is defined by reference to POSIX. How such behavior is actually implemented is unspecified.

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

Implementations are encouraged, but not required, to support such behavior as it is defined by POSIX. Implementations shall document any behavior that differs from the POSIX defined behavior. Implementations that do not support exact POSIX behavior are encouraged to provide behavior as close to POSIX behavior as is reasonable given the limitations of actual operating systems. 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 `basic_filesystem_error` exception, a special return value, or some other manner. --end note]

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

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

Add a new clause to the TR:

## Chapter (tbs) - Filesystem library

This clause describes components that C++ programs may use to interrogate and manipulate files (including directories), and certain of their attributes.

This clause applies only to hosted implementations (C++ Std, 1.4, Implementation compliance [intro.compliance]).

> [Note: This clause applies to any hosted implementation. Specific operating systems such as OpenMVS3, UNIX, and Windows4 are mentioned only for purposes of illustration or to give guidance to implementors. No slight to other operating systems is implied or intended. --end note.]

Unless otherwise specified, all components described in this clause are declared in namespace `std::tr2::sys`.

> [Note: The `sys` sub-namespace prevents collisions with names already in the standard library and emphasizes reliance on the operating system dependent behavior inherent in file system operations. -- end note]

The Effects and Postconditions of functions described in this clause may not be achieved in the presence of race conditions. No diagnostic is required.

If the possibility of race conditions makes it unreliable for a program to test for a precondition before calling a function described in this clause, Requires is not specified for the condition. Instead, the condition is specified as a Throws condition.

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

Footnote 3: OpenMVS® is a registered trademark of Hewlett-Packard Development Company.

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

### Definitions

The following definitions shall apply to this clause:

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

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

Filename: The name of a file. The format is as specified by the POSIX Filename base definition.

Path: A sequence of elements which identify a location within a filesystem. The elements are the root-name, root-directory, and each successive filename. See Pathname grammar.

Pathname: A character string that represents a path.

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

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

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

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

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

Slash: The character `'/'`, also known as solidus.

Dot: The character '.', also known as period.

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

### Requirements

#### Requirements on programs

The arguments for template parameters named `Path`, `Path1`, or `Path2` described in this clause shall be of type `basic_path`, or a class derived from `basic_path`, unless otherwise specified.

#### Requirements on implementations

Some function templates described in this clause have a template parameter named `Path`, `Path1`, or `Path2`. When called with a function argument `s` of type `char*` or `std::string`, the implementation shall treat the argument as if it were coded `path(s)`. When called with a function argument `s` of type `wchar_t*` or `std::wstring`, the implementation shall treat the argument as if it were coded `wpath(s)`. For functions with two arguments, implementations shall not supply this treatment when `Path1` and `Path2` are different types.

> [Note: This "do-the-right-thing" rule allows users to write `exists("foo")`, taking advantage of class `basic_path`'s string conversion constructor, rather than the lengthier and more error prone `exists(path("foo"))`. This is particularly important for the simple, script-like, programs which are an important use case for the library. Calling two argument functions with different types is a very rare usage, and may well be a coding error, so automatic conversion is not supported for such cases.

> The implementation technique is unspecified. One possible implementation technique, using `exists()` as an example, is:

> >     template <class Path>
> >       typename boost::enable_if<is_basic_path<Path>,bool>::type exists(const Path& p);
> >     inline bool exists(const path& p) { return exists<path>(p); }
> >     inline bool exists(const wpath& p) { return exists<wpath>(p); }

> The `enable_if` will fail for a C string or `std::basic_string` argument, which will then be automatically converted to a `basic_path` object via the appropriate `basic_path` conversion constructor. -- end note]

> The two overloads are not given in the normative text because:

> - Better techniques for achieving the desired affect may be developed, perhaps enabled by core language changes like Concepts.
> - Implementations may prefer techniques that work with legacy compilers that do not support enable_if.
> - Spelling out the overloads makes the text longer and harder to read without adding much benefit.
> - More overloads will probably be needed for char16_t and char32_t (or whatever they end up being called), making it even less attractive to actually spell out each one.

Implementations of functions described in this clause are permitted to call the applications program interface (API) provided by the operating system. If such an operating system API call results in an error, implementations shall report the error by throwing exception `basic_filesystem_error`, unless otherwise specified.

> [Note: Such exceptions and the conditions that cause them to be thrown are not explicitly described in each Throws element within this clause. Because hardware failures, network failures, race conditions, and a plethora of other errors occur frequently in file system operations, users should be aware that unless otherwise specified any file system operation, not matter how apparently innocuous, may throw an exception. -- end note]

Functions commonly used in contexts where errors are not exceptional have overloads taking an additional argument of type `system_error_code&` ec. Such overloaded functions shall not throw exceptions. If an error occurs, `ec` shall be set to the error code reported by the operating system, otherwise `ec` shall be set to 0. If an overload without an argument of type `system_error_code&` ec returns void, the other overload (without an argument of type `system_error_code&` ec) returns a `system_error_code` with the value of ec.

### Header `<filesystem>` synopsis

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class String, class Traits> class basic_path;

          template<class String, class Traits>
          void swap(basic_path<String, Traits> & lhs, basic_path<String, Traits> & rhs);

          template<class String, class Traits> bool operator<(a a, b b);
          template<class String, class Traits> bool operator==(a a, b b);
          template<class String, class Traits> bool operator!=(a a, b b);
          template<class String, class Traits> bool operator>(a a, b b);
          template<class String, class Traits> bool operator<=(a a, b b);
          template<class String, class Traits> bool operator>=(a a, b b);
          template<class String, class Traits> bool operator/(a a, b b);

          template<class Path>
            basic_ostream<typename Path::string_type::value_type, typename Path::string_type::traits_type> &
              operator<<(basic_ostream<typename Path::string_type::value_type, typename Path::string_type::traits_type>& os, const Path & ph);

          template<class Path>
            basic_istream<typename Path::string_type::value_type, typename Path::string_type::traits_type> &
              operator>>(basic_istream<typename Path::string_type::value_type, typename Path::string_type::traits_type>& is, Path & ph);

          struct path_traits;
          struct wpath_traits;

          typedef basic_path<std::string, path_traits>    path;
          typedef basic_path<std::wstring, wpath_traits>  wpath;

          template<class Path> struct is_basic_path;

          template<class Path> struct slash { static const char value = '/'; };
          template<class Path> struct dot   { static const char value = '.'; };
          template<class Path> struct colon { static const char value = ':'; };

          typedef int errno_type;  // type is determined by the C standard
          typedef implementation-defined system_error_type; // usually int or unsigned

          class filesystem_error;

          template <class Path> class basic_filesystem_error;

          typedef basic_filesystem_error<path> filesystem_path_error;
          typedef basic_filesystem_error<wpath> filesystem_wpath_error;

          template <class Path> class basic_directory_entry;

          typedef basic_directory_entry<path> directory_entry;
          typedef basic_directory_entry<wpath> wdirectory_entry;

          template <class Path> class basic_directory_iterator;

          typedef basic_directory_iterator<path> directory_iterator;
          typedef basic_directory_iterator<wpath> wdirectory_iterator;

          template <class Path> class basic_recursive_directory_iterator;

          typedef basic_recursive_directory_iterator<path> recursive_directory_iterator;
          typedef basic_recursive_directory_iterator<wpath> wrecursive_directory_iterator;

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

          class file_status;

          struct space_info  // returned by space function
          {
            uintmax_t capacity;
            uintmax_t free;
            uintmax_t available;
          };

          //  status functions
          template <class Path> file_status status(const Path& p);
          template <class Path> file_status status(const Path& p, system_error_code& ec);
          template <class Path> file_status symlink_status(const Path& p);
          template <class Path> file_status symlink_status(const Path& p, system_error_code& ec);

          //  predicate functions
          bool status_known( file_status s );
          bool exists( file_status s );
          bool is_regular( file_status s );
          bool is_directory( file_status s );
          bool is_symlink( file_status s );
          bool is_other( file_status s );

          template <class Path> bool exists(const Path& p);
          template <class Path> bool is_directory(const Path& p);
          template <class Path> bool is_regular(const Path& p);
          template <class Path> bool is_other(const Path& p);
          template <class Path> bool is_symlink(const Path& p);
          template <class Path> bool is_empty(const Path& p);

          template <class Path1, class Path2>
            bool equivalent(const Path1& p1, const Path2& p2);

          //  attribute functions
          template <class Path> Path current_path();
          template <class Path> const Path& initial_path();
          template <class Path> uintmax_t file_size(const Path& p);
          template <class Path> space_info space(const Path& p);
          template <class Path> std::time_t last_write_time(const Path& p);
          template <class Path>
            void last_write_time(const Path& p, const std::time_t new_time);

          //  operations functions
          template <class Path> bool create_directory(const Path& dp);
          template <class Path1, class Path2>
            void create_hard_link(const Path1& old_fp, const Path2& new_fp);
          template <class Path1, class Path2>
            system_error_type create_hard_link(const Path1& old_fp, const Path2& new_fp, system_error_code& ec);
          template <class Path1, class Path2>
            void create_symlink(const Path1& old_fp, const Path2& new_fp);
          template <class Path1, class Path2>
            system_error_type create_symlink(const Path1& old_fp, const Path2& new_fp, system_error_code& ec);
          template <class Path> bool remove(const Path& p);
          template <class Path1, class Path2>
            void rename(const Path1& from_p, const Path2& to_p);
          template <class Path1, class Path2>
            void copy_file(const Path1& from_fp, const Path2& to_fp);
          template <class Path> Path system_complete(const Path& p);
          template <class Path> Path complete(const Path& p, const Path& base=initial_path<Path>());
          errno_type lookup_errno(system_error_type code);
          void system_message(system_error_type code, std::string & target);
          void system_message(system_error_type code, std::wstring & target);

          //  convenience functions
          template <class Path> bool create_directories(const Path & p);
          template <class Path> typename Path::string_type extension(const Path & p);
          template <class Path> typename Path::string_type basename(const Path & p);
          template <class Path>
            Path replace_extension(const Path & p, const typename Path::string_type & new_extension);

        } // namespace sys
      } // namespace tr2
    } // namespace std

### Path traits

This subclause defines requirements on classes representing path behavior traits, and defines two classes that satisfy those requirements for paths based on `string` and `wstring`.. It also defines several path additional path traits structure templates, and defines several specializations of them.

Class template `basic_path` defined in this clause requires additional types, values, and behavior to complete the definition of its semantics.

For purposes of exposition, Traits behaves as if it is a class with private members bool m_locked, initialized false, and std::locale m_locale, initialized

Path Behavior Traits Requirements
Expression | Requirements
`Traits::external_string_type` | A typedef which is a specialization of `basic_string`. The `value_type` is a character type used by the operating system to represent pathnames.
`Traits::internal_string_type` | A typedef which is a specialization of `basic_string`. The `value_type` is a character type to be used by the program to represent pathnames. Required be the same type as the `basic_path String` template parameter.
`Traits::to_external( p, is )` | `is`, converted by the `m_locale` `codecvt` facet to `external_string_type`.
`Traits::to_internal( p, xs )` | `xs`, converted by the `m_locale` `codecvt` facet to to `internal_string_type`.
`Traits::imbue(loc)` | Effects: if `m_locked`, throw. Otherwise, `m_locked = true; m_locale = loc;` Returns: `void` Throws: `basic_filesystem_error`
`Traits::imbue(loc, std::nothrow)` | Effects: `if (!m_locked) m_locale = loc; bool temp(m_locked); m_locked = true;` Returns: `temp`

Type `is_basic_path` shall be a UnaryTypeTrait (TR1, 4.1). The primary template shall be derived directly or indirectly from `std::tr1::false_type`. Type `is_basic_path` shall be specialized for `path`, `wpath`, and any user-specialized `basic_path` types, and such specializations shall be derived directly or indirectly from `std::tr1::true_type`.

Structure templates `slash`, `dot`, and `colon` are supplied with values of type `char`. If a user-specialized `basic_path` has a `value_type` type which is not convertible from `char`, the templates `slash` and `dot` shall be specialized to provide `value` with type which is convertible to `basic_path::value_type`.

### Class template `basic_path`

Class template `basic_path` provides a portable mechanism for representing paths in C++ programs, using a portable generic pathname grammar. When portability is not a requirement, native file system specific formats can be used. Class template `basic_path` is concerned only with the lexical and syntactic aspects of a path. The path does not have to exist in the operating system's file system, and may contain names which are not even valid for the current operating system.

> [Note: If the library's functions trafficked only in C++ or C-style strings, they would provide only the illusion of portability since while the syntax of function calls would be portable, the semantics of the strings they operate on would not be portable. -- end note]

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class String, class Traits> class basic_path
          {
          public:
            typedef basic_path<String, Traits> path_type;
            typedef String string_type;
            typedef typename String::value_type value_type;
            typedef Traits traits_type;
            typedef typename Traits::external_string_type external_string_type;

            // constructors/destructor
            basic_path();
            basic_path(const basic_path& p);
            basic_path(const string_type& s);
            basic_path(const value_type* s);
            template <class InputIterator>
              basic_path(InputIterator first, InputIterator last);

           ~basic_path();

            // assignments
            basic_path& operator=(const basic_path& p);
            basic_path& operator=(const string_type& s);
            basic_path& operator=(const value_type* s);
            template <class InputIterator>
              basic_path& assign(InputIterator first, InputIterator last);

            // modifiers
            basic_path& operator/=(const basic_path& rhs);
            basic_path& operator/=(const string_type& s);
            basic_path& operator/=(const value_type* s);
            template <class InputIterator>
              basic_path& append(InputIterator first, InputIterator last);

            void swap( basic_path & rhs );
            basic_path& remove_leaf();

            // observers
            const string_type string() const;
            const string_type file_string() const;
            const string_type directory_string() const;

            const external_string_type external_file_string() const;
            const external_string_type external_directory_string() const;

            string_type  root_name() const;
            string_type  root_directory() const;
            basic_path   root_path() const;
            basic_path   relative_path() const;
            string_type  leaf() const;
            basic_path   branch_path() const;

            bool empty() const;
            bool is_complete() const;
            bool has_root_name() const;
            bool has_root_directory() const;
            bool has_root_path() const;
            bool has_relative_path() const;
            bool has_leaf() const;
            bool has_branch_path() const;

            // iterators
            class iterator;
            typedef iterator const_iterator;

            iterator begin() const;
            iterator end() const;

          };

        } // namespace sys
      } // namespace tr2
    } // namespace std

A `basic_path` object stores a possibly empty path. The internal form of the stored path is unspecified.

Functions described in this clause which access files or their attributes do so by resolving a `basic_path` object into a particular file in a file hierarchy. The pathname, suitably converted to the string type, format, and encoding required by the operating system, is resolved as if by the POSIX Pathname Resolution mechanism. The encoding of the resulting pathname is determined by the `Traits::to_external` conversion function.

> [Note: There is no guarantee that the path stored in a `basic_path` object is valid for a particular operating system or file system. -- end note]

Some functions in this clause return `basic_path` objects for paths composed partly or wholly of pathnames obtained from the operating system. Such pathnames are suitably converted from the actual format and string type supplied by the operating system. The encoding of the resulting path is determined by the `Traits::to_internal` conversion function.

For member functions described as returning "`const string_type`" or "`const external_string_type`", implementations are permitted to return "`const string_type&`" or "`const external_string_type&`" respectively.

> [Note: This allows implementations to avoid unnecessary copies. Return-by-value is specified as `const` to ensure programs won't break if moved to a return-by-reference implementation. -- end note]

#### Pathname formats

There are two formats for string or sequence arguments that describe a path:

- The portable pathname format as described in Pathname grammar and by the POSIX Filename, Pathname and Pathname Resolution definitions.

  > [Note: The POSIX format is the basis for the portable format because it is already an ISO standard, is the basis for the ubiquitous URL format, and is the native format or a subset of the native format for UNIX-like and Windows-like operating systems familiar to large numbers of programmers.

  > Use of the portable format does not alone guarantee portability; filenames must also be portable. See Filename conversions. Each operating system always follows its own rules. Use of the portable format does not change that. -- end note]

- A native pathname format as defined by the operating system.

  > [Note: If an operating system supports only the POSIX pathname format, the portable format and the native format are the same.

  > Identifying user-provided paths as native format is a common need, and ensures maximum portability, even though not strictly needed except on systems where the native format is not implicitly recognized.

  > Programs using hard-coding native formats are likely to be non-portable. -- end note]

All `basic_path` string or sequence arguments that describe a path shall accept the portable pathname format, and shall accept the native format if explicitly identified by a native format escape sequence prefix of `slash slash colon`.

> [Note: `slash slash colon` was chosen as the escape sequence because a leading `slash slash` is already implementation-defined by POSIX, `colon` is prohibited in a Windows filename, and on any system a single `slash` can be used when a filename beginning with a `colon` is desired. These factors eliminate the chance of collision with a real filename. -- end note]

Implementations are encouraged to implicitly recognize the native pathname format if it can be lexically identified. An implementation shall document whether or not the native pathname format is implicitly recognized.

> [Example:

> -- OpenVMS: `"SYS1::DISK1:[JANE.TYLER.HARRY]`" is treated as a native pathname with a system name, drive name, and three directory filenames, rather than a portable pathname with one filename.

> -- Windows: `"c:\\jane\\tyler\\harry"` is treated as a native pathname with a drive letter, root-directory, and three filenames, rather than a portable pathname with one filename.

> -- Counter-example 1: An operating system that allows slashes in filenames and uses dot as a directory separator. Distinguishing between portable and native format argument strings or sequences is not possible as there is no other distinguishing syntax. The implementation does not accept native format pathnames unless the `native` argument is present.

> -- Counter-example 2: An operating system that allows slashes in filenames and uses some unusual character as a directory separator. The implementation does accept native format pathnames without the additional `native` argument, which only has to be used for native format arguments containing slashes in filenames.

> -- end example]

> [Note: This duck-rule ("if it looks like a duck, walks like a duck, and quacks like a duck, it must be a duck") eliminates format confusion as a source of programmer error and support requests. -- end note]

If both the portable and native formats are accepted, implementations shall document what characters or character sequences are used to distinguish between portable and native formats.

> [Note: Windows implementations are encouraged to define colons and backslashes as the characters which distinguish native from portable formats. --end note]

#### Pathname grammar

The grammar for the portable pathname format is as follows:

> pathname:
> root-nameopt root-directoryopt relative-pathopt

> root-name:
> implementation-defined

> root-directory:
> slash
> root-directory slash
> implementation-defined

> relative-path:
> filename
> relative-path slash
> relative-path slash filename

> filename:
> name
> dot
> dot dot

> slash:
> `slash<Path>::value`

> dot:
> `dot<Path>::value`

The grammar is aligned with the POSIX Filename, Pathname and Pathname Resolution definitions. Any conflict between the grammar and POSIX is unintentional. This technical report defers to POSIX.

> The form of the above wording was taken from POSIX, which uses it in several places to defer to the C standard.

> [Note: Windows implementations are encouraged to define slash slash name as a permissible root-name. POSIX permits, but does not require, implementations to do the same. Windows implementations are encouraged to define an additional root-directory element root_directory name. It is applicable only to the slash slash name form of root-name.

> Windows implementations are encouraged to recognize a name followed by a colon as a native format root-name, and a backslash as a format element equivalent to slash. -- end note]

#### Filename conversion

When converting filenames to the native operating system format, implementations are encouraged, but not required, to convert otherwise invalid characters or character sequences to valid characters or character sequences. Such conversions are implementation-defined.

> [Note: Filename conversion allows much wider portability of both programs and filenames that would otherwise be possible.

> Implementations are encouraged to base conversion on existing standards or practice. Examples include the Uniform Resource Locator escape syntax of a percent sign (`'%'`) followed by two hex digits representing the character value. On OpenVMS, which does not allow percent signs in filenames, a dollar sign (`'$'`) followed by two hex digits is the existing practice, as is converting lowercase letters to uppercase. -- end note.]

> The Boost implementation for Windows currently does not map invalid characters. Pending feedback from the LWG, Boost may settle on % hex hex as the preferred escape sequence. If so, should there be normative encouragement?

#### Requirements

The argument for the template parameter named `String` shall be a class that includes members with the same names, types, values, and semantics as class template `basic_string`.

The argument for the template parameter named `Traits` shall be a class that satisfies the requirements specified in the Path Behavior Traits Requirements table.

The argument for template parameters named `InputIterator` shall satisfy the requirements of an input iterator (C++ Std, 24.1.1, Input iterators [lib.input.iterators]) and shall have a value type convertible to `basic_path::value_type`.

Some function templates with a template parameter named `InputIterator` also have non-template overloads. Implementations shall only select the function template overload if the type named by `InputIterator` is not `path_format_t`.

> [Note: This "do-the-right-thing" rule ensures that the overload expected by the user is selected. The implementation technique is unspecified - implementations may use enable_if or other techniques to achieve the effect. -- end note]

#### `basic_path` constructors

    basic_path();

> Postconditions: `empty()`.

    basic_path(const string_type& s);
    basic_path(const value_type * s);
    template <class InputIterator>
      basic_path(InputIterator s, InputIterator last);

> Remarks: The format of string `s` and sequence [`first`,`last`) is described in Pathname formats.

> Effects: The path elements in string `s` or sequence [`first`,`last`) are stored.

#### `basic_path` assignments

    basic_path& operator=(const string_type& s);
    basic_path& operator=(const value_type* s);
    template <class InputIterator>
      basic_path& assign(InputIterator first, InputIterator last);

> Remarks: The format of string `s` and sequence [`first`,`last`) is described in Pathname formats.

> Effects: The path elements in string `s` or sequence [`first`,`last`) are stored.

> Returns: `*this`

#### `basic_path` modifiers

    basic_path& operator/=(const basic_path& rhs);

> Effects: The path stored in `rhs` is appended to the stored path.

> Returns: `*this`

    basic_path& operator/=(const string_type& s);
    basic_path& operator/=(const value_type* s);
    template <class InputIterator>
    basic_path& append(InputIterator first, InputIterator last);

> Remarks: The format of string `s` and sequence [`first`,`last`) is described in Pathname formats.

> Effects: The path elements in string `s` or sequence [`first`,`last`) are appended to the stored path.

> Returns: `*this`

    void swap( basic_path & rhs );

> Effects: Swaps the contents of the two paths.

> Throws: nothing.

> Postcondition: `this->string()` contains the same sequence of characters that were in `rhs`, `rhs` contains the same sequence of characters that were is `this->string()`.

> Complexity: constant time.

    basic_path& remove_leaf();

> Effects: If `has_branch_path()` then remove the last filename from the stored path. If that leaves the stored path with one or more trailing slash elements not representing root-directory, remove them.

> Returns: `*this`

> [Note: This function is needed to efficiently implement `basic_directory_iterator`. It is made public to allow additional uses. -- end note]

#### `basic_path` observers

> See the Path decomposition table for examples for values returned by decomposition functions.

    const string_type string() const;

> Returns: The stored path, formatted according to the Pathname grammar rules.

    const string_type file_string() const;

> Returns: The stored path, formatted according to the operating system rules for regular file pathnames, with any Filename conversion applied.

> [Note: For some operating systems, including POSIX and Windows, the native format for regular file pathnames and directory pathnames is the same, so `file_string()` and `directory_string()` return the same string. On OpenMVS, however, the expression `path("/cats/jane").file_string()` would return the string `"[CATS]JANE"` while `path("/cats/jane").directory_string()` would return the string `"[CATS.JANE]"`. -- end note]

    const string_type directory_string() const;

> Returns: The stored path, formatted according to the operating system rules for directory pathnames, with any Filename conversion applied.

    const external_string_type external_file_string() const;

> Returns: The stored path, formatted according to the operating system rules for regular file pathnames, with any Filename conversion applied, and encoded by the `Traits::to_external` conversion function.

    const external_string_type external_directory_string() const;

> Returns: The stored path, formatted according to the operating system rules for directory pathnames, with any Filename conversion applied, and encoded by the `Traits::to_external` conversion function.

    string_type root_name() const;

> Returns: root-name, if the stored path includes root-name, otherwise `string_type()`.

    string_type root_directory() const;

> Returns: root-directory, if the stored path includes root-directory, otherwise `string_type()`.

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

    basic_path root_path() const;

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

    basic_path relative_path() const;

> Returns: A `basic_path` composed from the the stored path, if any, beginning with the first filename after root-path. Otherwise, an empty `basic_path`.

    string_type leaf() const;

> Returns: `empty() ? string_type() : *--end()`

    basic_path branch_path() const;

> Returns: `(string().empty() || begin() == --end()) ? path_type("") : br`, where `br` is constructed as if by starting with an empty `basic_path` and successively applying `operator/=` for each element in the range `begin()`, `--end()`.

    bool empty() const;

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

    bool is_complete() const;

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

    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_leaf() const;

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

    bool has_branch_path() const;

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

#### `basic_path` iterators

A `basic_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 `string_type`.

Calling any non-const member function of a `basic_path` object invalidates all iterators referring to elements of the object.

The forward traversal order is as follows:

- The root-name element, if present.
- The root-directory element, if present.
- Each successive filename element, if present.
- Dot, if one or more trailing non-root slash characters are present.

The backward traversal order is the reverse of forward traversal.

    iterator begin() const;

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

    iterator end() const;

> Returns: The end iterator.

#### basic_path non-member functions

    template<class String, class Traits>
    void swap( basic_path<String, Traits> & lhs, basic_path<String, Traits> & rhs )

> Effects: `lhs.swap( rhs )`

#### basic_path non-member operators

There are seven basic_path non-member operators (/, `==`, `!=`, `<`, `>`, `<=`, `>=`), each with five overloads. For brevity, the specifications are given in tabular form. Each of the resulting thirty-five signatures is a template, with template parameter list template`<class String, class Traits>`. The format of such arguments is described in Pathname formats.

Argument type overloads
`basic_path<String, Traits>& a, basic_path<String, Traits>& b`
`const typename basic_path<String, Traits>::string_type& a, basic_path<String, Traits>& b`
`const typename basic_path<String, Traits>::string_type::value_type* a, basic_path<String, Traits>& b`
`const basic_path<String, Traits>& a, typename basic_path<String, Traits>::string_type& b`
`const basic_path<String, Traits>& a, typename basic_path<String, Traits>::string_type::value_type* b`

In the basic_path non-member operators table, `a` and `b` are of the types given in the Argument type overloads table. If `a` or `b` is of type `const basic_path<String, Traits>&`, then a`'` or b' respectively is `a` or `b` respectively. Otherwise a`'` or b' respectively represent named or unnamed temporary `basic_path<String, Traits>` objects constructed from `a` or `b` respectively.

basic_path non-member operators
Expression | Return type | Semantics
`a / b` | `basic_path<String, Traits>` | `basic_path<String, Traits> tmp(a); return a /=` b'`;`
`a < b` | `bool` | `return lexicographical_compare(`a`'.begin(),` a`'.end(),` b'`.begin(),` b'`.end());`
`a == b` | `bool` | `return !(`a`' <` b'`) && !(`b'`<` a`');`
`a != b` | `bool` | `return !(`a`' ==` b'`);`
`a > b` | `bool` | `return` b'`<` a`';`
`a <= b` | `bool` | `return !(`b'`<` a`');`
`a >= b` | `bool` | `return !(`a`' <` b'`);`

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

> Equality is determined by basic_path's non-member `operator==`, which considers the two path's lexical representations only. Paths "abc" and "ABC" are never equal.

> Equivalence is determined by the equivalent() non-member function, which determines if two paths resolve to the same file system entity. Paths "abc" and "ABC" may or may not resolve to the same file, depending on the file system.

> 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]

#### `basic_path` inserter and extractor

    template<class Path>
      basic_istream<typename Path::string_type::value_type, typename Path::string_type::traits_type>&
        operator>>(basic_istream< typename Path::string_type::value_type, typename Path::string_type::traits_type>& is,
                   Path& ph );

> Effects: `typename Path::string_type str;
> is >> str;
> ph = str;`

> Returns: `is`

    template<class Path>
      basic_ostream<typename Path::string_type::value_type, typename Path::string_type::traits_type>&
        operator<<(basic_ostream< typename Path::string_type::value_type, typename Path::string_type::traits_type>& os,
                   const Path& ph );

> Effects: `os << ph.string()`

> Returns: `os`

### Class `filesystem_error`

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          class filesystem_error : public std::runtime_error
          {
          public:
            explicit filesystem_error(const std::string & what_arg, system_error_type ec=0);
            system_error_type system_error() const;
          };
        } // namespace sys
      } // namespace tr2
    } // namespace std

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

> [Note: A non-template base class allows users who are uninterested in path specifics to catch a single exception type. -- end note]

    explicit filesystem_error(const std::string & what_arg, system_error_type ec=0);

> Postcondition: `strcmp(what(), what_arg.c_str()) == 0 && system_error() == ec`

    system_error_type system_error() const;

> Returns: `ec`

### Class template `basic_filesystem_error`

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class Path> class basic_filesystem_error : public filesystem_error
          {
          public:
            typedef Path path_type;

            explicit basic_filesystem_error(const std::string& what_arg, system_error_type ec=0);
            basic_filesystem_error(const std::string& what_arg, const path_type& p1, system_error_type ec);
            basic_filesystem_error(const std::string& what_arg, const path_type& p1, const path_type& p2, system_error_type ec);

            const path_type& path1() const;
            const path_type& path2() const;
          };

        } // namespace sys
      } // namespace tr2
    } // namespace std

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

#### `basic_filesystem_error` constructors

    explicit basic_filesystem_error(const std::string& what_arg, system_error_type ec=0);

> Postconditions:

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

    basic_filesystem_error(const std::string& what_arg, const path_type& p1, system_error_type ec);

> Postconditions:

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

    basic_filesystem_error(const std::string& what_arg, const path_type& p1, const path_type& p2, system_error_type ec);

> Postconditions:

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

#### `basic_filesystem_error` observers

    const path_type& path1() const;

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

    const path_type& path2() const;

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

### Class template `basic_directory_entry`

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class Path> class basic_directory_entry
          {
          public:
            typedef Path path_type;
            typedef typename Path::string_type string_type;

            // constructors
            basic_directory_entry();
            explicit basic_directory_entry(const path_type& p,
              file_status st=file_status(), file_status symlink_st=file_status());

            // modifiers
            void assign(const path_type& p, file_status st=file_status(), file_status symlink_st=file_status());
            void replace_leaf(const string_type& s, file_status st=file_status(), file_status symlink_st=file_status());

            // observers
            const Path& path() const;
            operator const Path&() const;

            file_status  status() const;
            file_status  status(system_error_code& ec) const;
            file_status  symlink_status() const;
            file_status  symlink_status(system_error_code& ec) const;

            // comparisons
            bool operator<(const basic_directory_entry<Path>& rhs);
            bool operator==(const basic_directory_entry<Path>& rhs);
            bool operator!=(const basic_directory_entry<Path>& rhs);
            bool operator>(const basic_directory_entry<Path>& rhs);
            bool operator<=(const basic_directory_entry<Path>& rhs);
            bool operator>=(const basic_directory_entry<Path>& rhs);

          private:
            path_type            m_path;           // for exposition only
            mutable file_status  m_status;         // for exposition only; stat()-like
            mutable file_status  m_symlink_status; // for exposition only; lstat()-like
          };

        } // namespace sys
      } // namespace tr2
    } // namespace std

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

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

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

#### `basic_directory_entry` constructors

    basic_directory_entry();

> Postconditions:

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

    explicit basic_directory_entry(const path_type& p, file_status st=file_status(), file_status symlink_st=file_status());

> Postconditions:

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

#### `basic_directory_entry` modifiers

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

> Postconditions:

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

    void replace_leaf(const string_type& s, file_status st=file_status(), file_status symlink_st=file_status());

> Postconditions:

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

#### `basic_directory_entry` observers

    const Path& path() const;
    operator const Path&() const;

> Returns: `m_path`

    file_status status() const;

> Effects: As if,

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

> Throws: See `status` function.

> Returns: `m_status`

    file_status status(system_error_code& ec) const;

> Effects: As if,

> >     if ( !status_known( m_status ) )
> >     {
> >       if ( status_known(m_symlink_status) && !is_symlink(m_symlink_status) )
> >         { m_status = m_symlink_status; }
> >       else { m_status = status(m_path, ec); }
> >     }
> >     else ec = 0;

> Returns: `m_status`

    file_status symlink_status() const;

> Effects: As if,

> >     if ( !status_known( m_symlink_status ) )
> >     {
> >       m_symlink_status = symlink_status(m_path);
> >     }

> Throws: See `symlink_status` function.

> Returns: `m_symlink_status`

    file_status symlink_status(system_error_code& ec) const;

> Effects: As if,

> >     if ( !status_known( m_symlink_status ) )
> >     {
> >       m_symlink_status = symlink_status(m_path, ec);
> >     }
> >     else ec = 0;

> Returns: `m_symlink_status`

### Class template `basic_directory_iterator`

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class Path>
          class basic_directory_iterator :
            public iterator<input_iterator_tag, basic_directory_entry<Path> >
          {
          public:
            typedef Path path_type;

            // constructors
            basic_directory_iterator();
            explicit basic_directory_iterator(const Path& dp);
            basic_directory_iterator(const Path& dp, system_error_type& ec);
            basic_directory_iterator(const basic_directory_iterator& bdi);
            basic_directory_iterator& operator=(const basic_directory_iterator& bdi);
           ~basic_directory_iterator();

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

        } // namespace sys
      } // namespace tr2
    } // namespace std

`basic_directory_iterator` satisfies the requirements of an input iterator (C++ Std, 24.1.1, Input iterators [lib.input.iterators]).

A `basic_directory_iterator` reads successive elements from the directory for which it was constructed, as if by calling POSIX `readdir_r()`. After a `basic_directory_iterator` is constructed, and every time `operator++` is called, it reads and stores a value of `basic_directory_entry<Path>` and possibly stores associated status values. `operator++` is not equality preserving; that is, `i == j` does not imply that `++i == ++j`.

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

If the end of the directory elements is reached, the iterator becomes equal to the end iterator value. The constructor `basic_directory_iterator()` with no arguments always constructs an end iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end iterator is not defined. For any other iterator value a `const basic_directory_entry<Path>&` is returned. The result of `operator->` on an end iterator is not defined. For any other iterator value a `const basic_directory_entry<Path>*` is returned.

Two end iterators are always equal. An end iterator is not equal to a non-end iterator.

> The above wording is based on the Standard Library's istream_iterator wording. Commentary was shortened and moved into a note.

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

> [Example: This program accepts an optional command line argument, and if that argument is a directory pathname, iterates over the contents of the directory. For each directory entry, the name is output, and if the entry is for a regular file, the size of the file is output.

> >     #include <iostream>
> >     #include <filesystem>

> >     using std::tr2::sys;
> >     using std::cout;

> >     int main(int argc, char* argv[])
> >     {
> >       std::string p(argc <= 1 ? "." : argv[1]);

> >       if (is_directory(p))
> >       {
> >         for (directory_iterator itr(p); itr!=directory_iterator(); ++itr)
> >         {
> >           cout << itr->path().leaf() << ' '; // display filename only
> >           if (is_regular(itr->status())) cout << " [" << file_size(itr->path()) << ']';
> >           cout << '\n';
> >         }
> >       }
> >       else cout << (exists(p) : "Found: " : "Not found: ") << p << '\n';

> >       return 0;
> >     }

> -- end example]

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 `basic_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 `basic_directory_iterator` for the directory, it is unspecified whether or not subsequent incrementing of the iterator will ever result in an iterator whose value is the removed or added directory entry. See POSIX `readdir_r()`. --end note]

#### `basic_directory_iterator` constructors

`basic_directory_iterator();`

> Effects: Constructs the end iterator.

`explicit basic_directory_iterator(const Path& dp);`

> Effects: Constructs a iterator representing the first entry in the directory resolved to by `dp`, otherwise, the end iterator.

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

    basic_directory_iterator(const Path& dp, system_error_type& ec );

> Effects: Constructs a iterator representing the first entry in the directory resolved to by `dp`, otherwise, the end iterator. If an error occurs while establishing the results, the iterator constructed represents the end iterator and `ec` is set to the error code reported by the operating system, otherwise to 0.

### Class template `basic_recursive_directory_iterator`

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          template <class Path>
          class basic_recursive_directory_iterator :
            public iterator<input_iterator_tag, basic_directory_entry<Path> >
          {
          public:
            typedef Path path_type;

            // constructors
            basic_recursive_directory_iterator();
            explicit basic_recursive_directory_iterator(const Path& dp);
            basic_recursive_directory_iterator(const basic_recursive_directory_iterator& brdi);
            basic_recursive_directory_iterator& operator=(const basic_recursive_directory_iterator& brdi);
           ~basic_recursive_directory_iterator();

            // observers
            int level() const;

            // modifiers
            void pop();
            void no_push();

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

          private:
            int m_level; // for exposition only
          };

        } // namespace sys
      } // namespace tr2
    } // namespace std

The behavior of a `basic_recursive_directory_iterator` is the same as a `basic_directory_iterator` unless otherwise specified.

- When an iterator is constructed, `m_level` is set to 0;
- When an iterator `it` is incremented, if `it->is_directory()` is true and `no_push()` had not been called subsequent to the most recent increment operation (or construction, if no increment has occurred), then `m_level` is incremented, the directory is visited, and its contents recursively iterated over.
- When an iterator reaches the end of the directory currently being iterated over, or when `pop()` is called, `m_level` is decremented, and iteration continues with the parent directory, until the directory specified in the constructor argument is reached.
- `level()` returns `m_level`.
- `level()`, `pop()`, and `no_push()` all require that the iterator not be the end iterator.

> [Note: One of the uses of `no_push()` is to prevent unwanted recursion into symlinked directories. This may be necessary to prevent loops on some operating systems. --end note]

### Class file_status

    namespace std
    {
      namespace tr2
      {
        namespace sys
        {
          class file_status
          {
          public:
            explicit file_status( file_type v = status_unknown );

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

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

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

#### Members

    explicit file_status( file_type v = status_unknown );

> Effects: Stores `v`.

    file_type type() const;

> Returns: The stored `file_type`.

    void type( file_type v );

> Effects: Stores `v`, replacing the previously stored value.

### Non-member operational functions

#### Status functions

    template <class Path> file_status status(const Path& p, system_error_code& ec);
    template <class Path> file_status symlink_status(const Path& p, system_error_code& ec);

> Returns:

> > For `status,` determine the attributes of `p` as if by POSIX `stat()`, for `symlink_status` determine the attributes as if by POSIX `lstat()`.

> > > [Note: For symbolic links, `stat()` continues pathname resolution using the contents of the symbolic link, `lstat()` does not. -- end note]

> > If the operating system reports an error during attribute determination:

> > - If the error indicating that `p` could not be resolved, as if by POSIX error codes ENOENT or ENOTDIR, set ec to 0 and return `file_status(not_found_flag)`.
> > - Otherwise, set ec to the error code reported by the operating system and return `file_status(status_unknown)`.
> > Otherwise:
> > - If the attributes indicate a regular file, as if by POSIX S_ISREG(), return `file_status(regular_file)`.
> > - Else if the attributes indicate a directory, as if by POSIX S_ISDIR(), return `file_status(directory_file)`.
> > - Else if the attributes indicate a symbolic link, as if by POSIX S_ISLNK(), return `file_status(symlink_file)`. [Note: Only possible for `symlink_status`. --end note]
> > - Else if the attributes indicate a block special file, as if by POSIX S_ISBLK(), return `file_status(block_file)`.
> > - Else if the attributes indicate a character special file, as if by POSIX S_ISCHR(), return `file_status(character_file)`.
> > - Else if the attributes indicate a fifo or pipe file, as if by POSIX S_ISFIFO(), return `file_status(fifo_file)`.
> > - Else if the attributes indicate a socket, as if by POSIX S_ISSOCK(), return `file_status(socket_file)`.
> > - Else return `file_status(type_unknown)`.

> [Note: `directory_file` implies `basic_directory_iterator` on the file would succeed, and `regular_file` implies appropriate `<fstream>` operations would succeed, assuming no hardware, permission, access, or race condition errors. For `regular_file,` the converse is not true; lack of `regular_file` does not necessarily imply `<fstream>` operations would fail on a directory. -- end note]

    template <class Path> file_status status(const Path& p);

> Effects: `system_error_code ec;`
> `file_status stat(status(p, ec));`

> Throws: `basic_filesystem_error<Path>` if `ec != 0`

> Returns: `stat`

    template <class Path> file_status symlink_status(const Path& p);

> Effects: `system_error_code ec;`
> `file_status stat(symlink_status(p, ec));`

> Throws: `basic_filesystem_error<Path>` if `ec != 0`

> Returns: `stat`

#### Predicate functions

    bool status_known(file_status s);

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

    bool exists(file_status s);

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

    template <class Path> bool exists(const Path& p);

> Returns: `exists( status(p) )`

    bool is_regular(file_status s);

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

    template <class Path> bool is_regular(const Path& p);

> Returns: `is_regular( status(p) )`

    bool is_directory(file_status s);

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

    template <class Path> bool is_directory(const Path& p);

> Returns: `is_directory( status(p) )`

    bool is_symlink(file_status s);

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

    template <class Path> bool is_symlink(const Path& p);

> Returns: `is_symlink( symlink_status(p) )`

    bool is_other(file_status s);

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

    template <class Path> bool is_other(const Path& p);

> Returns: `is_other( status(p) )`

    template <class Path> bool is_empty(const Path& p);

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

> Throws: `basic_filesystem_error<Path>` if `!exist(s) || is_other(s)`.

> Returns: `is_directory(s)
> ? basic_directory_iterator<Path>(p) == basic_directory_iterator<Path>()
> : file_size(p) == 0;`

    template <class Path1, class Path2> bool equivalent(const Path1& p1, const Path2& p2);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

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

> Throws: `basic_filesystem_error<Path1>` if `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))`.

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

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

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

#### Attribute functions

[Note: A strictly limited number of attribute functions are provided because few file system attributes are portable. Even the functions provided will be impossible to implement on some file systems. --end note.]

    template <class Path> const Path& initial_path();

> Returns: `current_path()` at the time of entry to `main()`.

> [Note: These semantics turn a dangerous global variable into a safer global constant. --end note]

> [Note: Full implementation requires runtime library support. Implementations which cannot provide runtime library support are encouraged to instead store the value of `current_path()` at the first call of `initial_path``()`, and return this value for all subsequent calls. Programs using `initial_path``()` are encouraged to call it immediately on entrance to `main()` so that they will work correctly with such partial implementations. --end note]

    template <class Path> Path current_path();

> Returns: The current path, as if by POSIX `getcwd()`.

> Postcondition: `current_path().is_complete()`

> [Note: 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. Although dangerous, the function is useful in dealing with other libraries.. For a safer alternative, see `initial_path()`. The `current_path()` name was chosen to emphasize that the return is a complete path, not just a single directory name. -- end note]

    template <class Path> uintmax_t file_size(const Path& p);

> Returns: The size in bytes of the file `p` resolves to, determined as if by the value of the POSIX `stat` structure member `st_size` obtained as if by POSIX `stat()`.

    template <class Path> space_info space(const Path& p);

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

    template <class Path> std::time_t last_write_time(const Path& p);

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

    template <class Path> void last_write_time(const Path& p, const std::time_t new_time);

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

> [Note: The apparent postcondition `last_write_time(p) == new_time` is not specified since it would not hold for many file systems due to coarse time mechanism granularity. -- end note]

#### Other operations functions

    template <class Path> bool create_directory(const Path& dp);

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

> Throws: `basic_filesystem_error<Path>` if Effects fails for any reason other than because the directory already exists.

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

> Postcondition: `is_directory(dp)`

    template <class Path1, class Path2>
      system_error_type create_hard_link(const Path1& to_p, const Path2& from_p, system_error_code& ec);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: Establishes the postcondition, as if by POSIX `link()`.

> Returns: If the postcondition cannot be established, a system error code indicating the reason for the failure, otherwise 0.

> Postcondition:

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

> [Note: Some operating systems do not support hard links or support them only for regular files. Some operating systems limit the number of links per file. Some file systems that do not support hard links - the FAT system used on floppy discs, memory cards and flash drives, for example. Thus hard links should be avoided if wide portability is a concern. -- end note]

    template <class Path1, class Path2>
      void create_hard_link(const Path1& to_p, const Path2& from_p);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: As if `system_error_code ec( create_hard_link( to_p, from_p ) );`

> Throws: `basic_filesystem_error<Path1, Path2>` if `ec` is not zero.

    template <class Path1, class Path2>
      system_error_type create_symlink(const Path1& to_p, const Path2& from_p, system_error_code& ec);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: Establishes the postcondition, as if by POSIX `symlink()`.

> Returns: If the postcondition cannot be established, a system error code indicating the reason for the failure, otherwise 0.

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

> [Note: Some operating systems do not support symbolic links at all or support them only for regular files. Thus symbolic links should be avoided if code portability is a concern. -- end note]

    template <class Path1, class Path2>
      void create_symlink(const Path1& to_p, const Path2& from_p);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: As if `system_error_code ec( create_symlink( to_p, from_p ) );`

> Throws: `basic_filesystem_error<Path1, Path2>` if `ec` is not zero.

    template <class Path> bool remove(const Path& p);

> Precondition: `!p.empty()`

> Effects: Attempts to delete the file `p` resolves to, as if by POSIX `remove()`.

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

> Postcondition: `!exists(p)`

> Throws: `basic_filesystem_error<Path>` if:

> - `p.empty() || (exists(p) && is_directory(p) && !empty(p))`.
> - Effects fails for any reason other than because `p` does not resolve to an existing file.

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

    template <class Path1, class Path2> void rename(const Path1& from_p, const Path2& to_p);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: Renames `from_p` to `to_p`, as if by POSIX `rename()`.

> Postconditions: `!exists(from_p) && exists(to_p)`, and the contents and attributes of the file originally named `from_p` are otherwise unchanged.

> [Note: If `from_p` and `to_p` resolve to the same file, no action is taken. Otherwise, if `to_p` resolves to an existing file, it is removed. A symbolic link is itself renamed, rather than the file it resolves to being renamed. -- end note]

    template <class Path1, class Path2> void copy_file(const Path1& from_fp, const Path2& to_fp);

> Requires: `Path1::external_string_type` and `Path2::external_string_type` are the same type.

> Effects: The contents and attributes of the file `from_fp` resolves to are copied to the file `to_fp` resolves to.

> Throws: `basic_filesystem_error<Path>` if `from_fp.empty() || to_fp.empty() ||!exists(from_fp) || !is_regular(from_fp) || exists(to_fp)`

    template <class Path> Path complete(const Path& p, const Path& base=initial_path<Path>());

> Effects: Composes a complete path from `p` and `base`, using the following rules:

> | `p.has_root_directory()` | `!p.has_root_directory()`
> `p.has_root_name()` | `p` | precondition failure
> `!p.has_root_name()` | `base.root_name() / p` | `base / p`

> Returns: The composed path.

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

> Throws: If `!(base.is_complete() && (p.is_complete() || !p.has_root_name()))`

> [Note: When portable behavior is required, use complete(). When operating system dependent behavior is required, use system_complete().

> Portable behavior is useful when dealing with paths created internally within a program, particularly if the program should exhibit the same behavior on all operating systems.

> Operating system dependent behavior is useful when dealing with paths supplied by user input, reported to program users, or when such behavior is expected by program users. -- end note]

    template <class Path> Path system_complete(const Path& p);

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

> Returns: The composed path.

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

> Throws: If `p.empty()`.

> [Note: For POSIX, `system_complete(p)` has the same semantics as `complete(p, current_path())`.

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

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

    errno_type to_errno( system_error_type code );

> Returns: The value of the `errno` error number which corresponds to the operating system's error code `code`. The exact correspondence is implementation defined. Implementations are only required to support error codes reported by `basic_filesystem_error` exceptions thrown by functions defined in this clause.

    void system_message( system_error_type ec, std::string & target );
    void system_message( system_error_type ec, std::wstring & target );

> Effects: Appends a message corresponding to `ec` to `target`.

> [Note: Implementations are encouraged to supply a localized message. -- end note]

#### Convenience functions

    template <class Path> bool create_directories(const Path & p);

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

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

> Postcondition: `is_directory(p)`

> Throws: `basic_filesystem_error<Path>` if`exists(p) && !is_directory(p)`

    template <class Path> typename Path::string_type extension(const Path & p);

> Returns: if `p.leaf()` contains a dot, returns the substring of `p.leaf()` starting at the rightmost dot and ending at the string's end. Otherwise, returns an empty string.

> [Note: The dot is included in the return value so that it is possible to distinguish between no extension and an empty extension.

> Implementations are permitted but not required to define additional behavior for file systems which append additional elements to extensions, such as alternate data stream or partitioned dataset names. -- end note]

    template <class Path> typename Path::string_type basename(const Path & p);

> Returns: if `p.leaf()` contains a dot, returns the substring of `p.leaf()` starting at its beginning and ending at the last dot (the dot is not included). Otherwise, returns `p.leaf()`.

    template <class Path>
      Path replace_extension(const Path & p, const typename Path::string_type & new_extension);

> Postcondition: `basename(return_value) == basename(p) && extension(return_value) == new_extension`

> [Note: It follows from the semantics of `extension()` that `new_extension` should include dot to achieve reasonable results. -- end note]

### Additions to header `<cerrno>`

The header <cerrno> shall include an additional symbolic constant macro for each of the values returned by the to_errno function. The macro names shall be as defined in POSIX errno.h, with the additions below.

> This codifies existing practice. The required names are only a sub-set of those defined by POSIX, and are usually already supplied in <errno.h> (as wrapped by <cerrno>) as shipped with POSIX and Windows compilers. These implementations require no changes to their underlying C headers to conform with the above requirement.

> Name | Meaning
> `EBADHANDLE` | Bad operating system handle.
> `EOTHER` | Other error.

### Additions to header `<fstream>`

> These additions have been carefully specified to avoid breaking existing code in common operating environments such as POSIX, Windows, and OpenVMS. See Suggestions for `<fstream>` implementations for techniques to avoid breaking existing code in other environments, particularly on operating systems allowing slashes in filenames.

> [Note: The "do-the-right-thing" rule from Requirements on implementations does apply to header `<fstream>`.

> The overloads below are specified as additions rather than replacements for existing functions. This preserves existing code (perhaps using a home-grown path class) that relies on an automatic conversion to `const char*`. -- end note]

In 27.8.1.1 Class template basic_filebuf [lib.filebuf] synopsis preceding paragraph 1, add the function:

>     template <class Path> basic_filebuf<charT,traits>* open(const Path& p, ios_base::openmode mode);

In 27.8.1.3 Member functions [lib.filebuf.members], add the above to the signature preceding paragraph 2, and replace the sentence:

> It then opens a file, if possible, whose name is the NTBS s (“as if” by calling `std::fopen(s ,modstr ))`.

with:

> It then opens, if possible, the file that `p` or `path(s)` resolves to, “as if” by calling `std::fopen()` with a second argument of modstr.

In 27.8.1.5 Class template basic_ifstream [lib.ifstream] synopsis preceding paragraph 1, add the functions:

>     template <class Path> explicit basic_ifstream(const Path& p, ios_base::openmode mode = ios_base::in);
>     template <class Path> void open(const Path& p, ios_base::openmode mode = ios_base::in);

In 27.8.1.6 basic_ifstream constructors [lib.ifstream.cons] add the above constructor to the signature preceding paragraph 2, and in paragraph 2 replace

> `rdbuf()->open(s, mode | ios_base::in)`

with

> `rdbuf()->open(path(s), mode | ios_base::in)` or `rdbuf()->open(p, mode | ios_base::in)` as appropriate

In 27.8.1.7 Member functions [lib.ifstream.members] add the above open function to the signature preceding paragraph 3, and in paragraph 3 replace

> `rdbuf()->open(s, mode | ios_base::in)`

with

> `rdbuf()->open(path(s), mode | ios_base::in)` or `rdbuf()->open(p, mode | ios_base::in)` as appropriate

In 27.8.1.8 Class template basic_ofstream [lib.ofstream] synopsis preceding paragraph 1, add the functions:

>     template <class Path> explicit basic_ofstream(const Path& p, ios_base::openmode mode = ios_base::out);
>     template <class Path> void open(const Path& p, ios_base::openmode mode = ios_base::out);

In 27.8.1.9 basic_ofstream constructors [lib.ofstream.cons] add the above constructor to the signature preceding paragraph 2, and in paragraph 2 replace

> `rdbuf()->open(s, mode | ios_base::out)`

with

> `rdbuf()->open(path(s), mode | ios_base::out)` or `rdbuf()->open(p, mode | ios_base::out)` as appropriate

In 27.8.1.10 Member functions [lib.ofstream.members] add the above open function to the signature preceding paragraph 3, and in paragraph 3 replace

> `rdbuf()->open(s, mode | ios_base::out)`

with

> `rdbuf()->open(path(s), mode | ios_base::out)` or `rdbuf()->open(p, mode | ios_base::out)` as appropriate

In 27.8.1.11 Class template basic_fstream [lib.fstream] synopsis preceding paragraph 1, add the functions:

>     template <class Path> explicit basic_fstream(const Path& p, ios_base::openmode mode = ios_base::in|ios_base::out);
>     template <class Path> void open(const Path& p, ios_base::openmode mode = ios_base::in|ios_base::out);

In 27.8.1.12 basic_fstream constructors [lib.fstream.cons] add the above constructor to the signature preceding paragraph 2, and in paragraph 2 replace

> `rdbuf()->open(s, mode)`

with

> `rdbuf()->open(path(s), mode)` or `rdbuf()->open(p, mode)` as appropriate

In 27.8.1.13 Member functions [lib.fstream.members] add the above open function to the signature preceding paragraph 3, and in paragraph 3 replace

> `rdbuf()->open(s, mode)`

with

> `rdbuf()->open(path(s), mode)` or `rdbuf()->open(p, mode)` as appropriate

End of proposed text.

## Path decomposition table

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

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

Constructor argument | Elements found by iteration | `string()` | `file_ string()` | `root_ path() .string()` | `root_ name()` | `root_ directory()` | `relative_ path() .string()` | `branch_ path() .string()` | `leaf()`
`""` | `""` | `""` | `""` | `""` | `""` | `""` | `""` | `""` | `""`
`"."` | `"."` | `"."` | `"."` | `""` | `""` | `""` | `"."` | `""` | `"."`
`".."` | `".."` | `".."` | `".."` | `""` | `""` | `""` | `".."` | `""` | `".."`
`"foo"` | `"foo"` | `"foo"` | `"foo"` | `""` | `""` | `""` | `"foo"` | `""` | `"foo"`
`"/"` | `"/"` | `"/"` | `"/" "\"` | `"/"` | `""` | `"/"` | `""` | `""` | `"/"`
`"/foo"` | `"/","foo"` | `"/foo"` | `"/foo" "\foo"` | `"/"` | `""` | `"/"` | `"foo"` | `"/"` | `"foo"`
`"foo/"` | `"foo","."` | `"foo/"` | `"foo/" "foo\"` | `""` | `""` | `""` | `"foo/"` | `"foo"` | `"."`
`"/foo/"` | `"/","foo","."` | `"/foo/"` | `"/foo/" "\foo\"` | `"/"` | `""` | `"/"` | `"foo/"` | `"/foo"` | `"."`
`"foo/bar"` | `"foo","bar"` | `"foo/bar"` | `"foo/bar" "foo\bar"` | `""` | `""` | `""` | `"foo/bar"` | `"foo"` | `"bar"`
`"/foo/bar"` | `"/","foo","bar"` | `"/foo/bar"` | `"/foo/bar" "\foo\bar"` | `"/"` | `""` | `"/"` | `"foo/bar"` | `"/foo"` | `"bar"`
`"///foo///"` | `"/","foo","."` | `"///foo///"` | `"///foo///" "\foo\\\"` | `"/"` | `""` | `"/"` | `"foo///"` | `"///foo"` | `"."`
`"///foo///bar"` | `"/","foo","bar"` | `"///foo///bar"` | `"///foo///bar" "\foo\\\bar"` | `"/"` | `""` | `"/"` | `"foo///bar"` | `"///foo"` | `"bar"`
`"/."` | `"/","."` | `"/."` | `"/." "\."` | `"/"` | `""` | `"/"` | `"."` | `"/"` | `"."`
`"./"` | `".","."` | `"./"` | `"./" ".\"` | `""` | `""` | `""` | `"./"` | `"."` | `"."`
`"/.."` | `"/",".."` | `"/.."` | `"/.." "\.."` | `"/"` | `""` | `"/"` | `".."` | `"/"` | `".."`
`"../"` | `"..","."` | `"../"` | `"../" "..\"` | `""` | `""` | `""` | `"../"` | `".."` | `"."`
`"foo/."` | `"foo","."` | `"foo/."` | `"foo/." "foo\."` | `""` | `""` | `""` | `"foo/."` | `"foo"` | `"."`
`"foo/.."` | `"foo",".."` | `"foo/.."` | `"foo/.." "foo\.."` | `""` | `""` | `""` | `"foo/.."` | `"foo"` | `".."`
`"foo/./"` | `"foo",".","."` | `"foo/./"` | `"foo/./" "foo\.\"` | `""` | `""` | `""` | `"foo/./"` | `"foo/."` | `"."`
`"foo/./bar"` | `"foo",".","bar"` | `"foo/./bar"` | `"foo/./bar" "foo\.\bar"` | `""` | `""` | `""` | `"foo/./bar"` | `"foo/."` | `"bar"`
`"foo/.."` | `"foo",".."` | `"foo/.."` | `"foo/.." "foo\.."` | `""` | `""` | `""` | `"foo/.."` | `"foo"` | `".."`
`"foo/../"` | `"foo","..","."` | `"foo/../"` | `"foo/../" "foo\..\"` | `""` | `""` | `""` | `"foo/../"` | `"foo/.."` | `"."`
`"foo/../bar"` | `"foo","..","bar"` | `"foo/../bar"` | `"foo/../bar" "foo\..\bar"` | `""` | `""` | `""` | `"foo/../bar"` | `"foo/.."` | `"bar"`
`"c:"` | `"c:"` | `"c:"` | `"c:"` | `"" "c:"` | `"" "c:"` | `""` | `"c:" ""` | `""` | `"c:"`
`"c:/"` | `"c:","." "c:","/"` | `"c:/"` | `"c:/" "c:\"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:/" ""` | `"c:"` | `"." "/"`
`"c:foo"` | `"c:foo" "c:","foo"` | `"c:foo"` | `"c:foo"` | `"" "c:"` | `"" "c:"` | `""` | `"c:foo" "foo"` | `"" "c:"` | `"c:foo" "foo"`
`"c:/foo"` | `"c:","foo" "c:","/","foo"` | `"c:/foo"` | `"c:/foo" "c:\foo"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:/foo" "foo"` | `"c:" "c:/"` | `"foo"`
`"c:foo/"` | `"c:foo","." "c:","foo","."` | `"c:foo/"` | `"c:foo/" "c:foo\"` | `"" "c:"` | `"" "c:"` | `""` | `"c:foo/" "foo/"` | `"c:foo"` | `"."`
`"c:/foo/"` | `"c:","foo","." "c:","/","foo","."` | `"c:/foo/"` | `"c:/foo/" "c:\foo\"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:/foo/" "foo/"` | `"c:/foo"` | `"."`
`"c:/foo/bar"` | `"c:","foo","bar" "c:","/","foo","bar"` | `"c:/foo/bar"` | `"c:/foo/bar" "c:\foo\bar"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:/foo/bar" "foo/bar"` | `"c:/foo"` | `"bar"`
`"prn:"` | `"prn:"` | `"prn:"` | `"prn:"` | `"" "prn:"` | `"" "prn:"` | `""` | `"prn:" ""` | `""` | `"prn:"`
`"c:\"` | `"c:\" "c:","/"` | `"c:\" "c:/"` | `"c:\"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:\" ""` | `"" "c:"` | `"c:\" "/"`
`"c:foo"` | `"c:foo" "c:","foo"` | `"c:foo"` | `"c:foo"` | `"" "c:"` | `"" "c:"` | `""` | `"c:foo" "foo"` | `"" "c:"` | `"c:foo" "foo"`
`"c:\foo"` | `"c:\foo" "c:","/","foo"` | `"c:\foo" "c:/foo"` | `"c:\foo"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:\foo" "foo"` | `"" "c:/"` | `"c:\foo" "foo"`
`"c:foo\"` | `"c:foo\" "c:","foo","."` | `"c:foo\" "c:foo/"` | `"c:foo\"` | `"" "c:"` | `"" "c:"` | `""` | `"c:foo\" "foo/"` | `"" "c:foo"` | `"c:foo\" "."`
`"c:\foo\"` | `"c:\foo\" "c:","/","foo","."` | `"c:\foo\" "c:/foo/"` | `"c:\foo\"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:\foo\" "foo/"` | `"" "c:/foo"` | `"c:\foo\" "."`
`"c:\foo/"` | `"c:\foo","." "c:","/","foo","."` | `"c:\foo/" "c:/foo/"` | `"c:\foo/" "c:\foo\"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:\foo/" "foo/"` | `"c:\foo" "c:/foo"` | `"."`
`"c:/foo\bar"` | `"c:","foo\bar" "c:","/","foo","bar"` | `"c:/foo\bar" "c:/foo/bar"` | `"c:/foo\bar" "c:\foo\bar"` | `"" "c:/"` | `"" "c:"` | `"" "/"` | `"c:/foo\bar" "foo/bar"` | `"c:" "c:/foo"` | `"foo\bar" "bar"`

## Suggestions for `<fstream>` implementations

The change in semantics to functions taking `const char*` arguments can break existing code, but only on operating systems where implementations don't implicitly accept native format pathnames or operating systems that allow slashes in filenames. Thus on POSIX, Windows, and OpenVMS, for example, there is no problem if the implementation follows encouraged behavior.

For most of the Filesystem Library, there is no existing code, so the issue preserving existing code that uses slashes in filenames doesn't arise. New code simply must use basic_path constructors with `path_format_t` arguments of `native`. To preserve existing fstream code that uses slashes in filenames, an implementation may wish to provide a mechanism such as a macro to control selection of the old behavior.

Implementations are already required by the TR front-matter to provide a mechanism such as a macro to control selection of the old behavior (useful to guarantee protection of existing code) or new behavior (useful in new code, and code being ported from other systems) for headers. Because use of the rest of the Filesystem Library is independent of use of the `<fstream>` additions, affected implementations are encouraged to allow disabling the `<fstream>` additions separately from other TR features.

An rejected alternative was to supply new fstream classes in namespace `sys`, inheriting from the current classes, overriding the constructors and opens taking pathname arguments, and providing the additional overloads. In Lillehammer LWG members indicated lack of support for this alternative, feeling that costs outweigh benefits.

## Issues

### 1. Return type of certain basic_path members returning strings. [Howard Hinnant]

For member functions described as returning "`const string_type`" or "`const external_string_type`", implementations are permitted to return "`const string_type&`" or "`const external_string_type&`" respectively.

This allows implementations to avoid unnecessary copies. Return-by-value is specified as `const` to ensure programs won't break if moved to a return-by-reference implementation.

For example, the Boost implementation keeps the internal representation of a pathname in the portable format, so string() returns by reference and is inlined:

>     const string_type & string() const { return m_path; }

Howard Hinnant comments: This may inhibit optimization if rvalue reference is accepted. Const-qualified return types can't be moved from. I'd rather see either the return type specified as `const string_type&` or `string_type`.

Beman Dawes comments: I can't make up my mind. Removing the const will bite users, but not very often. OTOH, excessive copying is a real concern, and if move semantics can alleviate that, I'm all for it. What does the LWG think?

### 2. Basic_path canonize() and normalize() removed. [Beman Dawes]

The Boost implementation has basic_path functions canonize() and normalize() which return cleaned up string representations of a pathname. They have been removed from the proposal as messy to specify and implement, not hugely useful, and possible to implement by users as non-member functions without any loss of functionality or efficiency. There was also a concern the proposal was getting a bit large.

These functions can be added later as convenience functions if the LWG so desires..

### 3. Filename checking functions. [Beman Dawes]

Boost has a set of predicate functions that determine if a filename is valid for a particular operating or system. These can be used as building blocks for functions that determine if an entire pathname is valid for a particular operating or file system.

Users can use these functions to ensure that pathnames are in fact portable to target operating or file systems, without having to actually test on the target systems.

These functions are not included in the proposal because of lack of time, and uncertainty as to their fit with the Standard Library. They can be added later if the LWG so desires.

## Acknowledgements

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

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

Dietmar Kühl 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.

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

## References

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

## Revision History

N1841 | Initial version, August, 2005, pre-Tremblant mailing
N1889 Revision 1 | Missing argument name `fmt` added to several `basic_path` members. `is_empty()` name discrepancy between synopsis and description corrected. `file_size()` return type changed from `intmax_t` to `uintmax_t`. Wording slightly clarified. `struct space_info` and non-member function `space()` added. A paragraph was added to Important design decisions mentioning the need for both portable and platform specific semantics.
N1934 Revision 2 | Changed native path identification from constructor argument to `"//:"` escape prefix. Rationale: simplifies basic_path constructor interfaces, easier use for platforms needing explicit native format identification. Introduced a new base class, filesystem_error, to allow users to catch a single exception type if desired, or to deal with the case where the templated type is unknown. Rename filesystem_error and wfilesystem_error accordingly. Rewording basic_filesystem_error text to more closely follow the form of clause 19 of the standard. Removed dual specification of certain errors in both "Reguires" and "Throws" paragraphs. Since throwing an exception is well-defined behavior, the error condition does not result in undefined behavior as implied by "Requires". (Suggested by Dave Abrahams) Added a non-throwing version of create_hard_link(). Added two create_symlink() functions. Added basic_path inserter and extractor. (Suggested by Vladimir Prus) Added basic_path member and non-member swap() functions. Aligned basic_path operator functions with std::basic_string practice. Replaced status_flags with file_type enum and file_status class to improve encapsulation and allow for future expansion of file_status. Added predicate functions overloaded on file_status (Suggested by Martin Adrian). This change, coupled with the introduction of file_status, clarifies the meaning of file types and related predicate operations, and eliminates the need for user bit manipulation, which was a source of user error. Predicate function specification clarified accordingly. Revised and explicitly documented policy for non-throwing versions of functions to increase consistency. Added basic_directory_iterator constructor non-throwing overload (Suggested by Martin Adrian). Changed symlink awareness to separately name functions to cut clutter caused by addition of non-throwing overloads.

© Copyright Beman Dawes, 2002-2006

Revised 2006-02-06