2707. path construction and assignment should have "string_type&&" overloads

Section: 31.12.6 [fs.class.path] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.class.path].

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Discussion:

Currently construction of a path from the native string_type always performs a copy, even when that string is passed as a rvalue. This is a large pessimization as paths are commonly constructed from temporary strings.

One pattern I frequently see is:

path foo(path const& p) {
  auto s = p.native();
  mutateString(s);
  return s;
}

Implementations should be allowed to move from s and avoid an unnecessary allocation. I believe string_type&& constructor and assignment operator overloads should be added to support this.

Proposed resolution:

This wording is relative to N4582.

1. Change 31.12.6 [fs.class.path], class path synopsis, as indicated:

   [Drafting note: Making the string_type&& constructors and assignment operators noexcept would over-constrain implementations which may need to perform construct additional state]

   namespace std::filesystem {
     class path {
     public:
       […]
       // 27.10.8.4.1, constructors and destructor
       path() noexcept;
       path(const path& p);
       path(path&& p) noexcept;
       path(string_type&& source);
       template <class Source>
       path(const Source& source);
       […]
       
       // 27.10.8.4.2, assignments
       path& operator=(const path& p);
       path& operator=(path&& p) noexcept;
       path& operator=(string_type&& source);
       path& assign(string_type&& source);
       template <class Source>
       path& operator=(const Source& source);
       template <class Source>
       path& assign(const Source& source)
       template <class InputIterator>
       path& assign(InputIterator first, InputIterator last);    
       […]
     };
   }
   

2. Add a new paragraph following 31.12.6.5.1 [fs.path.construct]/3:

   path(string_type&& source);
   

   -?- Effects: Constructs an object of class path with pathname having the original value of source. source is left in a valid but unspecified state.

3. Add a new paragraph following 31.12.6.5.2 [fs.path.assign]/4:

   path& operator=(string_type&& source);
   path& assign(string_type&& source);
   

   -?- Effects: Modifies pathname to have the original value of source. source is left in a valid but unspecified state.

   -?- Returns:*this