3554. chrono::parse needs const charT* and basic_string_view<charT> overloads

Section: 30.13 [time.parse] Status: C++23 Submitter: Howard Hinnant Opened: 2021-05-22 Last modified: 2023-11-22

Priority: 2

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Discussion:

The chrono::parse functions take const basic_string<charT, traits, Alloc>& parameters to specify the format strings for the parse. Due to an oversight on my part in the proposal, overloads taking const charT* and basic_string_view<charT, traits> were omitted. These are necessary when the supplied arguments is a string literal or string_view respectively:

in >> parse("%F %T", tp);

These overloads have been implemented in the example implementation.

[2021-05-26; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative to N4885.

1. Modify 30.2 [time.syn], header <chrono> synopsis, as indicated:

   […]
   namespace chrono {
     // 30.13 [time.parse], parsing
     template<class charT, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp);
   
     template<class charT, class traits, class Parsable>
       unspecified
         parse(basic_string_view<charT, traits> fmt, Parsable& tp);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(basic_string_view<charT, traits> fmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev);
   
     template<class charT, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp, minutes& offset);
   
     template<class charT, class traits, class Parsable>
       unspecified
         parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(basic_string_view<charT, traits> fmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
               minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
     […]
   }
   […]
   

2. Modify 30.13 [time.parse] as indicated:

   template<class charT, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT>&>(), fmt, tp)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT>{fmt}, tp);

   template<class charT, class traits, class Parsable>
     unspecified
       parse(basic_string_view<charT, traits> fmt, Parsable& tp);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits>{fmt}, tp);

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev))
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(basic_string_view<charT, traits> fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev))
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);

   template<class charT, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp, minutes& offset);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT>&>(), fmt, tp,
               declval<basic_string<charT>*>(),
               &offset)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT>{fmt}, tp, offset);

   template<class charT, class traits, class Parsable>
     unspecified
       parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp,
               declval<basic_string<charT, traits>*>(),
               &offset)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits>{fmt}, tp, offset);

   template<class charT, class traits, class Alloc, class Parsable>
   unspecified
     parse(const charT* fmt, Parsable& tp,
           basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev), &offset)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(basic_string_view<charT, traits> fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   

   -?- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev), &offset)
   

   is well-formed when treated as an unevaluated operand.

   -?- Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);
   

   -2- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)
   

   is well-formed when treated as an unevaluated operand.

   -3- Returns: A manipulator such that the expression is >> parse(fmt, tp) has type I, has value is, and calls from_stream(is, fmt.c_str(), tp).

[2021-06-02 Tim comments and provides updated wording]

The current specification suggests that parse takes a reference to the format string (stream extraction on the resulting manipulator is specified to call from_stream on fmt.c_str(), not some copy), so we can't call it with a temporary string.

Additionally, since the underlying API (from_stream) requires a null-terminated string, the usual practice is to avoid providing a string_view overload (see, e.g., LWG 3430). The wording below therefore only provides const charT* overloads. It does not use "Equivalent to" to avoid requiring that the basic_string overload and the const charT* overload return the same type. As the manipulator is intended to be immediately consumed, the wording also adds normative encouragement to make misuse more difficult.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 30.2 [time.syn], header <chrono> synopsis, as indicated:

   […]
   namespace chrono {
     // 30.13 [time.parse], parsing
     template<class charT, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev);
   
     template<class charT, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp, minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
               minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const charT* fmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
     template<class charT, class traits, class Alloc, class Parsable>
       unspecified
         parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
     […]
   }
   […]
   

2. Modify 30.13 [time.parse] as indicated:

   -1- Each parse overload specified in this subclause calls from_­stream unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let is denote an object of type basic_­istream<charT, traits> and let I be basic_­istream<charT, traits>&, where charT and traits are template parameters in that context.

   -?- Recommended practice: Implementations should make it difficult to accidentally store or use a manipulator that may contain a dangling reference to a format string, for example by making the manipulators produced by parse immovable and preventing stream extraction into an lvalue of such a manipulator type.

   template<class charT, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp);
   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);
   

   -?- Let F be fmt for the first overload and fmt.c_str() for the second overload. Let traits be char_traits<charT> for the first overload.

   -2- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str()F, tp)
   

   is well-formed when treated as an unevaluated operand.

   -3- Returns: A manipulator such that the expression is >> parse(fmt, tp) has type I, has value is, and calls from_­stream(is, fmt.c_str()F, tp).

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev);
   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev);
   

   -?- Let F be fmt for the first overload and fmt.c_str() for the second overload.

   -4- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str()F, tp, addressof(abbrev))
   

   is well-formed when treated as an unevaluated operand.

   -5- Returns: A manipulator such that the expression is >> parse(fmt, tp, abbrev) has type I, has value is, and calls from_­stream(is, fmt.c_str()F, tp, addressof(abbrev)).

   template<class charT, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp, minutes& offset);
   
   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
             minutes& offset);
   

   -?- Let F be fmt for the first overload and fmt.c_str() for the second overload. Let traits be char_traits<charT> and Alloc be allocator<charT> for the first overload.

   -6- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(),
               fmt.c_str()F, tp,
               declval<basic_string<charT, traits, Alloc>*>(),
               &offset)
   

   is well-formed when treated as an unevaluated operand.

   -7- Returns: A manipulator such that the expression is >> parse(fmt, tp, offset) has type I, has value is, and calls:

   from_stream(is,
               fmt.c_str()F, tp,
               static_cast<basic_string<charT, traits, Alloc>*>(nullptr),
               &offset)
   

   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const charT* fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   
   template<class charT, class traits, class Alloc, class Parsable>
     unspecified
       parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
             basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
   

   -?- Let F be fmt for the first overload and fmt.c_str() for the second overload.

   -8- Constraints: The expression

   from_stream(declval<basic_istream<charT, traits>&>(),
               fmt.c_str()F, tp, addressof(abbrev), &offset)
   

   is well-formed when treated as an unevaluated operand.

   -9- Returns: A manipulator such that the expression is >> parse(fmt, tp, abbrev, offset) has type I, has value is, and calls from_­stream(is, fmt.c_str()F, tp, addressof(abbrev), &offset).