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

Title: chrono::parse needs const charT* and basic_string_view<charT> overloads
Status: c++23
Section: [time.parse]
Submitter: Howard Hinnant

Created on 2021-05-22.00:00:00 last changed 5 months ago

Messages

Date: 2021-10-14.09:56:08

Proposed resolution:

This wording is relative to N4885.

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

[…]
namespace chrono {
  // [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);

  […]
}
[…]

Modify [time.parse] as indicated:

-1- Each parse overload specified in this subclause calls from_stream unqualified, so as to enable argument dependent lookup ([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).

msg11960 (view)

Date: 2021-10-14.00:00:00

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

msg11864 (view)

Date: 2021-06-15.00:00:00

[ 2021-06-23; Reflector poll ]

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

msg11853 (view)

Date: 2021-06-02.00:00:00

[ 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.

msg11845 (view)

Date: 2021-05-15.00:00:00

[ 2021-05-26; Reflector poll ]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative to N4885.

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

[…]
namespace chrono {
  // [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);

  […]
}
[…]

Modify [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).

msg11844 (view)

Date: 2021-05-22.00:00:00

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.

History
Date	User	Action	Args
2023-11-22 15:47:43	admin	set	status: wp -> c++23
2021-10-14 09:56:08	admin	set	messages: + msg12123
2021-10-14 09:56:08	admin	set	status: voting -> wp
2021-09-29 12:57:28	admin	set	status: ready -> voting
2021-06-23 14:16:45	admin	set	messages: + msg11960
2021-06-23 14:16:45	admin	set	status: new -> ready
2021-06-02 05:33:21	admin	set	messages: + msg11864
2021-05-26 10:52:50	admin	set	messages: + msg11853
2021-05-22 22:17:34	admin	set	messages: + msg11845
2021-05-22 00:00:00	admin	create