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2137. Misleadingly constrained post-condition in the presence of exceptions

Section: 28.6.7.3 [re.regex.assign] Status: Open Submitter: Jonathan Wakely Opened: 2012-03-08 Last modified: 2024-10-03

Priority: 3

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Discussion:

The post-conditions of basic_regex<>::assign 28.6.7.3 [re.regex.assign] p16 say:

If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.

The default expectation in the library is that post-conditions only hold, if there is no failure (see also 2136⁽ⁱ⁾), therefore the initial condition should be removed to prevent any misunderstanding.

[ 2012-10 Portland: Move to Open ]

A favorable resolution clearly depends on a favorable resolution to 2136⁽ⁱ⁾. There is also a concern that this is just one example of where we would want to apply such a wording clean-up, and which is really needed to resolve both this issue and 2136⁽ⁱ⁾ is a paper providing the clause 17 wording that gives the guarantee for postcondition paragraphs, and then reviews clauses 18-30 to apply that guarantee consistently. We do not want to pick up these issues piecemeal, as we risk opening many issues in an ongoing process.

[2015-05-06 Lenexa: EricWF to write paper addressing 2136 and 2137]

[2024-10-03; Jonathan comments]

I could find no other cases in the entire standard where we say something like this in a Postconditions: element. 31.6.3.5.4 [streambuf.virt.pback] p2 says "Postconditions: On return, the constraints of [...]" which is probably redundant (postconditions are always "on return").

Proposed resolution:

This wording is relative to N3376.

template <class string_traits, class A>
  basic_regex& assign(const basic_string<charT, string_traits, A>& s,
    flag_type f = regex_constants::ECMAScript);

[…]

-15- Effects: Assigns the regular expression contained in the string s, interpreted according the flags specified in f. If an exception is thrown, *this is unchanged.

-16- Postconditions: If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.