2534. Constrain rvalue stream operators

Section: 30.7.5.5 [ostream.rvalue], 30.7.4.5 [istream.rvalue] Status: C++17 Submitter: Robert Haberlach Opened: 2015-09-08 Last modified: 2017-07-30

Priority: 3

View all other issues in [ostream.rvalue].

View all issues with C++17 status.

Discussion:

The rvalue stream insertion and extraction operators should be constrained to not participate in overload resolution unless the expression they evaluate is well-formed. Programming code that tests the validity of stream insertions (or extractions) using SFINAE can result in false positives, as the present declarations accept virtually any right-hand side argument. Moreover, there is no need for pollution of the candidate set with ill-formed specializations.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

  1. Modify 30.7.5.5 [ostream.rvalue] as indicated:

    template <class charT, class traits, class T>
      basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>&& os, const T& x);
    

    -1- Effects: os << x

    -2- Returns: os

    -?- Remarks: This function shall not participate in overload resolution unless the expression os << x is well-formed.

  2. Modify 30.7.4.5 [istream.rvalue] as indicated:

    template <class charT, class traits, class T>
      basic_istream<charT, traits>&
      operator>>(basic_istream<charT, traits>&& is, T& x);
    

    -1- Effects: is >> x

    -2- Returns: is

    -?- Remarks: This function shall not participate in overload resolution unless the expression is >> x is well-formed.