348. Minor issue with std::pair operator<

Section: 23.4 [pairs] Status: Dup Submitter: Andy Sawyer Opened: 2001-10-23 Last modified: 2016-02-10

Priority: Not Prioritized

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Duplicate of: 532

Discussion:

The current wording of 20.2.2 [lib.pairs] p6 precludes the use of operator< on any pair type which contains a pointer.

Proposed resolution:

In 23.4 [pairs] paragraph 6, replace:

    Returns: x.first < y.first || (!(y.first < x.first) && x.second <
        y.second).

With:

    Returns: std::less<T1>()( x.first, y.first ) ||
             (!std::less<T1>()( y.first, x.first) && 
             std::less<T2>()( x.second, y.second ) )

Rationale:

This is an instance of a much more general problem. If we want operator< to translate to std::less for pairs of pointers, where do we draw the line? The same issue applies to individual pointers, smart pointer wrappers, std::vector<T*>, and so on.

Andy Koenig suggests that the real issue here is that we aren't distinguishing adequately between two different orderings, a "useful ordering" and a "canonical ordering" that's used just because we sometimes need some ordering without caring much which ordering it is. Another example of the later is typeinfo's before.