2450. (greater|less|greater_equal|less_equal)<void> do not yield a total order for pointers

Section: 23.14.7 [comparisons] Status: C++17 Submitter: Joaquín M López Muñoz Opened: 2014-10-30 Last modified: 2017-07-30

Priority: 2

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Discussion:

less<void>::operator(t, u) (and the same applies to the rest of void specializations for standard comparison function objects) returns t < u even if t and u are pointers, which by 8.5.9 [expr.rel]/3 is undefined except if both pointers point to the same array or object. This might be regarded as a specification defect since the intention of N3421 is that less<> can substitute for less<T> in any case where the latter is applicable. less<void> can be rewritten in the following manner to cope with pointers:

template<> struct less<void>
{

  typedef unspecified is_transparent;

  template <class T, class U>
  struct pointer_overload : std::is_pointer<std::common_type_t<T, U>>
  {};

  template <
    class T, class U,
    typename std::enable_if<!pointer_overload<T, U>::value>::type* = nullptr
  >
  auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u))
  {
    return std::forward<T>(t) < std::forward<U>(u);
  } 

  template <
    class T, class U,
    typename std::enable_if<pointer_overload<T, U>::value>::type* = nullptr
  >
  auto operator()(T&& t, U&& u) const
    -> decltype(std::declval<std::less<std::common_type_t<T, U>>>()(std::forward<T>(t), std::forward<U>(u)))
  {
    std::less<std::common_type_t<T, U>> l;
    return l(std::forward<T>(t), std::forward<U>(u));
  }

};

This wording is relative to N4140.

  1. Change 23.14.7 [comparisons] p14 as indicated:

    -14- For templates greater, less, greater_equal, and less_equal, the specializations for any pointer type yield a total order, even if the built-in operators <, >, <=, >= do not. For template specializations greater<void>, less<void>, greater_equal<void>, and less_equal<void>, the call operator with arguments whose common type CT is a pointer yields the same value as the corresponding comparison function object class specialization for CT.

[2015-02, Cologne]

AM: Is there any way this will be resolved elsewhere? VV: No. AM: Then we should bite the bullet and deal with it here.

MC: These diamond operators are already ugly. Making them more ugly isn't a big problem.

JY found some issue with types that are convertible, and will reword.

Jeffrey suggests improved wording.

[2015-05, Lenexa]

STL: when diamond functions designed, this was on purpose
STL: this does go against the original design
STL: library is smarter and can give a total order
MC: given that the original design rejected this, give back to LEWG
STL: original proposal did not talk about total order
STL: don't feel strongly about changing the design
STL: no objections to taking this issue with some wording changes if people want it
MC: not happy with wording, comparing pointers — what does that mean?
STL: needs careful attention to wording
STL: want to guarantee that nullptr participates in total ordering
STL: all hooks into composite pointer type
MC: move from new to open with better wording
STL: to check updates to issue after Lenexa

[2015-06, Telecon]

MC: STL on the hook to update. He's shipping something today so not here.
MC: also add link to N4229

[2015-10, Kona Saturday afternoon]

STL was on the hook for wording, but STL: I don't care. The architecture on which this is an issue does not exist.
STL: We will also need to incorporate nullptr. TK: I think that's implied, since the wording is in terms of the resulting operation, not the deduced types.
STL: Seems legit. MC: I guess I'm OK with this. TK: I'm weakly in favour, so that we can get people to use transparent comparators without worrying.
STL: There's no change to implementations.
Move to Tentatively ready.

Proposed resolution:

This wording is relative to N4296.

  1. Change 23.14.7 [comparisons] p14 as indicated:

    -14- For templates greater, less, greater_equal, and less_equal, the specializations for any pointer type yield a total order, even if the built-in operators <, >, <=, >= do not. For template specializations greater<void>, less<void>, greater_equal<void>, and less_equal<void>, if the call operator calls a built-in operator comparing pointers, the call operator yields a total order.