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1231. weak_ptr comparisons incompletely resolved

Section: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The n2637 paper suggested several updates of the ordering semantics of shared_ptr and weak_ptr, among those the explicit comparison operators of weak_ptr were removed/deleted, instead a corresponding functor owner_less was added. The problem is that n2637 did not clearly enough specify, how the previous wording parts describing the comparison semantics of weak_ptr should be removed.

[ 2009-11-06 Howard adds: ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

1. Change 20.3.2.3 [util.smartptr.weak]/2 as described, the intention is to fix the now no longer valid requirement that weak_ptr is LessComparable [Note the deleted comma]:

   Specializations of weak_ptr shall be CopyConstructible, and CopyAssignable, and LessThanComparable, allowing their use in standard containers.

2. In 20.3.2.3.6 [util.smartptr.weak.obs] remove the paragraphs 9-11 including prototype:

   template<class T, class U> bool operator<(const weak_ptr<T>& a, const weak_ptr<U>& b);

   Returns: an unspecified value such that

   • operator< is a strict weak ordering as described in 25.4;
   • under the equivalence relation defined by operator<, !(a < b) && !(b < a), two weak_ptr instances are equivalent if and only if they share ownership or are both empty.

   Throws: nothing.

   [Note: Allows weak_ptr objects to be used as keys in associative containers. — end note]