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1297.unique_ptr's relational operator functions should induce a total order
Section: 20.3.1.6 [unique.ptr.special] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The comparison functions of unique_ptr currently directly delegate to the underlying comparison functions of unique_ptr<T, D>::pointer. This is disadvantageous, because this would not guarantee to induce a total ordering for native pointers and it is hard to define a total order for mixed types anyway.
The currently suggested resolution for shared_ptr comparison as of 1262(i) uses a normalization strategy: They perform the comparison on the composite pointer type (7.6.9 [expr.rel]). This is not exactly possible for unique_ptr in the presence of user-defined pointer-like types but the existing definition of std::duration comparison as of 30.5.7 [time.duration.comparisons] via common_type of both argument types demonstrates a solution of this problem. The approach can be seen as the general way to define a composite pointer type and this is the approach which is used for here suggested wording change.
For consistency reasons I would have preferred the same normalization strategy for == and !=, but Howard convinced me not to do so (now).
[ 2010-11-03 Daniel comments and adjustes the currently proposed wording changes: ]
Issue 1401(i) is remotely related. Bullet A of its proposed resolution provides an alternative solution for issue discussed here and addresses NB comment GB-99. Additionally I updated the below suggested wording in regard to the following: It is an unncessary requirement that the below defined effective composite pointer-like type CT satisfies the LessThanComparable requirements. All what is needed is, that the function object type less<CT> induces a strict weak ordering on the pointer values.
[2011-03-24 Madrid meeting]
Proposed resolution:
Change 20.3.1.6 [unique.ptr.special]/4-7 as indicated: [The implicit requirements and remarks imposed on the last three operators are the same as for the first one due to the normative "equivalent to" usage within a Requires element, see 16.3.2.4 [structure.specifications]/4. The effects of this change are that all real pointers wrapped in a unique_ptr will order like shared_ptr does.]
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);? Requires: Let
CTbecommon_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type. Then the specializationless<CT>shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.4 Returns:
less<CT>()(x.get(), y.get()) x.get() < y.get().? Remarks: If
unique_ptr<T1, D1>::pointeris not implicitly convertible toCTorunique_ptr<T2, D2>::pointeris not implicitly convertible toCT, the program is ill-formed.template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);5 Effects: Equivalent to
return !(y < x)Returns:x.get() <= y.get().template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);6 Effects: Equivalent to
return (y < x)Returns:x.get() > y.get().template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);7 Effects: Equivalent to
return !(x < y)Returns:x.get() >= y.get().
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