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2103.std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment

Section: 20.2.10[default.allocator]Status:C++14Submitter: Ai AzumaOpened: 2011-11-08Last modified: 2016-01-28

Priority:Not Prioritized

View otheractive issues in [default.allocator].

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Discussion:

"std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment::value"is specified as "false", according to (20.2.10[default.allocator]) and (20.2.9.2[allocator.traits.types]).However, according to (23.2.2[container.requirements.general]), this specification leads to the unneeded requirements(MoveInsertable andMoveAssignable of the value type) on the move assignment operator of containerswith the default allocator.

Proposed resolution:

Either of the following two changes;

1. adding the nested typedef like"typedef std::true_type propagate_on_container_move_assignment;"in the definition ofstd::allocator class template,
2. adding the explicit partial specialization of"std::allocator_traits" class template for "std::allocator"class template, in which "propagate_on_container_move_assignment"nested typedef is specified as "std::true_type".

Pablo prefers the first resolution.

[2011-12-02: Pablo comments]

This issue has potentially some overlap with2108⁽ⁱ⁾. Should the traitalways_compare_equalbeen added, this issue's resolution should be improved based on that.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 20.2.10[default.allocator], the class templateallocator synopsis as indicated:

namespace std {  template <class T> class allocator;// specialize forvoid:  template <> class allocator<void> {  public:    typedef void* pointer;    typedef const void* const_pointer;// reference-to-void members are impossible.    typedef void value_type;    template <class U> struct rebind { typedef allocator<U> other; };  };  template <class T> class allocator {  public:    typedef size_t size_type;    typedef ptrdiff_t difference_type;    typedef T* pointer;    typedef const T* const_pointer;    typedef T& reference;    typedef const T& const_reference;    typedef T value_type;    template <class U> struct rebind { typedef allocator<U> other; };typedef true_type propagate_on_container_move_assignment;    […]  };}