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3007. allocate_shared should rebind allocator to cv-unqualified value_type for construction

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Glen Joseph Fernandes Opened: 2017-08-06 Last modified: 2021-02-25

Priority: 0

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Discussion:

The remarks for the allocate_shared family of functions specify that when constructing a (sub)object of type U, it uses a rebound copy of the allocator a passed to allocate_shared such that its value_type is U. However U can be a const or volatile qualified type, and [allocator.requirements] specify that the value_type must be cv-unqualified.

[ 2017-11-01 Moved to Tentatively Ready after 6 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4687.

  1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

    template<class T, ...>
shared_ptr<T> make_shared(args);
template<class T, class A, ...>
shared_ptr<T> allocate_shared(const A& a, args);

    […]

    -7- Remarks:

    1. […]

    2. (7.5) — When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, allocate_shared shall initialize this (sub)object via the expression

      1. (7.5.1) — allocator_traits<A2>::construct(a2, pv, v) or

      2. (7.5.2) — allocator_traits<A2>::construct(a2, pv, l...)

      respectively, where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t<U>.

    3. (7.6) — When a (sub)object of non-array type U is specified to have a default initial value, make_shared shall initialize this (sub)object via the expression ::new(pv) U(), where pv has type void* and points to storage suitable to hold an object of type U.

    4. (7.7) — When a (sub)object of non-array type U is specified to have a default initial value, allocate_shared shall initialize this (sub)object via the expression allocator_traits<A2>::construct(a2, pv), where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t<U>.

    5. […]