4032. Possibly invalid types in the constraints of constructors of std::shared_ptr

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: New Submitter: Jiang An Opened: 2023-12-25 Last modified: 2024-01-13 14:20:50 UTC

Priority: Not Prioritized

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Discussion:

Currently, 20.3.2.2.2 [util.smartptr.shared.const]/3 and /9.1 says Y(*)[N] and Y(*)[], however, they may be invalid types when Y is an array type of unknown bound or a function type. Presumably, the constraints should be satisfied only when the mentioned Y(*)[N] or Y(*)[] is valid.

Proposed resolution:

This wording is relative to N4971.

  1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

    template<class Y> explicit shared_ptr(Y* p);

    -3- Constraints: When T is an array type, the expression delete[] p is well-formed and either T is U[N] and Y(*)[N] is a valid type and convertible to T*, or T is U[] and Y(*)[] is a valid type and convertible to T*. When T is not an array type, the expression delete p is well-formed and Y* is convertible to T*.

    […] 

    template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

    -9- Constraints: is_move_constructible_v<D> is true, and d(p) is a well-formed expression. For the first two overloads:

    1. (9.1) — If T is an array type, then either T is U[N] and Y(*)[N] is a valid type and convertible to T*, or T is U[] and Y(*)[] is a valid type and convertible to T*.

    2. (9.2) — If T is not an array type, then Y* is convertible to T*.

    […]