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4110. shared_ptr(nullptr_t, Deleter) is overconstrained, breaking some sensible deleters

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: New Submitter: Louis Dionne Opened: 2024-06-11 Last modified: 2025-10-22

Priority: 3

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Discussion:

The following code doesn't compile on conforming implementations:

    #include <memory>

    void f() {
        std::shared_ptr<int>(new int, [](auto pointer) { delete pointer; });
    }

This is caused by the constraint on shared_ptr(nullptr_t p, D d); being that d(p) is valid (20.3.2.2.2 [util.smartptr.shared.const] p9), which leads to a hard error inside the lambda since it is called with a nullptr_t. This seems unintended.

See LLVM issue 93071 comment for additional context.

[2025-10-22; Reflector poll.]

Set priority to 3 after reflector poll.

"I don't agree with the proposed resolution. As a general principle, shared_ptr<T>(p, d) always calls d(p) and never d(static_cast<T*>(p)).
If we really want to make this work, which is not unreasonable, even though the fix on the user side is trivial, we should make the nullptr_t constructor templated on same_as<nullptr_t> (or convertible_to<nullptr_t>?)."

"That would break passing NULL, only nullptr would work. It can be made to work by checking that d(p) is well-formed in a function parameter with a default argument, instead of as a template parameter:"

struct shared_ptr {
  template<class Y, class D>
  shared_ptr(Y* p, D d, std::void_t<decltype(d(p))>* = nullptr) {}
  template<class D>
  shared_ptr(std::nullptr_t p, D d, std::void_t<decltype(d(p))>* = nullptr) {}
};
shared_ptr s(new int, [](auto p) {delete p;});

"Ugh. We don't have to use it everywhere, only these two specific constructors."

Proposed resolution:

This wording is relative to N4981.

1. In 20.3.2.2.2 [util.smartptr.shared.const]/9, specify that shared_ptr(nullptr_t p, D d); checks whether d(static_cast<T*>(nullptr)) is well-formed. This requires expressing the the constraints for the Y* constructors and the nullptr_t constructors separately, which is mostly editorial:

   
   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 convertible to T*, or T is U[] and Y(*)[] is convertible to T*.
   2. (9.2) If T is not an array type, then Y* is convertible to T*.

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

   -?- Constraints: is_move_constructible_v<D> is true, and d(static_cast<T*>(p)) is a well-formed expression.