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4255. move_only_function constructor should recognize empty copyable_functions

Section: 22.10.17.4.3 [func.wrap.move.ctor] Status: Tentatively Ready Submitter: Tomasz Kamiński Opened: 2025-05-12 Last modified: 2025-10-22

Priority: Not Prioritized

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Discussion:

The standard currently requires that constructing move_only_function from empty copyable_function, creates an non-empty move_only_function, that contains an empty copyable_function as the target. For example:

std::copyable_function<int(int)> ce;
std::move_only_function<int(int)> me(ce);

We require that invoking me(1) is undefined behavior (as it leads to call to the ce(1)), however it cannot be detected in the user code, as me != nullptr is true.

We should require the move_only_function(F&& f) constructor to create an empty object, if f is an instantiation of copyable_function and f == nullptr is true, i.e. f does not contain target object.

This simplifies implementing avoidance of double wrapping per 22.10.17.1 [func.wrap.general] p2, as transferring the target produces an empty functor.

The copyable_function cannot be constructed from move_only_function, as it requires functor to be copyable. Invoking an empty std::function has well defined behavior (throws bad_function_call), and wrapping such object into other functors should reproduce that behavior.

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.10.17.4.3 [func.wrap.move.ctor] as indicated:

   template<class F> move_only_function(F&& f);
   

   […]

   -8- Postconditions:: *this has no target object if any of the following hold:

   1. (8.1) — f is a null function pointer value, or

   2. (8.2) — f is a null member pointer value, or

   3. (8.2) — remove_cvref_t<F> is a specialization of the move_only_function or copyable_function class template, and f has no target object.