Difference between revisions of "cpp/named req/Callable"

Revision as of 06:23, 19 September 2020

A Callable type is a type for which the INVOKE operation (used by, e.g., std::function, std::bind, and std::thread::thread) is applicable. This operation may be performed explicitly using the library function std::invoke.(since C++17)

Requirements

The type T satisfies Callable if

Given

• f, an object of type T
• ArgTypes, suitable list of argument types
• R, suitable return type

The following expressions must be valid:

where INVOKE<R>(f, t1, t2, ..., tN) is defined as static_cast<void>(INVOKE(f, t1, t2, ..., tN)) if R is possibly cv-qualified void, otherwise INVOKE(f, t1, t2, ..., tN), implicitly converted to R

where INVOKE(f, t1, t2, ..., tN) is defined as follows:

• if f is a pointer to member function of class T:

• If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.*f)(t2, ..., tN)
• otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.get().*f)(t2, ..., tN)(since C++17)
• otherwise, if t1 does not satisfy the previous items, then INVOKE(f, t1, t2, ..., tN) is equivalent to ((*t1).*f)(t2, ..., tN).

• otherwise, if N == 1 and f is a pointer to data member of class T:

• If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1) is equivalent to t1.*f
• otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1) is equivalent to t1.get().*f(since C++17)
• otherwise, if t1 does not satisfy the previous items, then INVOKE(f, t1) is equivalent to (*t1).*f

• otherwise, INVOKE(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, f is a FunctionObject)

Notes

For pointers to member functions and pointers to data members, t1 may be a regular pointer or an object of class type that overloads operator*, such as std::unique_ptr or std::shared_ptr.

Pointers to data members are Callable, even though no function calls take place.

Standard library

In addition, the following standard library facilities accept any Callable type (not just FunctionObject)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also