20 General utilities library [utilities]

20.9 Function objects [function.objects]

20.9.2 Requirements [func.require]

1

Define INVOKE(f, t1, t2, ..., tN) as follows:

  • (1.1)

    (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T;

  • (1.2)

    ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is not one of the types described in the previous item;

  • (1.3)

    t1.*f when N == 1 and f is a pointer to member data of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T;

  • (1.4)

    (*t1).*f when N == 1 and f is a pointer to member data of a class T and t1 is not one of the types described in the previous item;

  • (1.5)

    f(t1, t2, ..., tN) in all other cases.

2

Define INVOKE(f, t1, t2, ..., tN, R) as INVOKE(f, t1, t2, ..., tN) implicitly converted to R.

3

If a call wrapper ([func.def]) has a weak result type the type of its member type result_type is based on the type T of the wrapper's target object ([func.def]):

  • (3.1)

    if T is a pointer to function type, result_type shall be a synonym for the return type of T;

  • (3.2)

    if T is a pointer to member function, result_type shall be a synonym for the return type of T;

  • (3.3)

    if T is a class type with a member type result_type, then result_type shall be a synonym for T::result_type;

  • (3.4)

    otherwise result_type shall not be defined.

4

Every call wrapper ([func.def]) shall be MoveConstructible. A simple call wrapper is a call wrapper that is CopyConstructible and CopyAssignable and whose copy constructor, move constructor, and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. [ Note: In a typical implementation forwarding call wrappers have an overloaded function call operator of the form

template<class... UnBoundArgs>
R operator()(UnBoundArgs&&... unbound_args) cv-qual;

 — end note ]