std::invoke

 

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Defined in header <functional>
template< class F, class... ArgTypes> std::result_of_t<F&&(ArgTypes&&...)> invoke(F&& f, ArgTypes&&... args);		(since C++17)

Invoke the Callable object f with the parameters args. As by INVOKE(std::forward<F>(f), std::forward<Args>(args)...).

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

• if f is a pointer to member function of class T and t1 is an object of class T or reference to an object of class T or derived from T, then INVOKE(f, t1, t2, ..., tn) is equivalent to (t1.*f)(t2, ..., tn)
• otherwise, if f is a pointer to member function and t1 is not one of the types described above, 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 and t1 is an object of class T or reference to an object of class T or derived from T, then INVOKE(f, t1) is equivalent to t1.*f
• otherwise, if N == 1 and f is a pointer to data member of class T and t1 is not one of the types described above, 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)

Parameters

f	-	Callable object to be invoked
args	-	arguments to pass to f

Possible implementation

namespace detail {
template <class F, class... Args>
inline auto INVOKE(F&& f, Args&&... args) ->
    decltype(std::forward<F>(f)(std::forward<Args>(args)...)) {
      return std::forward<F>(f)(std::forward<Args>(args)...);
}
 
template <class Base, class T, class Derived>
inline auto INVOKE(T Base::*pmd, Derived&& ref) ->
    decltype(std::forward<Derived>(ref).*pmd) {
      return std::forward<Derived>(ref).*pmd;
}
 
template <class PMD, class Pointer>
inline auto INVOKE(PMD pmd, Pointer&& ptr) ->
    decltype((*std::forward<Pointer>(ptr)).*pmd) {
      return (*std::forward<Pointer>(ptr)).*pmd;
}
 
template <class Base, class T, class Derived, class... Args>
inline auto INVOKE(T Base::*pmf, Derived&& ref, Args&&... args) ->
    decltype((std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...)) {
      return (std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...);
}
 
template <class PMF, class Pointer, class... Args>
inline auto INVOKE(PMF pmf, Pointer&& ptr, Args&&... args) ->
    decltype(((*std::forward<Pointer>(ptr)).*pmf)(std::forward<Args>(args)...)) {
      return ((*std::forward<Pointer>(ptr)).*pmf)(std::forward<Args>(args)...);
}
} // namespace detail
 
template< class F, class... ArgTypes>
decltype(auto) invoke(F&& f, ArgTypes&&... args) {
    return detail::INVOKE(std::forward<F>(f), std::forward<ArgTypes>(args)...);
}

Example

Implement the basic functionality of std::mem_fn.

Run this code

#include <functional>
template< class PM >
class mem_fn_t {
    PM p;
public:
    mem_fn_t(PM p):p(p){}
    template<class... Args>
    decltype(auto) operator()(Args&&... args) {
        return std::invoke(p, std::forward<Args>(args)...);
    }
};
 
template< class R, class T >
auto mem_fn(R T::* pm){
    mem_fn_t<R T::*> t {pm};
    return t;
}

See also

mem_fn (C++11)	creates a function object out of a pointer to a member (function template)
result_of (C++11)	deduces the return type of a function call expression (class template)