decltype with two parameters, = decltype(a,b), for function return type

Question

I met a decltype() with two parameters as return value type of template function:

template<class C, class F>
auto test(C c, F f) -> decltype((void)(c.*f)(), void()) { }

Does someone knows what is the second parameter, void()? Thank you very much.

Answer 1

In the expression (void)(c.*f)(), void():

• (void)(c.*f)() serves to check that f is a member function in c that can be called with no arguments; it doesn't matter what the member-function return type is anyway, but it's nominally cast to void

• if the above is valid, the comma operator discards it and considers the second part, such that the overall effect is per decltype(void()), which yields a void type

Praetorian comments below that the trailing , void() is redundant, as the leading part is cast to void (the C-style cast (void)) anyway... I suspect the , void() is intended as documentation, highlighting the enable_if-like conditional selection of a return type, it's a style choice whether to shorten that further to decltype((c.*f)(), void()).

Further details / example

This can be used for SFINAE, though enable_if is more self-documenting. Consider this code, and the comments in main() (CT stands for Compile Time):

#include <iostream>

template<class C, class F>
auto test(C c, F f) -> decltype((void)(c.*f)(), void())
    { std::cout << "member function\n"; }

template<class C>
void test(C c, int)
    { std::cout << "int\n"; }

struct X {
    int f() { return 42; }
    double g(int) { return 3.14; }
};

int main()
{
    X x;
    test(x, &X::f);  // ok - outputs "member function\n"
    // test(x, &X::g);  // CT error - g needs an argument
    test(x, 99);   // ok - outputs "int\n"
}

Output:

member function
int

You can see and run the code here.