expression_SFINAE_void_t.cpp

// Expression SFINAE via void_t
// checks wether a class has a specific member function

#include <iostream>
#include <type_traits>
#include <utility>

struct Foo {
    void ham() const;
    void ham();
};

struct Bar {
    void ham() {}
};

template <typename...>
using void_t = void;

template <typename C, typename = void>
struct has_const_ham : std::false_type {};

template <typename C>
struct has_const_ham<C, void_t<decltype(std::declval<const C&>().ham())>>
    : std::true_type {};

int main() {
    std::cout << std::boolalpha;
    std::cout << has_const_ham<Foo>::value << std::endl;
    std::cout << has_const_ham<Bar>::value << std::endl;
}