Test if a type is a callable, i.e. objects of that type can be called as functions. Also shows how to do type tests with decltype instead of sizeof.

iscallable.cpp

#include <type_traits>

template<typename F, typename ...Ts>
class is_callable
{
    template<typename G, typename ...Us>
    static
    decltype(
        typename std::add_pointer<decltype(std::declval<G>()(std::declval<Us>()...))>::type{},
        std::true_type{})
    test(int);

    template<typename, typename ...>
    static
    std::false_type test(...);

public:
    static constexpr bool value = decltype(test<F, Ts...>(0))::value;
};

void test() {}
void test1(int) {}

int main()
{
    static_assert(is_callable<decltype(test)>::value, "test() - OK");
    static_assert(! is_callable<decltype(test), int>::value, "test(int) - error");
    static_assert(! is_callable<decltype(test1)>::value, "test1() - error");
    static_assert(is_callable<decltype(test1), int>::value, "test1(int) - OK");
}