Howto check a type for the existence of parameterless operator()

Question

I\'m trying to check whether a functor is compatible with a given set of parametertypes and a given return type (that is, the given parametertypes can be implicitely converted t

Answer 1

When I want to test if a given expression is valid for a type, I use a structure similar to this one:

template 
struct is_callable_without_parameters {
private:
    template 
    static decltype(std::declval()(), void(), 0) test(int);
    template 
    static void test(...);
public:
    enum { value = !std::is_void(0))>::value };
};