Consider the code:
#include <type_traits>
#include <iostream>
struct test1 {
void Invoke() {};
};
struct test2 {
template<typename> void Invoke() {};
};
enum class InvokableKind {
NOT_INVOKABLE,
INVOKABLE_FUNCTION,
INVOKABLE_FUNCTION_TEMPLATE
};
template<typename Functor, class Enable = void>
struct get_invokable_kind {
const static InvokableKind value = InvokableKind::NOT_INVOKABLE;
};
template<typename Functor>
struct get_invokable_kind<
Functor,
decltype(Functor().Invoke())
>
{
const static InvokableKind value = InvokableKind::INVOKABLE_FUNCTION;
};
template<typename Functor>
struct get_invokable_kind<
Functor,
decltype(Functor().Invoke<void>())
>
{
const static InvokableKind value = InvokableKind::INVOKABLE_FUNCTION_TEMPLATE;
};
int main() {
using namespace std;
cout << (get_invokable_kind<test1>::value == InvokableKind::INVOKABLE_FUNCTION) << endl;
cout << (get_invokable_kind<test2>::value == InvokableKind::INVOKABLE_FUNCTION_TEMPLATE) << endl;
}
What I'm trying to do is to create a metafunction for testing a specific definition of "invokability". And now I'm stuck on this compilation error on GCC 4.5.3:
prog.cpp:37:3: error: template argument 2 is invalid
What does it mean? Why can I specialize on decltype(Functor().Invoke()), but can not on decltype(Functor().Invoke<void>())?
