This is a follow up of this problem: Generic functor for functions with any argument list
I have this functor class (full code see link above):
template<typename... ARGS>
class Foo
{
std::function<void(ARGS...)> m_f;
public:
Foo( std::function<void(ARGS...)> f ) : m_f(f) {}
void operator()(ARGS... args) const { m_f(args...); }
};
In operator() I can access the args... easily with a recursive "peeling" function as described here http://www2.research.att.com/~bs/C++0xFAQ.html#variadic-templates
My problem is: I want to access the types of the arguments of f, i.e. ARGS..., in the constructor. Obviously I can't access values because there are none so far, but the argument type list is somehow burried in f, isn't it?
You can write function_traits
class as shown below, to discover the argument types, return type, and number of arguments:
template<typename T>
struct function_traits;
template<typename R, typename ...Args>
struct function_traits<std::function<R(Args...)>>
{
static const size_t nargs = sizeof...(Args);
typedef R result_type;
template <size_t i>
struct arg
{
typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
};
};
Test code:
struct R{};
struct A{};
struct B{};
int main()
{
typedef std::function<R(A,B)> fun;
std::cout << std::is_same<R, function_traits<fun>::result_type>::value << std::endl;
std::cout << std::is_same<A, function_traits<fun>::arg<0>::type>::value << std::endl;
std::cout << std::is_same<B, function_traits<fun>::arg<1>::type>::value << std::endl;
}
Demo : http://ideone.com/YeN29