How to Retrieve Argument Types of Function Pointers in Variadic Template Classes?

Retrieving Argument Types of Function Pointers in Variadic Template Classes

Generic functors are useful classes that provide a convenient way to handle functions with varying argument lists. However, accessing the argument types of function pointers within these classes can be a challenge.

Solution

Consider the following functor class:

<code class="cpp">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...); }
};</code>

While you can easily access the args... in the operator() method using a recursive "peeling" function, obtaining the argument types in the constructor is more complex.

To address this, you can implement a function_traits class:

<code class="cpp">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;
    };
};</code>

With this class, you can determine the argument types, return type, and number of arguments of a function pointer.

Test Code

The following code demonstrates the functionality:

<code class="cpp">struct R {};
struct A {};
struct B {};

void main() {
    typedef std::function<R(A, B)> fun;

    cout << std::is_same<R, function_traits<fun>::result_type>::value << endl;
    cout << std::is_same<A, function_traits<fun>::arg<0>::type>::value << endl;
    cout << std::is_same<B, function_traits<fun>::arg<1>::type>::value << endl;
}</code>

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