How can I access argument types from a function pointer within a variadic template class in C ?

Accessing Argument Types in a Variadic Template Class for Function Pointers

In an effort to create a generic functor for functions with any argument list, a developer seeks to extract the argument types of a function pointer within the class constructor.

Consider the following functor class:

<code class="cpp">template<typename... ARGS>
class Foo {
private:
    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>

To access the argument types within the constructor, the developer employs a technique of "peeling" the argument list recursively, as outlined by Stroustrup in his C 11 FAQ. However, the argument types are not readily accessible from the function pointer f.

By employing a function_traits class, it becomes possible to discover the argument types, return type, and the number of arguments associated with a function pointer.

<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>

Leveraging this function_traits class, the developer can now retrieve the argument types within the functor class constructor:

<code class="cpp">template<typename... ARGS>
class Foo {
private:
    std::function<void(ARGS...)> m_f;
    std::tuple<typename function_traits<decltype(m_f)>::arg<0>::type...> m_arg_types;
public:
    Foo(std::function<void(ARGS...)> f) : m_f(f), m_arg_types(std::make_tuple(typename function_traits<decltype(m_f)>::arg<0>::type()...)) {}
    void operator()(ARGS... args) const { m_f(args...); }
};</code>

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