How to Retrieve Function Argument Types in a Variadic Template Class?

Retrieving Function Argument Types in a Variadic Template Class

The specified functor class, Foo, provides a means to invoke functions with arbitrary argument lists. However, determining the argument types within the constructor presents a challenge.

To overcome this, you can utilize the function_traits class as follows:

<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>

Within the Foo class, you can employ function_traits to obtain the argument type information:

<code class="cpp">class Foo
{
    std::function<void(ARGS...)> m_f;
public:
    using result_type = typename function_traits<decltype(m_f)>::result_type;
    using arg0_type = typename function_traits<decltype(m_f)>::arg<0>::type;
    // ... Additional argument types as needed
};</code>

This approach allows you to leverage the type system to access and manipulate the function argument types in a generic and type-safe manner.

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