c++11 get type of first (second, etc…) argument, s

Question

Let's suppose I have a template function which takes an argument, which is a function (it can be a std::function, or a lambda, or actual function pointer). A silly example that illustrates the problem:

template<typename F,typename A,typename B = typename std::result_of<F(A)>::type>
B blabla(F &&f)
{
    return f(A())/3;
}

I can reference the return type of f with std::result_of::typename, given I have the type of A, but I would like the compiler to deduce type A from F's first argument. (If I write

template<typename A,typename B>
B blabla(const std::function<B(A)> &f)
{
    return f(A())/3;
}

the compiler have problems deducing A and B (especially if it's not an std::function but a lambda), so this is not the right way to do it.)

Answer 1

This won't work for generic lambdas or arbitrary functors whose operator() is overloaded or is a template.

// primary template.
template<class T>
struct function_traits : function_traits<decltype(&T::operator())> {
};

// partial specialization for function type
template<class R, class... Args>
struct function_traits<R(Args...)> {
    using result_type = R;
    using argument_types = std::tuple<Args...>;
};

// partial specialization for function pointer
template<class R, class... Args>
struct function_traits<R (*)(Args...)> {
    using result_type = R;
    using argument_types = std::tuple<Args...>;
};

// partial specialization for std::function
template<class R, class... Args>
struct function_traits<std::function<R(Args...)>> {
    using result_type = R;
    using argument_types = std::tuple<Args...>;
};

// partial specialization for pointer-to-member-function (i.e., operator()'s)
template<class T, class R, class... Args>
struct function_traits<R (T::*)(Args...)> {
    using result_type = R;
    using argument_types = std::tuple<Args...>;
};

template<class T, class R, class... Args>
struct function_traits<R (T::*)(Args...) const> {
    using result_type = R;
    using argument_types = std::tuple<Args...>;
};

// additional cv-qualifier and ref-qualifier combinations omitted
// sprinkle with C-style variadics if desired

Then

template<class T>
using first_argument_type = typename std::tuple_element<0, typename function_traits<T>::argument_types>::type;

Replace 0 with the desired number as needed, or write a separate alias that also take an index. Demo.

Answer 2

#include <boost/type_traits.hpp>

boost::function_traits<decltype(function)>::arg2_type