Can I get the Return Type of a Function From a Signature?

Question

So I have a ton of functions similar to these:

template <typename T>
bool Zero(const T, const T, const T);
template <typename T>
T One(const T, const T, const T, bool);
template <typename T>
T Three(const T, const T, const T, const T, const T, const T);

For each of these functions I have a wrapper which uses the return type of these functions so it looks something like this:

template <typename T>
decltype(Zero<decltype(declval<T>().x)>(decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()))) ZeroWrapper(const T);
template <typename T>
decltype(One<decltype(declval<T>().x)>(decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), bool())) OneWrapper(const T);
template <typename T>
decltype(Three<decltype(declval<T>().x)>(decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()), decltype(declval<decltype(declval<T>().x)>()))) ThreeWrapper(const T);

As you can see all those decltype(declval<T>().x)'s get disgustingly hard to read. Can I template a using or is there some standard function which will allow me to extract the return type from a function pointer without passing the argument types to decltype or result_of? So something like this:

template <typename T>
foo_t<Zero<decltype(declval<T>().x)>> ZeroWrapper(const T);
template <typename T>
foo_t<One<decltype(declval<T>().x)>> OneWrapper(const T);
template <typename T>
foo_t<Three<decltype(declval<T>().x)>> ThreeWrapper(const T);

Answer 1

In c++17 the function object has been endowed with a Deduction Guide which allows it to determine it's type from the argument passed to the constructor. So for example, given the function int foo() in c++11 we had to do:

function<int()> bar(foo);

In c++17 bar's function<int()> type will be derived if we simply:

function bar(foo);

Thus we can use the Deduction Guide to populate a temporary function with only the signature; thereby using function's result_type to find the result of your helper funcitons:

template <typename T>
typename decltype(function(Zero<decltype(declval<T>().x)>))::return_type ZeroWrapper(const T);
template <typename T>
typename decltype(function(One<decltype(declval<T>().x)>))::return_type OneWrapper(const T);
template <typename T>
typename decltype(function(Three<decltype(declval<T>().x)>))::return_type ThreeWrapper(const T);

Live Example

Answer 2

Can I template a using or is there some standard function which will allow me to extract the return type from a function pointer without passing the argument types to decltype or result_of?

Yes!

#include <tuple>
#include <functional>

template<class T>
struct callable_trait
{};

template<class R, class... Args>
struct callable_trait<std::function<R(Args...)>>
{
    using return_type    = R;
    using argument_types = std::tuple<Args...>;
};

template<auto callable>
using return_type = typename callable_trait<decltype(std::function{callable})>::return_type;

return_type<some_callable> is the type returned by some_callable when called with appropriate arguments. This uses a std::function in order to provide a specialization for each possible kind of callable (free function, function pointer, member function, functor object). This is explained in this StackOverflow answer.

---

In your case, you can use it like so:

template <typename T>
bool Zero(const T, const T, const T);
template <typename T>
T One(const T, const T, const T, bool);
template <typename T>
T Three(const T, const T, const T, const T, const T, const T);

template <typename T>
return_type<Zero<T>>  ZeroWrapper(const T);
template <typename T>
return_type<One<T>>   OneWrapper(const T);
template <typename T>
return_type<Three<T>> ThreeWrapper(const T);

Full demo