问题
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);
回答1:
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
decltypeorresult_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
回答2:
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
来源：https://stackoverflow.com/questions/54984669/can-i-get-the-return-type-of-a-function-from-a-signature