You can leverage std::function
here which will give you an alias for the functions return type. This does require C++17 support, since it relies on class template argument deduction, but it will work with any callable type:
using ReturnTypeOfFoo = decltype(std::function{foo})::result_type;
We can make this a little more generic like
template<typename Callable>
using return_type_of_t =
typename decltype(std::function{std::declval<Callable>()})::result_type;
which then lets you use it like
int foo(int a, int b, int c, int d) {
return 1;
}
auto bar = [](){ return 1; };
struct baz_
{
double operator()(){ return 0; }
} baz;
using ReturnTypeOfFoo = return_type_of_t<decltype(foo)>;
using ReturnTypeOfBar = return_type_of_t<decltype(bar)>;
using ReturnTypeOfBaz = return_type_of_t<decltype(baz)>;
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