The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
#include <utility> // for std::invoke()
#include <functional> // for std::forward()
#include <type_traits> // for std::is_same<> and invoke_result<>
template<typename Callable, typename... Args>
decltype(auto) call(Callable&& op, Args&&... args)
{
if constexpr(std::is_same_v<std::invoke_result_t<Callable, Args...>,
void>) {
// return type is void:
std::invoke(std::forward<Callable>(op),
std::forward<Args>(args)...);
//...
return;
}
else {
// return type is not void:
decltype(auto) ret{std::invoke(std::forward<Callable>(op),
std::forward<Args>(args)...)};
//...
return ret;
}
}