The following code example is taken from the book
C++17 - The Complete Guide
by Nicolai M. Josuttis,
Leanpub, 2017
The code is licensed under a
Creative Commons Attribution 4.0 International License.
#include <functional> // for std::forward()
#include <type_traits> // for std::is_same<> and std::invoke_result<>
template<typename Callable, typename... Args>
decltype(auto) call(Callable op, Args&&... args)
{
if constexpr(std::is_void_v<std::invoke_result_t<Callable, Args...>>) {
// return type is void:
op(std::forward<Args>(args)...);
//... // do something before we return
return;
}
else {
// return type is not void:
decltype(auto) ret{op(std::forward<Args>(args)...)};
//... // do something (with ret) before we return
return ret;
}
}