std::result_of parameter type

Question

I've been having this question for some time. Suppose we have a contrived function:

template<typename F>
std::result_of_t<std::decay_t<F>(???)> transform(F&& f)
{
    static const int num = 42;
    return std::forward<F>(f)(num);
}

The thing I'm not sure of is whether I should use int or const int& for the ??? part. Similarly, for this function:

template<typename F>
std::result_of_t<std::decay_t<F>(???)> transform(F&& f)
{
    ExpensiveType foo;
    return std::forward<F>(f)(std::move(foo));
}

Should I use ExpensiveType or ExpensiveType&& for the ??? part?

Answer 1

Use auto!

C++14:

template < typename F >
auto transform(F&& f)
{
    constexpr auto num = 42;
    return std::forward<F>(f)(num);
}

C++11:

template < typename F >
auto transform(F&& f) -> decltype(std::forward<F>(f)(42))
{
    // ... same body
}