auto return type on a function

How can we deduct the return type of a function? With unknown input type (templated).

First we have to know that

decltype(x + y)

gives us the type of the resulting expression x + y.

Now if we don’t have anything cool from modern C++, how we can implement “auto” return?

template<typename T, typename U>
decltype(x + y) add(T x, U y) {
    return x + y;
}

But this doesn’t work, as at the point of decltype our compiler doesn’t know anything about x and y¹.

So… what cursed thing we can do? In C++11 we can declare the return parameter as following:

template<typename T, typename U>
auto add(T x, U y) -> decltype(x + y) {
    return x + y;
}

Now our compiler knows what is x and y!

And thankfully, since C++14 you can just do the following:

template<typename T, typename U>
auto add(T x, U y) {
    return x + y;
}

And the compiler will do the rest for you.