Question

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What is “->” after function declaration?

I’ve just come across the following examples of C++ functions using the new auto keyword, and I was hoping someone could help me understand what the syntax means.

template <class T, class U>
auto add(T t, U u) -> decltype(t + u);

auto f = [](int a, int b) -> int {
   return a*b;
};

Specifically, I’m confused about the user of -> in the function signature and I would expect these to be written in the as

template <class T, class U>
auto add(T t, U u)
{
    decltype(t + u);
}

auto f = [](int a, int b){
    return a*b;
};

What’s the -> operator doing in there, and where can I learn more about this syntax?

Answer 1

What’s the -> operator doing in there?

That’s a trailing return type. Instead of:

int f();

you can equivalently write:

auto f() -> int;

If the return type depends on the function parameter types, then you need to use this form; the parameters aren’t available until after they’ve been declared:

decltype(t+u) add(T t, U u); // Error: uses `t` and `u` before they're declared
auto add(T t, U u) -> decltype(t + u); // OK

Also, if you want to specify the return type of a lambda, then you must use this form; although, as you point out, in many cases (including this one) you don’t need to specify that at all.