what is -> in c++ in a function declaration [duplicate]

Question

In the Wikipedia article on decltype http://en.wikipedia.org/wiki/Decltype I came across this example:

int& foo(int& i);
float foo(float& f);

template <class T> auto transparent_forwarder(T& t) −> decltype(foo(t)) {
  return foo(t);
}

Though I understood the motive behind this function, I didnot understand the syntax it uses and specifically the -> in the declaration. What is -> and how is it interpreted?

EDIT 1

Based on the above: What is wrong here?

template <typename T1, typename T2>
auto sum(T1 v1, T2 v2) -> decltype(v1 + v2) {
    return v1 + v2;
}

The error is:

error: expected type-specifier before ‘decltype’
error: expected initializer before ‘decltype

Answer to EDIT 1:

OOPS! I forgot to use -std=c++11 compiler option in g++.

EDIT 2

Based on the below answer. I have a related question: Look at the declaration below:

template <typename T1, typename T2>
decltype(*(T1 *) nullptr + *(T2 *) nullptr) sum2(T1 v1, T2 v2);

It is using decltype without the need for -> in the function declaration. So Why do we need ->

Answer 1

This uses trailing return type notation. This:

auto f() -> T { ... }

Is equivalent to this:

T f() { ... }

The advantage is that with a trailing return type notation you can express the type of a function based on expressions that involve the arguments, which is not possible with the classical notation. For instance, this would be illegal:

    template <class T>
    decltype(foo(t)) transparent_forwarder(T& t) {
//  ^^^^^^^^^^^^^^^^
//  Error! "t" is not in scope here...

        return foo(t);
    }

Concerning your edit:

Based on the above: What is wrong here?

template <typename T1, typename T2>
auto sum(T1 v1, T2 v2) -> decltype(v1 + v2) {
    return v1 + v2;
}

Nothing.

Concerning your second edit:

[...] It is using decltype without the need for -> in the function declaration. So Why do we need ->

In this case you don't need it. However, the notation that does use trailing return type is much clearer, so one may prefer it to make the code easier to understand.