In C++, what does & mean after a function's return type?

Question

In a C++ function like this:

int& getNumber();

what does the & mean? Is it different from:

int getNumber();

Answer 1

It's different.

int g_test = 0;

int& getNumberReference()
{
     return g_test;
}

int getNumberValue()
{
     return g_test;
}

int main()
{
    int& n = getNumberReference();
    int m = getNumberValue();
    n = 10;
    cout << g_test << endl; // prints 10
    g_test = 0;
    m = 10;
    cout << g_test << endl; // prints 0
    return 0;
}

the getNumberReference() returns a reference, under the hood it's like a pointer that points to an integer variable. Any change applyed to the reference applies to the returned variable.

The getNumberReference() is also a left-value, therefore it can be used like this:

getNumberReference() = 10;