c++ - Return value of operator++

I have the following code that is broken. I can fix it by modifying certain line in code (see the comment). What is the cause of the problem?

#include 
using namespace std;

class Number{
public:

    int n;
    Number(int a):n(a){}

    //when I change the following to
    //friend Number& operator++(Number& source, int i)
    //then it compiles fine and correct value is printed
    friend Number operator++(Number& source, int i){
        ++source.n;
        return source;
    }

};

int main() {

    Number x(5);
    x++++; //error: no 'operator++(int)' declared for postfix '++' [-fpermissive]
    cout<
    return 0;
}

Answer

You attempt to apply the second ++ to the temporary object returned by the first invocation. However, the operand must be passed by reference, and you can't bind a temporary to a non-constant lvalue reference.

You probably don't want to "fix" this, since there's little reason to modify a temporary value like that. However, you should return a copy of the value before incrementing it, to give the expected post-increment behaviour.

The prefix operator should return a reference, which can be happily bound to another reference so that ++++x; should work as expected.