initialize reference in initialization list

Question

I was told the reference variable must be initialized in the initialization list, but why this is wrong?

   class Foo
    {
    public: 
        Foo():x(0) {      
         y = 1;
        }
    private:
        int& x;
        int y;
    };

Because 0 is a temporary object? If so, what kind of object can reference be bound? The object which can take an address?

Thanks!

Answer 1

0 is not an lvalue, it's an rvalue. You cannot modify it, but you're trying to bind to a reference where it could be modified.

If you make your reference const, it will work as expected. Consider this:

int& x = 0;
x = 1; // wtf :(

This obviously is a no-go. But const&'s can be bound to temporaries (rvalues):

const int& x = 0;
x = 1; // protected :) [won't compile]

Note that the life-time of the temporary is ended at the completion of the constructor. If you make static-storage for your constant, you'll be safe:

class Foo
{
public:
    static const int Zero = 0;

    Foo() : x(Zero) // Zero has storage
    {
        y = 1;
    }
private:
    const int& x;
    int y;
};