Assign a C++ out reference to something that was destroyed?

Question

Hello,

So I'm looking through some code, and I see this:

class whatever 
{
public:
    void SomeFunc(SomeClass& outVal)
    {
        outVal = m_q.front();
        m_q.pop();
    }

private:
    std::queue<SomeClass> m_q;
};

This doesn't seem like outVal would be a valid reference any more... However, it appears to work.

I've seen this in other code before too, is this valid? Thanks

Answer 1

What I wrote in my previous reply was complete nonsense. (Whoever upvoted my original response, please take it back :)

In this example the reference is not bound to a dying object, but rather the value of the object in the front is copied to another object (referred to by the reference). The copy continues to exist independently of the queue an the fact that the front of the queue is destroyed has no adverse effects on the copy.

Please, refer to Crashworks reply for a great explanation of what is happening here.

Answer 2

Remember that references are not like pointers: they cannot be rebound after their creation. That means that if I do

int a;
int b;
int &c = a;

Then throughout that scope, an assignment to c will actually mean an assignment to a. So,

int a = 2;
{
   int b = 3;
   int &c = a;
   c = b;
   b = -5;
}
printf("%d",a); // prints "3".

So, in this case, the reference is not being pointed at a deleted object. Rather, the return value of m_q.front() is copied into whatever outVal references, via the assignment operator.

Answer 3

It's valid. You are not reseating the outVal reference to refer to m_q.front(), that is not something supported by references, instead you are assigning m_q.front() to the variable (actually lvalue) that outVal refers to.

  SomeClass c;
  someWhatever.SomeFunc(c);

Can be thought of as behaving like:

  SomeClass c;
  c = someWhatever.m_q.front();
  someWhater.m_q.pop();