Call by reference in C++

Question

What is actually passed in call by reference to a function?

void foo(int &a,int &b)

when I write

foo(p,q)

what is actually passed to the function. Is it the address of p and q?

Answer 1

What's actually passed to the function is a reference. The named parameter b becomes a synonym for the argument object q.

How the compiler probably implements this that the caller places the address of q on the stack or in a register before calling, and the callee uses that value to effect all accesses to b. But it could be misleading to describe that as "actually passing" a pointer, because parameter passing is a concept at the level of the C++ language, and at that level it is not the same concept as passing a pointer. For instance, when you pass a pointer you can pass a null pointer, but when you pass a reference you cannot (validly). So it'd be wrong to say they're same thing.

That said, the person implementing the compiler might describe it as "actually passing a pointer", and you know what they mean. For comparison, if char variables occupy 4-byte stack slots in the calling convention, they might say that the compiler is "actually passing an int". So it depends what "actually" is supposed to mean.

Answer 2

The other answers mention the semantic difference between a reference and a pointer.

In practice, every single compiler I've ever worked with implements them the same way -- passing a reference is really passing a pointer at the assembly level. This isn't specified in any standard, it's just the case in practice everywhere.

The question's come up on SO before: What's the low-level difference between a pointer an a reference?