What is a reference?, C++ FAQ

C++ FAQ / Section 8 / FAQ 8.1

Section 8:

8.1	What is a reference?
8.2	What happens if you assign to a reference?
8.3	What happens if you return a reference?
8.4	What does `object.method1().method2()` mean?
8.5	How can you reseat a reference to make it refer to a different object?
8.6	When should I use references, and when should I use pointers?
8.7	What does it mean that a reference must refer to an object, not a dereferenced `NULL` pointer?
8.8	What is a handle to an object? Is it a pointer? Is it a reference? Is it a pointer-to-a-pointer? What is it?

[8.1] What is a reference?

An alias (an alternate name) for an object.

References are frequently used for pass-by-reference:

void swap(int& i, int& j)
{
  int tmp = i;
  i = j;
  j = tmp;
}

int main()
{
  int x, y;
  ...
  swap(x,y);
  ...
}

Here i and j are aliases for main's x and y respectively. In other words, i is x — not a pointer to x, nor a copy of x, but x itself. Anything you do to i gets done to x, and vice versa.

OK. That's how you should think of references as a programmer. Now, at the risk of confusing you by giving you a different perspective, here's how references are implemented. Underneath it all, a reference i to object x is typically the machine address of the object x. But when the programmer says i++, the compiler generates code that increments x. In particular, the address bits that the compiler uses to find x are not changed. A C programmer will think of this as if you used the C style pass-by-pointer, with the syntactic variant of (1) moving the & from the caller into the callee, and (2) eliminating the *s. In other words, a C programmer will think of i as a macro for (*p), where p is a pointer to x (e.g., the compiler automatically dereferences the underlying pointer; i++ is changed to (*p)++; i = 7 is automatically changed to *p = 7).

Important note: Even though a reference is often implemented using an address in the underlying assembly language, please do not think of a reference as a funny looking pointer to an object. A reference is the object. It is not a pointer to the object, nor a copy of the object. It is the object.