TITLE: implementation notes on references

(Newsgroups: comp.lang.c++, 28 Aug 99)


PHILEMON: Philemon

> blargg wrote:
> > Nope. References aren't objects, and this does have implications.
> I didn't get that. References aren't objects and pointers aren't
> objects. What are the implications you had in mind?


BECKER: Pete Becker <petebecker@acm.org>

Pointers are objects with a small 'o', in the same way that ints are
objects: they occupy storage and there are rules that define the valid
operations on those regions of storage. References do not have to occupy
storage, and even when they do, there are no operations that the user
can perform on that storage. Conceptually, a reference is a name for
some object, and nothing else. References are often implemented with
pointers, but often they are not. For example;

void f(int i)
{
int &ir = i;
printf("%d\n", ir);
}

This can be implemented without allocating any storage for ir. The
compiler just needs to look through the reference to the object it
refers to, and pass the value of i to printf.

UNKNOWN#1:

>>> References are *required* only in copy (ctor and operators.)

UNKNOWN#2:

>> They are needed for overloading built-in operators, such as those you
>> mention, and also other operators (operator +, operator *, etc.)

PHILEMON:

> No, they are not required there. They _can_ be used, of course, and
> syntactically they are a good fit there--but again, it is syntax, and
> syntax only.

BECKER: 

Well, in a sense that's true, but such trivializations gloss over
important differences. How would you write the equivalent of the
following code, preserving the use of overloaded operators, without
references?

cout << "Hello, " << "world" << endl;

Two more important difference between references and pointers are that
references always refer to an object and that the object they refer to
cannot be changed. The first is a useful guarantee, because you don't
need to check for null pointers. The second is also useful because you
can cache values without having to worry that the values will be
invalidated by changing what object a reference refers to.