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From: gilley@ndl.com (Greg Gilley)
Newsgroups: comp.lang.c++
Subject: distinguishing operator[] on left and right
Message-ID: <1991Feb28.212419.20920@ndl.com>
Date: 28 Feb 91 21:24:19 GMT
Sender: gilley@ndl.com (Greg Gilley)
Organization: Numerical Design Limited
Lines: 58

Is there any way to distinguish when the operator[] is used as an
lvalue as opposed to an rvalue?

The problem is that I am attempting to do a delayed copy of contents
with a reference count (a sort of copy-on-write).  So what I have is
a class of the form:

class X
{
    struct Xrep
    {
	float *data;
	int refcnt;
	int length;
    } *p;
    .
    .
    .
};

and operator= looks like:
void X::operator=(const X &x)
{
    x.p->refcnt++;

    if (--p->refcnt == 0)
    {
	delete p->data;
	delete p;
    }

    p = x.p;
}

Which is all well and good.  If I do other operators, they create new
ones, etc.  The problem arises with [].  Suppose we have:

     X a(1);
     X b(1);

     a[0] = 1.0;
     b = a;
     b[0] = 10.0;

What you would like to have is a[0] == 1.0 and b[0] == 10.0 (which means
that b has to be "detached" from a before it changes).  However,
I can't find a way to distinguish when the result of [] is being used
as an lvalue or rvalue.

Any suggestions?  Thanks,

			Greg

-- 
-------------------------------------------------------
  Greg Gilley
  gilley@ndl.COM   [Numerical Design Limited]
  919-929-2917 (voice)