Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!tut.cis.ohio-state.edu!att!dptg!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Lvalue checking for operator= ??
Message-ID: <10045@alice.UUCP>
Date: 23 Oct 89 15:35:52 GMT
References: <33433@cornell.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 39

In article <33433@cornell.UUCP>, deb@svax.cs.cornell.edu (David Baraff) writes:
> Suppose you have a standard matrix class, which implements '+',
> and also has its own assignment operator defined i.e.

> 	matrix &matrix::operator= (matrix &m)

> exists. Then,

> 	matrix	a, b, c;

> 		...

> 	a+b = c;

> appears to be legal. (As least it compiled under 1.2.) Is it legal under
> 2.0 ? What does it really mean? Shouldn't  the '=' operator be forced
> to only accept an lvalue as its left-hand operand?

How about making operator+ return a const matrix?
Then you won't be able to assign to it.


To tell the truth, I hadn't thought about this issue until this
question forced me to do so.  There are zillions of things like
string classes out there that say

	extern String operator+(const String&, const String&);

and apparently they really should say

	extern const String operator+(const string&, const String&);

The point is that unless you say otherwise, you can't call operator=
on behalf of a const object (well, 2.0 will let you get away with
it but will give a warning).  I think that's almost exactly
what you want.
-- 
				--Andrew Koenig
				  ark@europa.att.com