Path: utzoo!attcan!uunet!mcsun!inria!timpit0!cnetlu!ranson
From: ranson@cnetlu.UUCP (Ranson)
Newsgroups: comp.lang.c++
Subject: Defining operators
Message-ID: <1253@cnetlu.UUCP>
Date: 20 Nov 89 13:19:52 GMT
Reply-To: ranson@cnetlu.UUCP (Ranson)
Organization: C.N.E.T. Lannion 22301 LANNION FRANCE
Lines: 36


When defining operators on objects of a class in C++ (e.g. + on matrices),
there are currently two possibilities (leaving aside the equivalent using
member functions, and assuming args passed by reference):

matrix& operator+(matrix& x, matrix& y) { ... }

matrix operator+(matrix& x, matrix& y) { ... }

The first one looks promising, but since returning a reference to a local
variable is an error, one has to allocate a new matrix for the result, and
disposing of it afterwards may not be easy, especially in complex expressions
(e.g. how does one dispose of the result of A+B in (A+B)+C ?).

Stroustrup recommends using the second form, where intermediate results
will be dealt with automatically by C++, but this also means that the
result needs to be copied for each operation. As I understand it, an
expression like: X = (A+B)+C; is translated into:
	matrix U;
	U = operator+(A,B);
	X = operator+(U,C);

Why not allow a third form, where the caller would pass as a third argument
a reference to the place where the result should be put?

void operator+(matrix& x, matrix& y, matrix& result) { ... }

The previous example would translate into:
	matrix U;
	operator+(A,B,U);
	operator+(U,C,X);
and no copying would be necessary. Am I missing something obvious?

     Daniel Ranson
     X400: ranson@lannion.cnet.fr
     uucp: ranson@cnetlu.fr, or ...!mcvax!inria!cnetlu!ranson