Path: utzoo!utgpu!watmath!iuvax!mailrus!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim Adcock)
Newsgroups: comp.lang.c++
Subject: Re: Defining operators
Message-ID: <9182@microsoft.UUCP>
Date: 27 Nov 89 20:29:02 GMT
References: <1253@cnetlu.UUCP>
Reply-To: jimad@microsoft.UUCP (Jim Adcock)
Organization: Microsoft Corp., Redmond WA
Lines: 58

In article <1253@cnetlu.UUCP> ranson@cnetlu.UUCP (Ranson) writes:
...
>matrix operator+(matrix& x, matrix& y) { ... }
>Stroustrup recommends using <the above> 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?

The C++ compilers I am familiar with do not necessarily work as you 
assume.  If I declare as follows:

matrix(const matrix&);
void friend operator=(matrix&, const matrix&);
matrix friend operator+(const matrix& a, const matrix& b);

...then my 1.2 compatible compiler does what you want -- I can
write:

	matrix D = A + B + C;

and no matrix copying will be done.  BUT NOTE:  This is not the
same as if you write:

	D = A + B + C;

where D was declared previously.  When one writes "matrix D =" one
is telling the compiler to *construct* a new matrix out of the results
of "A + B + C".  Whereas when one writes "D = " you tell the compiler
you want to have D *assigned* the value of some matrix -- but what matrix?
-- one constructed from "A + B + C" ---- sooo you force the compiler to 
introduce a temporary that then can be copied into D:  

	D = A + B + C;

means:

	D = (matrix Tmp(A + B + C));

In C++ construction
vs assignment are very much *not* the same thing.  Unfortunately, a 
plethora of syntaxes (syntacti?) for construction vs assignment
confuses the issue.  Even if one does not optimally use construction
verses assignment, C++ compilers can avoid most of the unnecessary
copying by creating op+ in the three parameter form as you suggest.