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From: pj@hrc63.co.uk (Mr P Johnson "Baddow")
Newsgroups: comp.lang.c++
Subject: Re: Generating temporaries
Summary: Arrggggh
Message-ID: <567@hrc63.co.uk>
Date: 17 Apr 89 12:15:43 GMT
References: <7.UUL1.2#261@persoft.UUCP> <9431@claris.com>
Organization: GEC Hirst Research Centre, Wembley, England.
Lines: 49

In article <9431@claris.com>, hearn@claris.com (Bob Hearn) writes:

> I think this is a good question whose answer should be posted rather than
> just emailed.
> 
> One thing you can do is have operator+ have a static local object of class
> M that it puts the result into.  You can return a reference to this, and
> after you do the copy
> 
> a = b + c
> 
> to a from the temp the temp is not referenced anymore, so it is free to be
> used again whenever it is needed.  You don't have to use new or delete,
> and in fact the storage for the temp never has to get thrown away.
> 
> Bob Hearn
> hearn@claris.com


What happens when you do something like

    a = (b + c) + d;

C++ calls "operator+(b, c).  This puts the result in your internal static &
returns a reference.  This reference then goes into the outer addition, which
(without realising it) will overwrite its argument with its result.

I know how you feel: the way C++ does this is not very efficient.  It would
be nice if operators could be told where to put their results.

My only solution is to suggest providing a "+=" operator.  This would add to
its first argument and then return a reference to that argument.  Much
better.

It might be possible to pull some trick with the "M::M( M& )" constructor
to avoid this: if you define this constructor then it will get called for
function argument/return values instead of the default bit-copy function,
but you do have to be careful how this affects your other functions.  This
constructor is explained in Stroustrup section 6.6.

Also, remember that "=" can be defined, otherwise it is also a bit-copy.
Some compilers are intelligent and avoid doing two bit-copies.

Paul

BTW, I am not an expert, so this could be wrong.

If this has been dealt with, sorry.  We have problems with our mail feed
right now.