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From: ahodgson@hstbme.mit.edu (Antony Hodgson)
Newsgroups: comp.lang.c++
Subject: Re: friend operator +(l,r) vs. operator +(r)
Summary: Warning about using foo& operator+( foo& f )
Message-ID: <1990Nov22.001208.26092@athena.mit.edu>
Date: 22 Nov 90 00:12:08 GMT
References: <11759@hubcap.clemson.edu> <1990Nov21.140621.4229@ssd.kodak.com> <11782@hubcap.clemson.edu>
Sender: daemon@athena.mit.edu (Mr Background)
Reply-To: ahodgson@hstbme.mit.edu.UUCP (Antony Hodgson)
Organization: Massachusetts Institute of Technology
Lines: 27

Just a warning that the use of the expression:

	foo& operator + ( foo& f )
or
	friend foo& operator + ( foo& f1, foo& f2 )

will almost invariably lead to a run-time error.  The obvious interpretation
of the + operator is that you are creating a new object from the two arguments.
This new object will therefore require its own storage.  You can either do
this by defining a foo variable local to the + function, or by using "new"
to create a new one inside +.  If you do the former, you return a reference
to an object which is susceptible to being immediately deallocated, so any
use of the foo& outside of + will give garbage.  If you do the latter, you
must keep track of the returned reference in order to properly deallocate
it later;  if you use the returned value as an intermediate result (as in
foo c = a + b + d) you will lose the ability to deallocate the result of
b+d, so you'll have permanently trapped some memory.  Do this often enough
and you'll run out of memory.  It's much better to define the operator as

	foo operator+(foo& f)

and let the system deal with deallocating it when it goes out of scope.

Good luck,

Tony Hodgson
ahodgson@hstbme.mit.edu