Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!accuvax.nwu.edu!tank!uwvax!persoft!ericf
From: ericf@persoft.UUCP (Eric R. Feigenson)
Newsgroups: comp.lang.c++
Subject: Generating temporaries
Message-ID: <7.UUL1.2#261@persoft.UUCP>
Date: 4 Apr 89 14:10:37 GMT
Organization: Persoft, Inc.
Lines: 49

[I'm rather new to C++, so this may be a rather naive query.  Please bear
 with me]

I have a class M for which I want to define operator+.  My question(s) have
to do with how I generate the temporary that contains the result, and how
such a temporary gets destroyed.

First, if my operator+ returns an object of class M, it has to create
an object to return. If I understand things right, this will call
operator+, which will return an object of class M (which was
pushed onto the stack), and do a structure copy into c.  All the destructors
get called just fine, and no stray memory is left around, but there's the
overhead of the structure copy.

Now I've also fiddled around with trying to return a reference to an object
M, in order to avoid the structure copy.  The code for operator+() has to
allocate a pointer to an object of type M using the "new" operator.
So, if we have some code that looks like:

    M a, b, c;

    // some code to give a and b values

    c = a + b;  // calls "M& M::operator+(M& x)"

and we define operator=(M&) to do the assignment, how can we free the
space allocated by operator+() for the temporary result?  If operator=()
does the "delete" then it may destroy some space that we meant to keep
around.  Also, in an expression such as "a + b + c", how can the
intermediate temporaries be freed?  The destructors get called, but how
can they know to delete the space on the free store?

To summarize:  I want to be able to define operator+ (or any other operator
for that matter) using references (pointers) to avoid the overhead of
structure copy.  The problem seems to be knowing when and how to free the
space that the operator must dynamically allocate in order to generate a
result.

I hope this makes sense.  Thanks in advance for your help.  Please e-mail
any responses.

                        -Eric


---------------------------------------------------------------------------
                        -Eric Feigenson
                        Persoft, Inc.
            (Internet)  ericf@Persoft.COM
            (UUCP)      {ihnp4, seismo, uunet, allegra}!uwvax!persoft!ericf