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From: beshers@sylvester.columbia.edu (Clifford Beshers)
Newsgroups: comp.lang.c++
Subject: The supplied vector.h and operator= function.
Message-ID: <4509@columbia.UUCP>
Date: Wed, 1-Apr-87 12:04:08 EST
Article-I.D.: columbia.4509
Posted: Wed Apr  1 12:04:08 1987
Date-Received: Sun, 5-Apr-87 05:17:29 EST
Sender: nobody@columbia.UUCP
Reply-To: beshers@sylvester.columbia.edu.UUCP (Clifford Beshers)
Organization: Columbia University CS Department
Lines: 95

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I've been perusing the files supplied with C++ v1.2 and have come
across something I don't understand.

In the file vector.h, there is an assignment operator defined like
this:


class vector(type) {
	type* v;
	int sz;
public:
	//...
	vector(type)& operator=(vector(type)&);
	//...
};

vector(type)& vector(type)::operator=(vector(type)& a)
{
	register i = a.sz;
	if (i != sz)
		callerror(vector,type,3,"different vector sizes in assignment");
	register type* vv = &v[i];
	register type* av = &a.v[i];
	while (i--) *--vv = *--av;

	// ???????????????
	delete[i] v;
	// ???????????????

	return *this;
}


I thought I understood everything until I got to the delete call
that I have marked off.  In the immortal words of Cindy Lou Who,
"Why Santy Claus, why?"

It seems to me there are exactly two objects in this operation,
namely 'a' and 'this', both of which should have been initialized.
At least they *can* have been initialized, as in the code below.
Therefore each has some storage allocated for the array v.  The
purpose of the assignment is to copy the contents of a->v onto
this->v, isn't it?  Why, after we go to the trouble of copying this,
do we then delete it?

I could understand this if there was a new copy of v created, but
the only new variables are the walking pointers.  I thought this
code would remove one of two v's, leaving only one for two objects.


I tried this out with the following code and it worked; I looked
at the C code produced and it seemed to say the same thing.
What's going on?

/*******  Code follows. *********/



#include <CC/stream.h>
#include <CC/vector.h>

#define FLOAT float

declare(vector,FLOAT)
implement(vector,FLOAT)


ostream& operator<<(ostream& s, vector(FLOAT) v)
{
	register int sz = v.size();
	s << "(";
	for (int i=0; i < sz-1; i++)
		s << v.elem(i) << ",";

	return s << v.elem(sz-1) << ")";
}

main()
{
	int i;
	vector(FLOAT) fred(5);
	vector(FLOAT) bob(5);

	for (i=0; i<fred.size(); i++)
	{
		fred[i] = i;
		bob[i] = bob.size() - i;
	}

	cout << (fred = bob);
}

Cliff Beshers
Columbia University Computer Science Department
beshers@sylvester.columbia.edu