Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!princeton!allegra!alice!bs
From: bs@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: The supplied vector.h and operator= function.
Message-ID: <6786@alice.uUCp>
Date: Tue, 7-Apr-87 21:30:27 EST
Article-I.D.: alice.6786
Posted: Tue Apr  7 21:30:27 1987
Date-Received: Sat, 11-Apr-87 07:57:03 EST
References: <4509@columbia.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 39
Summary: delete[i]




(beshers @ Columbia University CS Department) writes:

 > I've been perusing the files supplied with C++ v1.2 and have come
 > across something I don't understand.
 > 
 > vector(type)& vector(type)::operator=(vector(type)& a)
 > {
 >	register i = a.sz;
 ...
 > 	// ???????????????
 > 	delete[i] v;
 > 	// ???????????????
 ...
 > }
 > 
 > 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?"S
 > 
 > 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?

The short answer is look at pages 92 and 259 in the C++ book (where the
index directs you to). The longer answer is that a pointer type
doesn't allow the compiler to determine whether you point to an object
or an array of objects. This typically doesn't matter since the free
store ``knows''. However, the free store doesn't know about destructors,
so consider a vector(T) where T::~T() has been declared. Then, when we
delete a vector(T) with 10 elements the destructor T::~T() needs to be
called 10 times - once for each element. ``The system'' is too dump to
know, so you must tell it, hence the ``spurious'' [i] in the example.