NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/cplus/18324

Text File | 1992-12-22 | 2.0 KB | 53 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!psinntp!cci632!dwr
From: [e-mail] (Donald W. Rouse II)
Subject: Re: Can't I initialize a dynamically allocated object in its constructor?
Message-ID: <[e-mail]>
Organization: [Computer Consoles, Inc., Rochester, NY
References: <[e-mail]>
Date: Tue, 22 Dec 1992 19:14:46 GMT
Lines: 42

In article <[e-mail]> [e-mail] (Cocotte Minute) writes:
>Hello,
>
>I have the following problem.
>
[ problem deleted ]

My off-the-top-of-my-head guess is that your problem is caused
by lack of a copy constructor.
When operator+ returns, the compiler probably invokes
a copy constructor to return the value of the operation.
Since you have not defined one, it creates a default copy
constructor which just copies item and size, but does not
allocate memory for *item, nor does it copy anything.
Now, the returned value.item and retVect.item point to the
same memory, so when retVect gets destructed, the array
gets returned to the heap.  Now (v1+v2).item points to free space.
Next, operator+ is called again.  When retVect is constructed,
it gets memory from the heap, which just happens to be the
memory that was previously freed.  If you do not initialize
the array, it retains its old values, and everything looks great.
(It isn't great, though; it just looks that way.)
If you do initialize, then of course the old values get scrogged.

All of this is speculation on my part, except for that fact that
you should define your own copy constructor:

Vector::Vector(Vector& v) {
   item = new double[v.size] ;
   size = v.size ;
   printf("copy constructing\n");
   for(int i=0;i<size;i++)item[i]=v.item[i];
}

Hope this helps.

PS.  IMO, compilers should warn about this type of thing.
I understand that g++ detects T operator=(T&) with no T(T&).
Kudos to them.  All compilers should do this.
It would also be nice if all compilers warned about
creating default copy constructors when other constructors
use "new".  Every little bit helps.