NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/10430

Text File | 1996-08-05 | 1.6 KB | 45 lines

Newsgroups: comp.lang.c++
Path: in2.uu.net!allegra!alice!ark
From: [e-mail] (Andrew Koenig)
Subject: Re: Can copy constructor and operator= share code?
Message-ID: <[e-mail]>
Organization: AT&T Research, Murray Hill NJ
References: <4h2kcn$[e-mail]> <[e-mail]> <4hk3bh$[e-mail]>
Date: Thu, 7 Mar 1996 01:19:25 GMT

In article <4hk3bh$[e-mail]> [e-mail] (Michael Anthony Porcelli) writes:

> Here's a style tip from a senior level C++/OO class on exactly this topic.
> In fact our teacher *required* this for all of our classes and frankly I
> believe this is a GREAT C++ tip for good OO/SoftEng.

Well, almost.  See below.

> X(X& x)
>  { Duplicate(x); } //one-statement, in-line copy constructor.

> operator= (X& x)
>  { Delete(); Duplicate(x); } //two-statement, in-line op=

Oops!  This fails when you try to assign an object to itself,
because it destroys the objects value and then assigns the destroyed
value to itself.  It also implicitly returns an undefined integer
value, instead of returning a reference to the left-hand side
the way the built-in assignment operator does.  Finally, the
operand should be const.

My revised version:

  X& operator=(const X& x)
   { if (this != &x) { Delete(); Duplicate(x); } return *this; }

> ~X()
>  { Delete(); } //one-statement, in-line destructor.

> Pretty nifty, eh?  I believe that all class design should be done this way.

It's a good strategy if you fix the bugs described above.
--
                --Andrew Koenig
                  [e-mail]