Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!mailrus!umix!umich!mibte!gamma!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: Critique of Wiener/Pinson
Message-ID: <7820@alice.UUCP>
Date: 21 Apr 88 20:31:16 GMT
References: <8063@apple.Apple.Com> <6590039@hplsla.HP.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 42
Summary: using access functions you can change the internal representation of a class without changing the interface.

In article <6590039@hplsla.HP.COM>, jima@hplsla.UUCP writes:
> If you refuse to stick to your original definition of blah,
> and you make blah public, and someone uses blah, and then you change
> your definition of what blah means, then that user of your class is
> going to be hosed.
> 
> BUT this is true whether or not blah is an access function or a public
> instance variable.  If you change your definition of what a public
> anything means, you will hose your users.

(This should be obvious, but ... )

Suppose you have

	class Complex {
		double	re, im;
	public:
		double real() { return re; }
		// ...
	};

and you decide you want to switch to polar coordinates.  Then you
can write

	class Complex {
		double	rho, theta;
	public:
		double real() { return <trig expression>; }
		// ...
	};

and users will not be affected.

> ...  Change the meaning of 
> any part of your public interface, either instance variable or function, 
> and your class's users will be hosed.

But use access functions so you can change the internal structure of your
class without changing the meaning of its public interface.
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Murray Hill, NJ  07974
		research!shopiro   (201) 582-4179