Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c++
Subject: access functions
Message-ID: <11152@mimsy.UUCP>
Date: 21 Apr 88 06:03:31 GMT
References: <8063@apple.Apple.Com> <6590039@hplsla.HP.COM>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 48

In article <6590039@hplsla.HP.COM> jima@hplsla.HP.COM (Jim Adcock) 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 is true.  The point, however, is that a variable is always
a variable, while a function is arbitrary.  So if you declare that
henceforth and forever, anyone can take complexvar.realpart() and
complexvar.imaginarypart(), and you decide you prefer polar coordinates,
people can still take the two, while if you declare that anyone
can simply take .realpart and .imaginarypart, you are stuck with
rectangular.

To make the example slightly more interesting, consider a system
in which complex numbers are needed in rectangular form about half
the time, and in polar about half, but `clustered'.  Then if you
used access functions you can write something like

	class complex {
		union {
			struct { double re, im; } r; // if rectangular
			struct { double r, theta; } p; // if polar
		} u;
		boolean	polar;	// true => polar, else rectangular
		void flip();	// guess...
	public:
		inline double re() { if (this->polar) flip();
					return (this->u.r.re); }
		inline double theta() { if (!this->polar) flip();
					return (this->u.p.theta); }
		// etc
	}

which has the nice side effect of leaving it in the last-used
format, eliminating unnecessary conversions.

If you never need the polar representation, this is just excess
baggage.  The question becomes `do I want to leave myself the
option of using polar representation?'  If you intend to reuse
the implmentation later, perhaps it would be wise to do so.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris