Path: utzoo!utgpu!water!watmath!clyde!att-cb!osu-cis!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: Critique of Wiener/Pinson
Summary: arguments for access functions
Message-ID: <7814@alice.UUCP>
Date: 17 Apr 88 02:26:28 GMT
References: <8063@apple.Apple.Com> <6590037@hplsla.HP.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 51
Posted: Sat Apr 16 22:26:28 1988

In article <6590037@hplsla.HP.COM>, jima@hplsla.UUCP writes:
> I don't buy the idea that instance variables of an object must always
> be accessed via an access method.
> To suggest that:
> 
> 	x = myComplexNumber.re();
> 	y = myComplexNumber.im();
> 
> is in any way better than:
> 
> 	x = myComplexNumber.re;
> 	y = myComplexNumber.im;
> 
> seems ludicrous to me.

Here are two reasons you might prefer access functions.

1) Access functions allow you to change the internal representation.
You might someday want to represent complex numbers in polar
coordinates.  Admittedly, rectangular coordinates are usually
preferred, but there undoubtedly are applications in which polar
coordinates would be better.  Use access functions and all your code
will continue to run.

2) Access functions support read-only access to instance variables.
You might someday want a redundant represention of complex numbers
using both rectangular and polar coordinates.  You then need to
preserve the consistency of the two parts of the representation.  If
you allowed the user direct access to the instance variables, that
consistency could be destroyed by any code that accessed a complex
number.  If you use access functions, you only have to worry about
member functions.

I should also mention that there is no performance penalty for access
functions.  If they are expanded inline the generated code is exactly
the same as reading a public instance variable.

On the other hand C++ does allow public instance variables, so you can
make your own decision.

> In fact, I believe that in many situations similar to
> the complex number example, allowing direct access is preferrable to forcing
> the class's user to always tag on the "meaningless" () parenthesis.

I will agree that the required () for access functions may seem annoying,
but if you agree with me that all syntax is painful, then I hope you'll
also agree that this is only a minor turn of the screw.  (:-)
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Murray Hill, NJ  07974
		research!shopiro   (201) 582-4179