NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/17768

Text File | 1996-08-05 | 3.0 KB | 67 lines

Path: news.compuserve.com!newsmaster
From: 76623,[e-mail]  (Bobby Martin)
Newsgroups: comp.lang.c++
Subject: Re: [Q]Virtualizing/Deriving
Date: 17 Apr 1996 13:42:14 GMT
Organization: CompuServe Incorporated
Message-ID: <4l2sfm$[e-mail]>
References: <[e-mail]> <4knlec$[e-mail]>
Reply-To: 76623,[e-mail] (Bobby Martin)
NNTP-Posting-Host: dd21-037.compuserve.com
X-Newsreader: IBM NewsReader/2 v1.03

In <4knlec$[e-mail]>, [e-mail] (Drew Eckhardt) writes:
>In article <[e-mail]>,
>Dspse Bedford <[e-mail]> wrote:
>>I just started using C++ so bare with me.
>>
>>Lets say I have a base class, BASE, with lots and lots of operators and
>>member functions virtualized, like 10,000. Now I create 100 derived classes
>>with BASE as their parent.  The first 99 derived classes use all of the
>>operators and member function which exist in BASE, of course, if they need
>>to redefine any they will.  The last derived class does not use some of the
>>operators defined, lets say 2 out of 10,000. Now if I don't define these 2
>>operators, the compiler will go pick up the parent definition if a user of
>>the last derived class attempts to use any of these two operators.
>>  What can I do such that the compiler will give an error to the user?  I
>>do not want to create another base class if possible.
>
>You can declare the functions you don't want called as private, and fail
>to define them.  Ie, given
>
>   class foo : public base {
>   public:
>      // stuff
>
>   private:
>      void dont_call ();
>   };
>
>a user will get an error message if he attempts to call dont_call
>(scope is resolved before overloading, so this will keep the user
>from getting at a dont_call function with _any_ type signature in
>the base class).
>
>--
><a href="http://www.poohsticks.org/drew/">Home Page</a>
>Four boxes : soap, ballot, jury, ammo.  | Work: [e-mail]
>Use in that order.                      | Play: [e-mail]

    I wouldn't try this, if you have declared the methods as virtual then you
must be calling them from a pointer to the base class.  Any code compiled with
a pointer to the base class will know nothing about the method being redeclared
private.  I believe the lack of a definition of a virtual function should cause
a link error if that virtual function is ever used, *even if only from a base
class pointer*.
    If you need some methods to not be available to some children, you
should solve it as the design problem it is, not as a coding problem.  Leave
those methods out of the class BASE, and add a subclass of BASE that defines
those methods (maybe called BaseAddOn).  Then inherit any of the children that
need those methods from BaseAddOn and inherit the other children from BASE.
Any function that needs only Base's functionality should use a BASE* to receive
objects, and any function that also needs the extra methods should use a
BaseAddOn* to receive objects.

Hope that helps,
Bobby Martin