NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/958

Internet Message Format | 1996-08-06 | 2.6 KB

Path: news.ahc.ameritech.com!datalytics!news
From: Rob Stewart <[e-mail]>
Newsgroups: comp.lang.c++
Subject: Re: Creating a pointer to a function "void (*ptrFunction)()"  inside a class
Date: 8 Jan 1996 17:35:42 GMT
Organization: Datalytics, Inc
Message-ID: <4crkle$[e-mail]>
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Jason Gresh <[e-mail]> wrote:
>    I am trying to create a base class that has a function that the
>derived class needs to create.  In order to do this, I want to create a
>pointer to a function in the base class that the derived class can then
>define.  This is not a case where an override will work because the
>number and names of the derived functions is not known.  >
>class BaseClass
>{
>    int (*ptrFunction)();
>}
>class DerivedClass : BaseClass
>{
>    DerivedClass::DerivedClass();
>    int myFunction(int, int);
>}
>DerivedClass::DerivedClass()
>{
>    ptrFunction = myFunction;
>}
>DerivedClass::myFunction(int, int)
>{
>// code ....
>}
>If I don't make myFunction part of the derived class (global),
>everything works fine.  As soon as I include it in the class, I cannot
>assign a pointer to it.  I am aware that pointers to functions inside
>the class include the class name in some way ( this is not an issue for
>global functions).  What is the casting (or other) mechanism to make
>this work?
>
This behavior will only work for static member functions.  By
declaring a member function static, you are asking the
compiler to avoid mangling the name.

On the other hand, by making a member function static, you
lose the this pointer in the function call and the member
function can no longer operate on an instance of the class.
If you wish to restore that capability, you must pass a this
pointer explicitly.  That is, the function can take a pointer
to the appropriate class parameter.

If that "this" pointer is to be of a specific type (to match
the member function's derived type, you'll need to use RTTI to
validate that the "this" pointer is of the correct type.  (The
function pointer type will necessarily take a pointer to the
base class, as must the derived class member functions.  If
you need a derived class pointer, you need to determine if a
cast to the desired derived type is valid.  RTTI is the only
way to do that--whether you or the compiler implements RTTI is
moot.)

--
Robert Stewart        | My opinions are usually my own.
Datalytics, Inc.
[phone]
[e-mail]