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From: "Sam Inala" <sami@microsoft.com>
Subject: Re: What is wrong with OO ?
Date: 1997/02/17
Message-ID: <01bc1d22$e144fe60$928d389d@miyu>#1/1
X-Deja-AN: 219466598
References: <JSA.97Jan3143208@alexandria> <fxtu3oysbzp.fsf@isolde.mti.sgi.com>
 <JSA.97Jan6190704@alexandria> <E3uK9F.CzG@research.att.com>
 <E5Dx8v.6y3@tip.nl> <cjolley-1002971634080001@news.zippo.com>
 <dewar.855671030@merv>
Organization: Microsoft
Newsgroups: 
 comp.lang.c++,comp.lang.smalltalk,comp.lang.eiffel,comp.lang.ada,comp.object,comp.software-eng
Date: 1997-02-17T00:00:00+00:00
List-Id: <comp.lang.ada>


Robert Dewar <dewar@merv.cs.nyu.edu> wrote in article
<dewar.855671030@merv>...
> Charles said
> <<C++ actually does not bundle code with the structs. If a member
function
> is not virtual, the compiler will just binds a obj.foo() reference to its
> obj::foo() counterpart.  On the other hand, if a member function is
> virtual, then function pointers, like you mentioned above, are used.
> Anyway, just a minor detail; they're still just glorified structs. ;)>>
> 
> YOu should better say "typical C++ implementations" rather than C++, I
see
> nothing in the draft standard that would support your claim that this is
> implied by the language definition.

Just for clarification: even typical C++ implementations differ from
structs
containing function pointers. Consider the following:

#include <stdio.h>

struct One {
   int x;
};

struct Two {
   int x;
   void foo() {}
};

struct Three {
   int x;
   virtual void foo() {}
};

struct Four {
   int x;
   virtual void foo() {}
   virtual void bar() {}
};

int
main() {
   printf( "sizes = %d %d %d %d\n", sizeof( One ), sizeof( Two ),
	sizeof( Three ), sizeof( Four ) );
   return 0;
}

Output from gcc:
sizes = 4 4 8 8

With an implementation of class Four like this:

struct Four {
	int x;
	void (*foo)();
	void (*bar)();
};

...we would expect the sizeof( Four ) to be 12. Instead, it most
likely contains a single pointer to a table of function pointers,
which are shared by all the instances of the class:

struct Four {
	void* vtbl;
	int x;
};

<return type> (*Four::function[ cMethods ])(...);

I only mention this because I perceive people thinking
that object instances carry around their own virtual 
function tables.