🚀 go-pugleaf

In article <3vqr6h$8nj@imag.imag.fr>, Gilles Serasset
<Gilles.Serasset@imag.fr> wrote:

>Hi all,
>
...
>B3 inherits from B2 and redefines DoThat().
...
>When I call A2::DoThis(), B2::DoThat() is called because of the casting.
>And I would like B3::DoThat() to be called instead.
>
>How can I do this ? Am I missing something in C++ hierarchies principles ?

If you want B3::DoThat() to 'override' B2::DoThat(), then declare DoThat
as virtual.

class B2 : public B1 {
   ...
   virtual void DoThat(void);
   ...
};

class B3 : public B2 {
   ...
   virtual void DoThat(void);
   ...
};

Now, when you have a B3 being pointed to by a B2 *, B3::DoThat is called.

void foo(void) {
   B2 *pb2 = new B3;
   pb2->DoThat();  /* Calls B3::DoThat(), since pb2 is 'really' a B3.  */
};

It seems like you will need the cast in A2, unless you could declare
virtual DoThat in B1, and then A1 or A2 would be able to call
bb->DoThat(), since B1::DoThat() is virtual and defined in B1.

>
>Thanks in advance,
>
>Gilles Serasset,
>
>======================================================
> Au paradis, on est assis a la droite de Dieu.
> C'est normal, c'est la place du mort.    P. Desproges
>-----  Gilles.Serasset@imag.fr (Gilles Serasset)  ----
>GETA IMAG, UJF et CNRS, BP 53 F-38041 GRENOBLE CEDEX 9

-Justin
jcv6@columbia.edu

jcv6@columbia.edu (Justin Vallon) wrote:
>In article <3vqr6h$8nj@imag.imag.fr>, Gilles Serasset
><Gilles.Serasset@imag.fr> wrote:
>
>>Hi all,
>>
>...
>>B3 inherits from B2 and redefines DoThat().
>...
>>When I call A2::DoThis(), B2::DoThat() is called because of the casting.
>>And I would like B3::DoThat() to be called instead.
>>
>>How can I do this ? Am I missing something in C++ hierarchies principles ?
>
>If you want B3::DoThat() to 'override' B2::DoThat(), then declare DoThat
>as virtual.
>
>class B2 : public B1 {
>   ...
>   virtual void DoThat(void);
>   ...
>};
>
>class B3 : public B2 {
>   ...
>   virtual void DoThat(void);
>   ...
>};
>
>Now, when you have a B3 being pointed to by a B2 *, B3::DoThat is called.
>
>void foo(void) {
>   B2 *pb2 = new B3;
>   pb2->DoThat();  /* Calls B3::DoThat(), since pb2 is 'really' a B3.  */
>};
>
>It seems like you will need the cast in A2, unless you could declare
>virtual DoThat in B1, and then A1 or A2 would be able to call
>bb->DoThat(), since B1::DoThat() is virtual and defined in B1.

You should also think about having

class B1 {

virtual void DoThat (void) = 0;
};

If you never make a B1 this will allow you to make a B1.

- rmgw

http://www.punchdeck.com/hawkfish/PunchDeck.html

----------------------------------------------------------------------------
Richard Wesley  hawkfish@punchdeck.com | "'Hand it round first, and cut it
Punch Deck Consulting pnchdeck@aol.com |  afterwards.'" - Lewis Carroll,
     Macintosh Software Development    |    "Through the Looking Glass"
----------------------------------------------------------------------------

On Fri, 4 Aug 1995, Richard Wesley wrote:

> You should also think about having
> 
> class B1 {
> 
> virtual void DoThat (void) = 0;
> };
> 
> If you never make a B1 this will allow you to make a B1.

If B1 has pure virtual methods, B1 cannot be instantiated.  The B1 as 
declared cannot be instantiated.

> - rmgw

-Justin
jcv6@columbia.edu

In article <3vqr6h$8nj@imag.imag.fr>,
Gilles Serasset  <Gilles.Serasset@imag.fr> wrote:
>I face a problem related to C++ classes. And I would like to know how to
>get rid of it.
>
>void A2::DoThis(void) { ((B2 *)(this->bb))->DoThat(); }
>
>B3 inherits from B2 and redefines DoThat().
>A2 is not modified (but its bb slot contains a B3 instance).

If I understand your system correctly, then making B2::DoThat() virtual
ought to give you what you want. (Might as well declare A2::DoThis()
virtual as well, for symmetry if nothing else.)

cheers,

Roger Pantos                 Telesis North, Inc.           telesisnorth
Mac Software Guy             telesis@ecf.toronto.edu       (AppleLink)
I've got a Mac instead of a DOS box because I need a tool, not a hobby.