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Hello,

I have two classes A and B. B is a subclass of A.
I then have another class C. This one has the following methods :

int doSomething(A *obj);
int doSomething(B *obj);

C also has as a member an instance of A,

private:
A *obj;

In one of C's methods I have in the previous attribute an instance of
B, with which I will call :

doSomething(obj);

Stepping through this, I noticed that the "int doSomething(A *obj)" is
called instead of the other, although the instance at runtime is of
class B. I guess that's because obj was declared as being of type A. I
was expecting that the other method would be called.

Is there some other way of doing this ?

Daniel Luis dos Santos wrote:
> Hello,
>
> I have two classes A and B. B is a subclass of A.
> I then have another class C. This one has the following methods :
>
> int doSomething(A *obj);
> int doSomething(B *obj);
>
> C also has as a member an instance of A,
>
> private:
> A *obj;
>
> In one of C's methods I have in the previous attribute an instance of B,
> with which I will call :
>
> doSomething(obj);
>
> Stepping through this, I noticed that the "int doSomething(A *obj)" is
> called instead of the other, although the instance at runtime is of
> class B. I guess that's because obj was declared as being of type A. I
> was expecting that the other method would be called.
>
> Is there some other way of doing this ?
>

Off course there is.

This might help you solve the problem:
http://www.parashift.com/c++-faq-lit...t.html#faq-5.8

Daniel Luis dos Santos wrote:
> Hello,
>
> I have two classes A and B. B is a subclass of A.
> I then have another class C. This one has the following methods :
>
> int doSomething(A *obj);
> int doSomething(B *obj);
>
> C also has as a member an instance of A,
>
> private:
> A *obj;
>
> In one of C's methods I have in the previous attribute an instance of B,
> with which I will call :
>
> doSomething(obj);
>
> Stepping through this, I noticed that the "int doSomething(A *obj)" is
> called instead of the other, although the instance at runtime is of
> class B. I guess that's because obj was declared as being of type A. I
> was expecting that the other method would be called.
>
> Is there some other way of doing this ?
>
Google for "double dispatch".

On Apr 13, 11:10 pm, Daniel Luis dos Santos <daniel.d...@gmail.com>
wrote:

> I have two classes A and B. B is a subclass of A.
> I then have another class C. This one has the following
> methods :

> int doSomething(A *obj);
> int doSomething(B *obj);

> C also has as a member an instance of A,

> private:
> A *obj;

> In one of C's methods I have in the previous attribute an
> instance of B, with which I will call :

> doSomething(obj);

> Stepping through this, I noticed that the "int doSomething(A
> *obj)" is called instead of the other, although the instance
> at runtime is of class B. I guess that's because obj was
> declared as being of type A. I was expecting that the other
> method would be called.

Why? doSomething isn't a virtual function of A.

> Is there some other way of doing this ?

Make doSomething a virtual function of A.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

On Apr 14, 3:42 pm, red floyd <no.spam.h...@example.com> wrote:
> Daniel Luis dos Santos wrote:

> > I have two classes A and B. B is a subclass of A. I then
> > have another class C. This one has the following methods :

> > int doSomething(A *obj);
> > int doSomething(B *obj);

> > C also has as a member an instance of A,

> > private:
> > A *obj;

> > In one of C's methods I have in the previous attribute an
> > instance of B, with which I will call :

> > doSomething(obj);

> > Stepping through this, I noticed that the "int doSomething(A
> > *obj)" is called instead of the other, although the instance
> > at runtime is of class B. I guess that's because obj was
> > declared as being of type A. I was expecting that the other
> > method would be called.

> > Is there some other way of doing this ?

> Google for "double dispatch".

Except that there's no double dispatch involved. He wants
dynamic dispatch on a non-member function, which C++ doesn't
support. (It's possible to emulate it, of course, if you really
have to, but the correct solution is almost always to use
virtual member functions.)

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34