Re: I don't get it

From:
Jo <jo@mutools.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 18 Jun 2007 21:22:55 GMT
Message-ID:
<PICdi.22046$ZM7.1241190@phobos.telenet-ops.be>
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Andre Kostur wrote:

Jo <jo.langie@telenet.be> wrote in
news:faBdi.21838$nN5.1513633@phobos.telenet-ops.be:

 

John Harrison wrote:

   

Jo wrote:

     


[snip]

 

Perhaps you are expecting an ordinary cast to work like a
dynamic_cast? When you write

bp2=(B*)p; // resulting bp2 is WRONG!

you are expecting the program to 'discover' that there a B object
hidden inside the object that p is pointing to?

If so google for dynamic_cast, or better still read a good book.
     


I understand that a dynamic_cast cannot do a base-to-derived
conversion, so that's not the solution here.
   


No, that's exactly what dynamic_cast is for, _if_ the pointer is pointing
to an object that actually is of the derived type.

 

And a static_cast doesn't seem to make any difference, because i'm
already doing static casts right, just in the 'old' way.

So i'm still looking for the solution for this situation

class A {

 public:
 virtual long GetClassID() { return(' A'); }
};

class B {

 public:
 // some class data & funx
};

class C : public class A, public class B {

 public:
 virtual long GetClassID() { return(' C'); }
};

foo1(C *cp) {
 ...
 foo2(cp);
 ...
}

foo2(A *ap) {

 if (ap->GetClassID==' C') {
   C *cp=(C*)ap; // IS THIS LEGAL and PORTABLE C++
   ?
 }
}
   


In theory that would work... of course so would:

class A {
 virtual ~A() {};
};

class B {
 virtual ~B() {};
};

class C : public class A, public class B {
 virtual ~C() {};

 void cfn() {};
};

foo1(C *cp) {
 fooA(cp);
}

fooA(A *ap) {
 C * cp = dynamic_cast<C*>(ap);

 if (cp != NULL) {
   cp->cfn();
 }
}

No "getclassid" function required.


Indeed.

But i prefer not to use RTTI for the reasons mentioned in the earlier post.

However, note that there's no mention
of B anywhere in there (other than the inheritance).... so how does B
get involved in this question?


I guess i was confused:

Because i ran into the casting problem (due to the use of the void*), i
started thinking that it had to do with the multiple inheritance...

I really didn't know that using a void* abandons all the class relation
info. (and that after all these years of programming, damn damn damn)

Thanks a lot for your help!

A dummy got a bit less dummy :)

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<br>
Andre Kostur wrote:<br>
<blockquote type="cite"
 cite="midXns99538A2AA71CCnntpspamkosutrnet@209.135.99.21">
  <pre wrap="">Jo <a class="moz-txt-link-rfc2396E" href="mailto:jo.langie@telenet.be">&lt;jo.langie@telenet.be&gt;</a> wrote in
<a class="moz-txt-link-freetext" href="news:faBdi.21838$nN5.1513633@phobos.telenet-ops.be:">news:faBdi.21838$nN5.1513633@phobos.telenet-ops.be:</a>

  </pre>
  <blockquote type="cite">
    <pre wrap="">John Harrison wrote:

    </pre>
    <blockquote type="cite">
      <pre wrap="">Jo wrote:

      </pre>
    </blockquote>
  </blockquote>
  <pre wrap=""><!---->
[snip]
 
  </pre>
  <blockquote type="cite">
    <blockquote type="cite">
      <pre wrap="">Perhaps you are expecting an ordinary cast to work like a
dynamic_cast? When you write

bp2=(B*)p; // resulting bp2 is WRONG!

you are expecting the program to 'discover' that there a B object
hidden inside the object that p is pointing to?

If so google for dynamic_cast, or better still read a good book.
      </pre>
    </blockquote>
    <pre wrap="">
I understand that a dynamic_cast cannot do a base-to-derived
conversion, so that's not the solution here.
    </pre>
  </blockquote>
  <pre wrap=""><!---->
No, that's exactly what dynamic_cast is for, _if_ the pointer is pointing
to an object that actually is of the derived type.

  </pre>
  <blockquote type="cite">
    <pre wrap="">And a static_cast doesn't seem to make any difference, because i'm
already doing static casts right, just in the 'old' way.

So i'm still looking for the solution for this situation

class A {

  public:
  virtual long GetClassID() { return(' A'); }
};

class B {

  public:
  // some class data &amp; funx
};

class C : public class A, public class B {

  public:
  virtual long GetClassID() { return(' C'); }
};

foo1(C *cp) {
  ...
  foo2(cp);
  ...
}

foo2(A *ap) {

  if (ap-&gt;GetClassID==' C') {
    C *cp=(C*)ap; // IS THIS LEGAL and PORTABLE C++
    ?
  }
}
    </pre>
  </blockquote>
  <pre wrap=""><!---->
In theory that would work... of course so would:

class A {
  virtual ~A() {};
};

class B {
  virtual ~B() {};
};

class C : public class A, public class B {
  virtual ~C() {};

  void cfn() {};
};

foo1(C *cp) {
  fooA(cp);
}

fooA(A *ap) {
  C * cp = dynamic_cast&lt;C*&gt;(ap);

  if (cp != NULL) {
    cp-&gt;cfn();
  }
}

No "getclassid" function required. </pre>
</blockquote>
<br>
<tt>Indeed.<br>
<br>
But i prefer not to use RTTI for the reasons mentioned in the earlier
post.<br>
<br>
</tt>
<blockquote type="cite"
 cite="midXns99538A2AA71CCnntpspamkosutrnet@209.135.99.21">
  <pre wrap="">However, note that there's no mention
of B anywhere in there (other than the inheritance).... so how does B
get involved in this question?</pre>
</blockquote>
<tt><br>
I guess i was confused: <br>
<br>
Because i ran into the casting problem (due to the use of the void*), i
started thinking that it had to do with the multiple inheritance...<br>
<br>
I really didn't know that using a void* abandons all the class relation
info. (and that after all these years of programming, damn damn damn)<br>
<br>
Thanks a lot for your help!<br>
<br>
A dummy got a bit less dummy :)<br>
</tt>
</body>
</html>

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