Re: I don't get it

From:

Bharath <bharath.donnipad@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 18 Jun 2007 16:36:57 -0000

Message-ID:

<1182184617.381272.46900@z28g2000prd.googlegroups.com>

On Jun 18, 11:17 am, Jo <jo.lan...@telenet.be> wrote:

class A {

  public:

  char text_a[100];

          A() { *text_a=0; }
          ~A() {}};

//-----------------------------------------------------------------------------
class B {

  public:

  char text_b[100];

          B() { *text_b=0; }
          ~B() {}};

//-----------------------------------------------------------------------------
class C : public A, public B {

  public:

  char text_c[100];

          C() { *text_c=0; }
          ~C() {}};

//-------------------------------------------------------------------------------
void test() {

B *bp1,*bp2;
C c,*cp1,*cp2,*cp3;
void *p;

  strcpy(c.text_a,"hello a");
  strcpy(c.text_b,"hello b");
  strcpy(c.text_c,"hello c");
  cp1=&c;
  p=cp1;
  bp1=cp1; // ok
  bp2=(B*)p; // resulting bp2 is WRONG!
  cp2=(C*)p; // ok
  cp3=(C*)bp2; // resulting cp3 is WRONG! Which is logical because bp2
is already wrong.}

//-----------------------------------------------------------------------------

So the hot spot is the bp2=(B*)p;

What's wrong with that???

I can imagine someone saying "p is not pointing to a B object".
But if you think about it, conceptually, it does, imho.

So is it more a technical matter of compiling this!?

Maybe i'm stupid and/or missing something essential about C++.

If so, please give me a link to where i can study this right.

Cheers,

Jo

I re-wrote your test function like this -
int main() {

B *bp1,*bp2;
C c,*cp1;
void *p;
int a;

  strcpy(c.text_a,"hello a");
  strcpy(c.text_b,"hello b");
  strcpy(c.text_c,"hello c");
  cp1=&c;
  p=cp1;
  bp2=(B*)p;
  std::cout<<p<<"\n";
  std::cout<<bp2;
  std::cin>>a;
}

I printed the value in p and also in bp2. Both are one and the same.
On my m/c the O/P was -
0x22fe28
0x22fe28

Why do you think, in your program, resulting bp2 is WRONG?