Base class pointer in dervied class pointer

[amitwagh]

class BaseEx
{
public :
virtual void Display()
{
cout<<"BaseEx:: Display()"<<endl;
}
};

class DervEx : publjc BaseEx
{
public :
void Display()
{
cout<<"Derv:: Display()"<<endl;
}

void Print()
{
cout<<"Derv:: Print()"<<endl;
}
};

int main()
{
Derv *d = reinterpret_cast<Derv *> ( new BaseEx);

d->Print();
return 0;

}

I got the output as Derv:: Print() and not a crash as expected.

How does it work??

[Banfa]

I suspect this not a good thing to do having read the documentation of reinterpret_cast.

You need to remember that a simple class like this with no data members is just a list of function pointers that are the same for all instances of the class. Since there are no member variables for DervEx::Print to access the code works.

If you change DervEx to

class DervEx : public BaseEx
{
public :
  int i;
 
  DervEx(void) : i(5) {}
  void Display() 
  {
    cout<<"Derv:: Display()"<<endl;
  }
 
  void Print()
  {
    cout<<"Derv:: Print(): "<<i<<endl;
  }
};

You will see that the output is garbage because the data actually pointed at is garbage and not an actual DervEx class.

You would only get a crash if you attempted to access a piece of memory outside the legal bounds for your platform, in MSVC (what I am using) since the heap starts at several k and all the used data is on the heap and very small the extra location accessed by DervEx::i is still valid even though it is not an actual part of the class.

The documentation that I have of reinterpret_cast states that you should not use pointers cast in this way as you have no garantee about there validity in fact you code works if you change

DervEx *d = reinterpret_cast<DervEx *> ( new BaseEx);

to

DervEx *d = reinterpret_cast<DervEx *> (new int);

I think the point is that you should only use reinterpret_cast in very special circumstances not in normal use and having used it if you have converted pointer types you should not be using the new pointer.