C++ Programming - OOPS Concepts - Discussion

The dynamic_cast can only be used with pointers and references to classes (or with void*). Its purpose is to ensure that the result of the type conversion points to a valid complete object of the destination pointer type.

This naturally includes pointer upcast (converting from pointer-to-derived to pointer-to-base), in the same way as allowed as an implicit conversion.

But dynamic_cast can also downcast (convert from pointer-to-base to pointer-to-derived) polymorphic classes (those with virtual members) if -and only if- the pointed object is a valid complete object of the target type.

For example:
// dynamic_cast
#include <iostream>
#include <exception>
using namespace std;

class Base { virtual void dummy() {} };
class Derived: public Base { int a; };

int main () {
try {
Base * pba = new Derived;
Base * pbb = new Base;
Derived * pd;

pd = dynamic_cast<Derived*>(pba);
if (pd==0) cout << "Null pointer on first type-cast.\n";

pd = dynamic_cast<Derived*>(pbb);
if (pd==0) cout << "Null pointer on second type-cast.\n";

} catch (exception& e) {cout << "Exception: " << e.what();}
return 0;
}

Output: Null pointer on second type-cast.

The actual reason is - a derived class has all information about a base class and also some extra bit of information. Now a pointer to a derived class will require more space and that is not sufficient in base class. So the a pointer to a derived class cannot point to it. While on the other hand the reverse is true.

The statement is correct because, derived class has no knowledge of its parent.

Already derived from that class only, so this can use that class directly, again why to point. So to avoid this efficiency problem language developers already created in that way.

I agree with srikant! there is no way a pointer should be able to point to an object of its parent class type because derived class not know anything about the new members that might have declared in parent class.

I totally agree with Vinod. Derived class object can access members of base class[Depending upon type of inheritance], so need to do that, because C++ Developers have developed compiler in this way only.

When base class is made as virtual then only derived class pointer points to the base class otherwise derived class pointer can't point to the base class.

Because base class allready has its pointer n with this pointer we can access the derived class so no need to declare a pointer in derived class.

Derived classes are derived from their base class so base class has total knowledge of it's derived class n can able to point them but derived class has no ability to point to its base class. Because it's a top down approach.

There is no need of derived class to point to its base class or to it's parent class as it is inherited from it and it would be quite violating the property of inheritance.