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Tip of the Day
Language: C++
Expertise: All
May 20, 1999

Perform Safe Downcasts

A downcast is a cast from a base to a derived object. Before the introduction of RTTI to the language, downcasts were regarded as bad programming practice--they were unsafe and some even considered the reliance on the dynamic type of an object to be a violation of object-oriented principles. You can perform safe downcasts from a virtual base to its derived object using dynamic_cast.
 
struct V
{
  virtual ~V (){} //ensure polymorphism
};
struct A: virtual V {};
struct B: virtual V {};
struct D: A, B {};

#include <iostream>
using namespace std;
int main()
{
 V *pv = new D;
 A* pa = dynamic_cast<A*> (pv); // downcast
 cout<< "pv: "<< pv << " pa: " << pa <<endl;  // pv and pa have different addresses
 return 0;
}
V is a virtual base for classes A and B. D is multiply-inherited from A and B. Inside main(), pv is declared as a "pointer to V" and its dynamic type is "pointer to D". The dynamic type of pv is needed in order to properly downcast it to a pointer to A. Using a static_cast<> in this case would be rejected by the compiler. As the output of the program shows, pv and pa indeed point to different memory addresses.
Danny Kalev
 
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