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dynamic_cast can be used only with pointers and references to objects. Its purpose is to ensure that the result of the type conversion is a valid complete object of the requested class.
- Anonymous September 27, 2011Therefore, dynamic_cast is always successful when we cast a class to one of its base classes:
class CBase { };
class CDerived: public CBase { };
CBase b; CBase* pb;
CDerived d; CDerived* pd;
pb = dynamic_cast<CBase*>(&d); // ok: derived-to-base
pd = dynamic_cast<CDerived*>(&b); // wrong: base-to-derived
The second conversion in this piece of code would produce a compilation error since base-to-derived conversions are not allowed with dynamic_cast unless the base class is polymorphic.
When a class is polymorphic, dynamic_cast performs a special checking during runtime to ensure that the expression yields a valid complete object of the requested class:
// dynamic_cast
#include <iostream>
#include <exception>
using namespace std;
class CBase { virtual void dummy() {} };
class CDerived: public CBase { int a; };
int main () {
try {
CBase * pba = new CDerived;
CBase * pbb = new CBase;
CDerived * pd;
pd = dynamic_cast<CDerived*>(pba);
if (pd==0) cout << "Null pointer on first type-cast" << endl;
pd = dynamic_cast<CDerived*>(pbb);
if (pd==0) cout << "Null pointer on second type-cast" << endl;
} catch (exception& e) {cout << "Exception: " << e.what();}
return 0;
}