Gotcha #35: Unchecked Downcasting

Casting a base class pointer to a derived class pointer ("downcasting") may result in a bad address, as illustrated in the example below and in Figure 4-3. The delta arithmetic performed by the compiler on the casted pointer assumes that the base class address belongs to a base class part of the derived class:

class A { public: virtual ~A(); }; 


class B { public: virtual ~B(); };


class D : public A, public B {};


class E : public B {};


B *bp = getMeAB(); // get an object derived from B


D *dp = static_cast<D*>(bp); // safe???

The best approach is to design so that downcasting is unnecessary; systematic use of downcasts is often an indication of bad design. If a downcast really is required, it's often a good idea to use a dynamic_cast, which will perform a runtime check to ensure that the cast is correct:

if( D *dp = dynamic_cast<D *>(bp) ) { 


   // cast succeeded


}


else {


   // cast failed


}