Why converting a base class pointer to a derived class pointer is consider dangerous by the compiler?

Answer 1

There are two methods of casting one type to another: static casting and dynamic casting (both of which apply to pointers and references to objects). Statically casting a derived class to a base class is typesafe as the base class is guaranteed to exist if the derived class exists. However, static casting from a base class to a derived class is always considered dangerous as the conversion is not typesafe. Dynamic casting exists to cater for this scenario, however it is only possible when the base class is polymorphic (thus ensuring the required runtime information is available). If the conversion is not possible, the return value is NULL. However, it is considered bad programming practice to dynamically cast from a base class to a derived class. If the base class is polymorphic (which it must be), there is NEVER any need to dynamically cast between types. Virtual methods ensure correct behaviour. Whenever you are forced to dynamically cast from a base class to a derived class, consider redesigning the base class interface instead, as it is a clear sign of bad design. It is not dangerous, however; only static casting from a base class to a derived class is considered dangerous.