Inside the C++ Object Model

Chapter 5. Semantics of Construction, Destruction, and Copy

Consider the following abstract base class declaration:

class Abstract_base { 
public: 
   virtual ~Abstract_base() = 0; 
   virtual void interface() const = 0; 
   virtual const char* 
      mumble () const { return _mumble; } 
protected: 
   char *_mumble; 
}; 

Do you see any problems? Although the class is designed to serve as an abstract base class (the presence of a pure virtual function disallows independent instances of Abstract_base to be created), the class still requires an explicit constructor in order to initialize its one data member, _mumble. Without that initialization, a local object of a class derived from Abstract_base will have its instance of _mumble uninitialized. For example:

 class Concrete_derived ...