If no other classes can derive from a class, then you know that all the pointers to that class point to objects of a uniform size. This can simplify memory management and file storage. The idea is to first define a dummy class with a private default constructor. Then virtually derive all the other classes for which you want to inhibit derivation from the dummy class and declare them as friends of the dummy class:
class CDummy{private: //Private default constructor CDummy() {} friend class CX1; friend class CX2; //...};The class CDummy is a virtual base for the classes CX1, CX2.
class CX1 : virtual public CDummy{public: //Constructor, can invoke CDummy's private constructor. CX1() {}};The constructor of CDummy is private, but CX1 is a friend, so it can invoke that private constructor. But here’s what happens if you try to derive a class CY1 from CX1:
class CY1 : public CX1{public: //Constructor, cannot invoke CDummy's private constructor CY1() {}};Because CY1 has CDummy as a virtual base, it must initialize it, but it cannot. You get a compile time error, for example in the case of the VC++ compiler you get the message:
"'CDummy::CDummy' : cannot access private member declared in class'CDummy'"
