Use Protected Constructors to Block Undesirable Object Instantiation

Use Protected Constructors to Block Undesirable Object Instantiation

In order to block creation of class instances, you can declare its constructor as protected.

 
class CommonRoot {
	protected: CommonRoot(){}//no objects of this class can be instantiated
};

class Derived: public CommonRoot {
public: Derived() {}
};

Derived d; // OK, constructor of d has access to any protected member in its base class
CommonRoot cr; //compilation error: attempt to access a protected member of CommonRoot

The same effect of blocking instantiation of a class can be achieved with pure virtual functions. However, these add runtime and space overhead. When pure virtual functions aren't needed, you can use a protected constructor instead.

Danny Kalev