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Tip of the Day
Language: C++
Expertise: Intermediate
Oct 6, 1998



Building the Right Environment to Support AI, Machine Learning and Deep Learning

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
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