When working with object-oriented programming languages like C++, constructors play a crucial role in creating and initializing objects. Constructors allow you to define how objects are instantiated and set up their initial state. By default, constructors are public, meaning they can be accessed and called from anywhere in the program.

However, there are cases when you want to restrict the access to a constructor and limit its invocation to certain situations, such as within derived classes or friend functions. In such cases, you can use protected constructors in C++.

What is a protected constructor?

A protected constructor is a member function of a class that has restricted access. It can only be accessed by the derived classes of the same class or friend functions. The constructor is not accessible to any other classes or functions outside the class hierarchy.

Here’s an example to illustrate how to use a protected constructor in C++:

class Parent {
protected:
    Parent() {
        // Constructor code goes here
    }
};

class Child : public Parent {
public:
    Child() {
        // Child class can access the protected constructor
    }
};

int main() {
    // Parent parent; // Error: Cannot access protected constructor
    Child child; // Child class can access protected constructor
    return 0;
}

In the example above, we have a Parent class with a protected constructor. The Child class, derived from Parent, can access this protected constructor and instantiate objects of the Child class. However, trying to directly instantiate the Parent class outside its hierarchy will result in a compilation error.

Why use protected constructors?

Protected constructors provide a way to control the instantiation of objects, ensuring that only derived classes or friend functions can create instances of a class. This restriction can be useful in several scenarios, including:

• Implementing class hierarchies: By making the constructor protected, you can enforce that objects of a particular class are only created within the hierarchy, preventing external code from creating objects that don’t adhere to the class hierarchy.

• Applying object creation policies: You can use protected constructors to enforce certain rules or initialization procedures when creating objects. Only derived classes or friend functions that are aware of these rules can create objects and ensure proper initialization.

• Preventing direct instantiation of base classes: In some cases, you may want to prevent direct instantiation of a base class and ensure that only derived classes are used. By making the base class constructor protected, you can enforce this restriction and promote the use of derived classes instead.

Conclusion

Protected constructors in C++ provide a way to restrict access to the construction of objects. By making the constructor protected, you can ensure that only derived classes or friend functions can create instances of a class. This feature allows for better control over object instantiation and can be valuable in managing class hierarchies and enforcing object creation policies.

Remember to use protected constructors when it aligns with your design and encapsulation requirements. Happy coding!

#CPlusPlus #ProtectedConstructors