C++ Tutorial Private Inheritance - 2020

Private Inheritance is one of the ways of implementing the has-a relationship.

With private inheritance, public and protected member of the base class become private members of the derived class. That means the methods of the base class do not become the public interface of the derived object. However, they can be used inside the member functions of the derived class.

Because there are implications of using private inheritance, in his book, "Effective C++ 55 ...", Scott Meyers gave a separate item for private inheritance: item 39: "Use private inheritance judiciously."
Let's briefly look at the head of that section showing not is-a but has-a property of private inheritance.

class Person {};
class Student:private Person {};	// private
void eat(const Person& p){}		// anyone can eat
void study(const Student& s){}		// only students study

int main() 
{
	Person p;	// p is a Person
	Student s;	// s is a Student
	eat(p);		// fine, p is a Person
	eat(s);		// error! s isn't a Person
	return 0;
}

He explains, in contrast to public inheritance, compilers will generally not convert a derived class object (Student) into a base class object (Person) if the inheritance relationship between the classes is private. That's why the call to eat() fails for the object s.

With public inheritance, the public methods of the base class become public methods of the derived class. In other words, the derived class inherits the base-class interface (the interface is still visible to outside and can use it). This is the is-a relationship. But with the private inheritance, the public methods of the base class become private methods of the derived class, even if they were protected or public in the base class. So, the derived class does not inherit the base-class interface.

But we should be careful when we talk about private inheritance. Sometimes it is very confusing. The inherit does not mean "own". Suppose, a parent gave a child a secret recipe for a candy under the condition of not releasing the recipe. The child can give variety of candies to other people but not the recipe. With private inheritance, the derived class does enjoy(implement) the inherited interface but does not own the method. Therefore, derived class cannot show the interface to outside world. The only thing that they can show off to the outside is the product whose inner secret workings are hidden.

A class does inherit the implementation with private inheritance.

In his new book, "Programming Principles and Practice Using C++", Stroustrup described the implementation and interface as follows: The interface is the part of the class's declaration that its users access directly. The implementation is the part of the class's declaration that its users access only indirectly through the interface.

Let's look at the following example:

#include <iostream>

using namespace std;

class Engine 
{
 public:
	Engine(int nc){
		 cylinder = nc;
	}

	void start() {
		cout << getCylinder() <<" cylinder engine started" << endl;
	};

	int getCylinder() {
		return cylinder;
	}
	
private:
	int cylinder;
 };
 
 class Car : private Engine
{    // Car has-a Engine
 public:
   Car(int nc = 4) : Engine(nc) { }        
   void start() {
	cout << "car with " << Engine::getCylinder() <<
		   " cylinder engine started" << endl;
	Engine:: start();
   }
 }; 

int main( ) 
{ 
	Car c(8);
	c.start();
	return 0; 
}

The output is:

car with 8 cylinder engine started
8 cylinder engine started

As we see from the example, the Car class winds up with an inherited Engine component such as cylinder and the Car method can use the Engine method, getCylinder(), internally to access the Engine component, cylinder.

In short, private inheritance does acquire the implementation, but does not acquire interface.

From Scott Meyers'book :

1. "Private inheritance is most likely to be a legitimate design strategy when you're dealing with two classes not related by is-a where one either needs access to the protected members of another or needs to redefine one or more its virtual functions."
2. "Private inheritance means is-implemented-in-terms-of. It's usually inferior to composition"
3. "If you make a class D privately inherit from a class B, you do so because you are interested in taking advantage of some of the features available in class B, not because there is any conceptual relationship between objects of types B and D."
4. "Private inheritance means nothing during software design, only during software implementation."

As you may know, the private inheritance is a variant of composition, aggregation, or containment. So, the has-a relationship can be achieved using composition as in the example below.

#include <iostream>

using namespace std;

class Engine 
{
 public:
	Engine(int nc){
		 cylinder = nc;
	}

	void start() {
		cout << getCylinder() <<" cylinder engine started" << endl;
	};

	int getCylinder() {
		return cylinder;
	}

private:
	int cylinder;
 };
 
 class Car 
{
 public:
	Car(int n = 4): eng(n) { }	

	void start() {
	   cout << "car with " << eng.getCylinder() <<
		   " cylinder engine started" << endl;
		eng.start();
	}
private:
	Engine eng;	// Car has-a Engine
 }; 

int main( ) 
{ 
	Car c(8);
	c.start();
	return 0; 
}

This produced the same output as the example for private inheritance.

car with 8 cylinder engine started
8 cylinder engine started

So, what are the similarities and differences between private inheritance and composition?

1. Similarities
   
   1. In both cases, there is exactly one Engine member object contained in every Car object.
   2. In both cases the Car class has a start() method that calls the start() method on the contained Engine object.
   3. In neither case can users (outsiders) convert a Car* to an Engine*.


2. differences
   
   1. The composition is needed if you want to contain several Engines per Car.
   2. The private inheritance can introduce unnecessary multiple inheritance.
   3. The private inheritance allows members of Car to convert a Car* to an Engine*.
   4. The private inheritance allows access to the protected members of the base class.
   5. The private inheritance allows Car to override Engine's virtual functions.

Let's look at the following example which shows the transitions of designs from private inheritance to composition.

Private inheritance lets us inherit the functionality, but not the public interface of another class. In the following code, Circle does not expose any of the member functions of Ellipse.

class Circle : private Ellipse
{
public:
	Circle();
	explicit Circle(float r);

	void setRadius(float r);
	float getRadius() const;
};

But objects of type Circle cannot be passed to code that accepts an Ellipse because the Ellipse base type is not publicly accessible. If we really want to expose a public or protected method of Ellipse in Circle, then we can do this as in the example below:

class Circle : private Ellipse
{
public:
	Circle();
	explicit Circle(float r);

	using Ellipse::getMajorRadius;
	using Ellipse::getMinorRadius;

	void setRadius(float r);
	float getRadius() const;
};

However, preferred is to use composition. This simply means that instead of class A inheriting from B, A declares B as a private data member (has-a) or A declares a pointer or reference to B as a member variable (holds-a):

class Circle
{
public:
	Circle();
	explicit Circle(float r);

	void setRadius(float r);
	float getRadius() const;

private:
	Ellipse mEllipse:
};


void Circle::setRadius(float r)
{
	mEllipse.setMajorRadius(r);
	mEllipse.setMinorRadius(r);
}

float Circle::getRadius()const
{
	return mEllipse.getMajorRadius();
}

The interface for Ellipse is not exposed in the interface for Circle, however, Circle still builds upon the functionality of Ellipse by creating a private instance of Ellipse. Therefore, composition provides the functional equivalent of private inheritance.

Use compostion over private inheritance because inheritance produces a more tightly coupled design.

Given that we can achieve a has-a relationship either with composition or with private inheritance, which should we use? Use composition when you can, but use private inheritance when you have to.

When we look at the class declaration, we see explicitly named objects representing the contained classes, and our code can refer to these objects by name. While the private inheritance makes the relationship seem more abstract. Also, inheritance can raise problems of multi-inheritance.

In short, we're less likely to run into trouble if we use composition. Also, as we summarized in the previous section, composition allows us to include several objects.

On the other hand, private inheritance does offer features that composition can't. Suppose, for example, that a class has protected members. Such members are available to derived classes but not to the world. If we include such a class in another class by using composition, the new class becomes part of that world, not a derived class. So, it can't access protected members. But by using inheritance makes the new class a derived class, so it can access protected members.

Another case that gives private inheritance advantage is if we want to redefine virtual functions. This is a privilege awarded to a derived class but not to a containing class.

I'll use Scott Meyers' statements as a summary.
"Private inheritance means is-implemented-in-terms-of. If you make a class D privately inherit from a class B, you do so because you are interested in taking advantage of some of the features available in class B, not because there is any conceptual relationship between objects of types B and D. As such, private inheritance is purely an implementation technique. (That's why everything you inherit from a private base class becomes private in your class: it's all just implementation detail.)
....
private inheritance means that implementation only should be inherited; interface should be ignored. If D privately inherits from B, it means that D objects are implemented in terms of B objects, nothing more. Private inheritance means nothing during software design, only during software implementation."