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Significance of private inheritance in C++...

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The article describes about private inheritance in C++.

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Significance of private inheritance in C++...

  1. 1. SOM-ITSOLUTIONS C++ Private Inheritance SOMENATH MUKHOPADHYAY som-itsolutions  #A2 1/13 South Purbachal Hospital Road Kolkata 700078 Mob: +91 9748185282 Email: ​som.mukhopadhyay@som-itsolutions.com​ / ​som.mukhopadhyay@gmail.com Website:​ ​http://www.som-itsolutions.com/ Blog: ​www.som-itsolutions.blogspot.com
  2. 2. Private inheritance in C++ is an important subject and should be studied thoroughly in order to understand different nuances of C++. The way private inheritance is done is as follows: Class Derived : ​private​ Base{ ….. ….. }; From a programmer’s point of view it means that the public and the protected members of the Base class which will be inherited by the Derived class will become private in the Derived class. However there is a greater significance of private inheritance in C++ from an Object Oriented Designer’s perspective. Private Inheritance actually means that we are not inheriting the interface of the Base class, but we are inheriting the implementation. It means that in the derived class, we can create member functions which can be implemented in terms of the Base class functions. So it is something like the Derived class HAS the Base class and this is also known as Composition. One very useful application of private inheritance is in the Adapter Pattern specifically in the Class Adapter pattern (and not the Object Adapter). Another aspect of private inheritance I must add here. Normally in any OOPS, we can pass an object of a derived class in a function which takes a type of the Base class as a parameter. This is possible because as we say in UML, the subclass is of type SuperClass. Or Subclass IS A SuperClass. But in case of Private Inheritance, we cannot do that because the derived class does not become a subtype of the base class. Hence in case we pass an Object of a privately derived subclass to a method which expects an Object of type Superclass, the compiler will not be able to do the automatic type conversion and will throw a Compile time error. Lets see the following example to understand this. #include​ ​<iostream> #include​ ​<string> using​ ​namespace​ std; class​ ​Fruit​{ private​: string color; int​ calory; protected​: bool​ toBePeeled; public​: Fruit()​:​color(​"RED"​), calory(​100​), toBePeeled(​false​){ }
  3. 3. string getColor(){ return​ color; } void​ setColor(string color){ this​->​color ​=​ color; } int​ getCalory(){ return​ calory; } void​ setCalory(​int​ calory){ this​->​calory ​=​ calory; } bool​ getToBePeeled(){ return​ toBePeeled; } void​ setToBePeeled(​bool​ peel){ this​->​toBePeeled ​=​ peel; } bool​ isToBePeeled(){ return​ (toBePeeled ​==​ ​true​); } }; class​ ​Orange​ ​:​ ​private​ Fruit{ public​: Orange(){ toBePeeled ​=​ ​true​; } }; class​ ​Banana​ ​:​ ​public​ Fruit{ public​: Banana(){ toBePeeled ​=​ ​true​; } }; class​ ​Human​{ public​:
  4. 4. //Human can eat any fruit void​ eatFruit(Fruit​&​ fruit){ string text; if​(fruit.isToBePeeled()){ text ​=​ ​"after peeling it"​; } else​{ text ​=​ ​"without peeling it"​; } cout​<<​"I have eaten a "​ ​<<​fruit.getColor()​<<​" colored fruit " <<​text​<<​" and taken in "​<<​fruit.getCalory()​<<​" calories "​<<​endl; } }; int​ ​main​() { Human me; Orange orange; //i am eating an Orange. As orange is privately derived. Hence the passed object will not be automatically converted to Fruit. hence it will give error... me.eatFruit(orange);//error Banana banana; banana.setColor(​"YELLOW"​); banana.setCalory(​500​); me.eatFruit(banana); //okay return​ ​0​; } As shown in the above example, Orange has been ​privately derived​ from Fruit. Hence Orange ​cannot be considered as a subtype of Fruit​. As a result, in the eatFruit method we cannot pass an orange object in place of the Fruit data type. On the other hand Banana has been ​publicly derived​ from Fruit. Hence​ Banana IS A Fruit​. And hence we can pass a banana object as a parameter of the eatFruit method. Now let us see a practical use of private inheritance to implement Adapter Pattern as described in the Gang of Four book. There are two ways through which the adapter pattern can be implemented - ● Class Adapter ● Object Adapter In class adapter, the Adapter class inherits publicly from the Target class and privately from the Adaptee class as shown below:
  5. 5. So the Adapter class inherits the interface of the Target (here Request()), but it implements the functionality using the privately inherited Adaptee’s functions (here specificRequest). So the client calls the Adapter’s interface which in turn calls Adaptee’s functions.

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