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C++ Inheritance

Definition

Inherit Definition - Derive quality and characteristics from parents or ancestors. Like you inherit features of your parents.

Example: "She had inherited the beauty of her mother"

Inheritance in Object Oriented Programming can be described as a process of creating new classes from existing classes.

New classes inherit some of the properties and behavior of the existing classes. An existing class that is "parent" of a new class is called a base class. New class that inherits properties of the base class is called a derived class.

Inheritance is a technique of code reuse. It also provides possibility to extend existing classes by creating derived classes.

Inheritance Syntax

The basic syntax of inheritance is:

class DerivedClass : accessSpecifier BaseClass

Access specifier can be public, protected and private. The default access specifier is private. Access specifiers affect accessibility of data members of base class from the derived class. In addition, it determines the accessibility of data members of base class outside the derived class.

Inheritance Access Specifiers

Public Inheritance

This inheritance mode is used mostly. In this the protected member of Base class becomes protected members of Derived class and public becomes public.

class DerivedClass : public BaseClass
 Accessing Base class members public protected private
 From Base class  Yes  Yes  Yes
 From object of a Base class  Yes  No  No
 From Derived classes  Yes (As Public)  Yes (As Protected)  No
 From object of a Derived class  Yes  No  No
 From Derived class of Derived Classes  Yes (As Public)  Yes (As Protected)  No

Derived class of Derived Classes: If we are inheriting a derived class using a public inheritance as shown below

class B : public A

class C : public B

then public and protected members of class A will be accessible in class C as public and protected respectively.

Protected Inheritance

In protected mode, the public and protected members of Base class becomes protected members of Derived class.

class DerivedClass : protected BaseClass
 Accessing Base class members public protected private
 From Base class  Yes  Yes  Yes
 From object of a Base class  Yes  No  No
 From Derived classes  Yes (As Protected)  Yes (As Protected)  No
 From object of a Derived class  No  No  No
 From Derived class of Derived Classes  Yes (As Protected)  Yes (As Protected)  No

Derived class of Derived Classes: If we are inheriting a derived class using a public inheritance as shown below

class B : protected A

class C : protected B

then public and protected members of class A will be accessible in class C as protected

Private Inheritance

In private mode the public and protected members of Base class become private members of Derived class.

class DerivedClass : private BaseClass
class DerivedClass : BaseClass   // By default inheritance is private
 Accessing Base class members public protected private
 From Base class  Yes  Yes  Yes
 From object of a Base class  Yes  No  No
 From Derived classes  Yes (As Private)  Yes (As Private)  No
 From object of a Derived class  No  No  No
 From Derived class of Derived Classes  No  No  No

Derived class of Derived Classes: If we are inheriting a derived class using a public inheritance as shown below

class B : private A

class C : private B

then public and protected members of class A will not be accessible in class C


Types of Inheritance

There are different types of inheritance:

  1. Single Inheritance
  2. Multiple Inheritance
  3. Multilevel Inheritance
  4. Hierarchical Inheritance
  5. Hybrid (Virtual) Inheritance

Single Inheritance

Single inheritance represents a form of inheritance when there is only one base class and one derived class. For example, a class describes a Person:

single-inheritance

Example of Single Inheritance

//base class
class Person
{
public:
	Person(string szName, int iYear)
	{
		m_szLastName = szName;
		m_iYearOfBirth = iYear;
	}
	string m_szLastName;
	int m_iYearOfBirth;
	void print()
	{
		cout << "Last name: " << szLastName << endl;
		cout << "Year of birth: " << iYearOfBirth << endl;
	}
protected:
	string m_szPhoneNumber;
};

We want to create new class Student which should have the same information as Person class plus one new information about university. In this case, we can create a derived class Student:

//derived class
class Student:public Person
{
public:
	string m_szUniversity;
};

Class Student is having access to all the data members of the base class (Person).

Since class Student does not have a constructor so you can create a constructor as below

//will call default constructor of base class automatically
Student(string szName, int iYear, string szUniversity)
{
	m_szUniversity = szUniversity;
}

If you want to call the parameterized(user defined) constructor of a base class from a derived class then you need to write a parameterized constructor of a derived class as below

Student(string szName, int iYear, string szUniversity) :Person(szName, iYear)
{
	m_szUniversity = szUniversity;
}

Person(szName, iYear) represents call of a constructor of the base class Person. The passing of values to the constructor of a base class is done via member initialization list.

We can access member functions of a base class from a derived class. For example, we can create a new print() function in a derived class, that uses print() member function of a base class:

void print()
{
	//call function print from base class
	Person::print();
	cout << "University " << m_szUniversity << endl;
}

If you want to call the member function of the base class then you have to use the name of a base class

Multiple Inheritance

Multiple inheritance represents a kind of inheritance when a derived class inherits properties of multiple classes. For example, there are three classes A, B and C and derived class is D as shown below:

multiple-inheritance

If you want to create a class with multiple base classes, you have to use following syntax:

Class DerivedClass: accessSpecifier BaseClass1, BaseClass2, …, BaseClassN

Example of Multiple Inheritance

class A
{
	int m_iA;
	A(int iA) :m_iA(iA)
	{
	}
};

class B
{
	int m_iB;
	B(int iB) :m_iB(iB)
	{
	}
};

class C
{
	int m_iC;
	C(int iC) :m_iC(iC)
	{
	}
};

You can create a new class that will inherit all the properties of all these classes:

class ABC :public A, public B, public C
{
	int m_iABC;
	//here you can access m_iA, m_iB, m_iC 
};

Multilevel Inheritance

Multilevel inheritance represents a type of inheritance when a Derived class is a base class for another class. In other words, deriving a class from a derived class is known as multi-level inheritance. Simple multi-level inheritance is shown in below image where Class A is a parent of Class B and Class B is a parent of Class C

Example of Multi-Level Inheritance

Below Image shows the example of multilevel inheritance

As you can see, Class Person is the base class of both Student and Employee classes. At the same time, Class Student is the base class for ITStudent and MathStudent classes. Employee is the base class for Driver and Engineer classes.

The code for above example of multilevel inheritance will be as shown below

class Person 
{ //content of class person }; class Student :public Person { //content of Student class }; class Employee : public Person { //content of Employee class }; class ITStundet :public Student { //content of ITStudent class }; class MathStundet :public Student { //content of MathStudent class }; class Driver :public Employee { //content of class Driver }; class Engineer :public Employee { //content of class Engineer };

Hierarchical Inheritance

When there is a need to create multiple Derived classes that inherit properties of the same Base class is known as Hierarchical inheritance

class base
{
	//content of base class
};

class derived1 :public base
{
	//content of derived1
};

class derived2 :public base
{
	//content of derived
};

class derived3 :public base
{
	//content of derived3
};

class derived4 :public base
{
	//content of derived4
};

Hybrid Inheritance (also known as Virtual Inheritance)

Combination of Multi-level and Hierarchical inheritance will give you Hybrid inheritance.

Diamond Problem

When you have a hybrid inheritance then a Diamond problem may arise. In this problem a Derived class will have multiple paths to a Base class. This will result in duplicate inherited members of the Base class. This kind of problem is known as Diamond problem

Virtual Inheritance

We can avoid Diamond problem easily with Virtual Inheritance. Child classes in this case should inherit Grandparent class by using virtual inheritance:

class Grandparent
{
	//content of grandparent class
};

class Child1 :public virtual Grandparent
{
	//content of Child1 class
};

class Child2 :public virtual Grandparent
{
	//content of Child2 class
};

class grandson :public Child1, public Child2
{
	//content of grandson class
};

Now grandson class will have only one copy of data members of the Grandparent class.

Order of Constructor Call

When a default or parameterized constructor of a derived class is called, the default constructor of a base class is called automatically. As you create an object of a derived class, first the default constructor of a base class is called after that constructor of a derived class is called.

To call parameterized constructor of a base class you need to call it explicitly as shown below.

Student(string szName, int iYear, string szUniversity) :Person(szName, iYear)
{

}

Below program will show the order of execution that the default constructor of base class finishes first after that the constructor of a derived class starts. For example, there are two classes with single inheritance:

//base class
class Person
{
public:
	Person()
	{
		cout  << "Default constructor of base class called" << endl;
	}
	Person(string lName, int year)
	{
		cout  << "Parameterized constructor of base class called" << endl;
		lastName = lName;
		yearOfBirth = year;
	}
	string lastName;
	int yearOfBirth;
};

//derived class
class Student :public Person
{
public:
	Student()
	{
		cout  << "Default constructor of Derived class called" << endl;
	}
	Student(string lName, int year, string univer)
	{
		cout  << "Parameterized constructor of Derived class called" << endl;
		university  = univer;
	}
	string university;
};

There is no explicit call of constructor of a base class. But on creating two objects of Student class using default and parameterized constructors, both times default constructor of a base class get called.

Student student1; //Using default constructor of Student class
Student student2("John", 1990, "London School of  Economics"); //calling parameterized constructor of Student class

In both the above cases, default constructor of a base class is called before the constructor of a derived class.

Default constructor of base class called
Default constructor of Derived class called
Default constructor of base class called
Parameterized constructor of Derived class called

When multiple inheritance is used, default constructors of base classes are called in the order as they are in inheritance list. For example, when a constructor of derived class is called:

class derived: public class1, public class 2

the order of constructors calls will be

class1 default constructor
class2 default constructor
derived constructor

If you want to call a parameterized constructor of the base class then this can be done using initializer list as shown below.

Student(string lName, int year, string univer) :Person(lName, year)
{
	cout << "Parameterized constructor of Derived class works" << endl;
	university  = univer;
}

Above code means that you are calling parametrized constructor of the base class and passing two parameters to it. Now the output will be

Default constructor of base class works
Default constructor of Derived class works
Parameterized constructor of base class works
Parameterized constructor of Derived class works

Now you can see that parameterized constructor of the base class is called from derived class constructor.

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