InheritanceThe mechanism by which one class

can inherit the properties of another.

It allows a hierarchy of classes to be built, moving from the most general to the most specific.

Base Class, Derived ClassBase Class

Defines all qualities common to any derived classes.

Derived Class Inherits those general properties and

adds new properties that are specific to that class.

Example: Base Class

class base {int x;

public:void setx(int n) { x = n; }void showx() { cout << x << ‘\n’ }

};

Example: Derived Class

// Inherit as publicclass derived : public base {

int y;public:

void sety(int n) { y = n; }void showy() { cout << y << ‘\n’;}

};

Access Specifier: publicThe keyword public tells the compiler

that base will be inherited such that: all public members of the base class

will also be public members of derived.

However, all private elements of base will remain private to it and are not directly accessible by derived.

Example: main()

int main() {derived ob;ob.setx(10);ob.sety(20);ob.showx();ob.showy();

}

An incorrect example

class derived : public base {int y;

public:void sety(int n) { y = n; }/* Error ! Cannot access x, which is private member of base. */void show_sum() {cout << x+y; }

};

Access Specifier: privateIf the access specifier is private:

public members of base become private members of derived. these members are still accessible by member functions of derived.

Example: Derived Class

// Inherit as privateclass derived : private base {

int y;public:

void sety(int n) { y = n; }void showy() { cout << y << ‘\n’;}

};

Example: main()

int main() {derived ob;ob.setx(10); // Error! setx() is private.ob.sety(20); // OK!ob.showx(); // Error! showx() is private.ob.showy(); // OK!

}

Example: Derived Classclass derived : private base {int y;public:// setx is accessible from within derivedvoid setxy(int n, int m) { setx(n); y = m; }// showx is also accessiblevoid showxy() { showx(); cout<<y<< ‘\n’;}};

Protected MembersSometimes you want to do the following:

keep a member of a base class private allow a derived class access to it

Use protected members!If no derived class, protected

members is the same as private members.

Protected Members

The full general form of a class declaration:class class-name {// private membersprotected:// protected memberspublic:// public members};

3 Types of Access SpecifiersType 1: inherit as private

Base Derived

private members inaccessible

protected members private members

public members private members

3 Types of Access SpecifiersType 2: inherit as protected

Base Derived

private members inaccessible

protected members protected members

public members protected members

3 Types of Access SpecifiersType 3: inherit as public

Base Derived

private members inaccessible

protected members protected members

public members public members

Constructor and DestructorIt is possible for both the base class

and the derived class to have constructor and/or destructor functions.

The constructor functions are executed in order of derivation. i.e.the base class constructor is executed

first.The destructor functions are executed

in reverse order.

Passing arguments What if the constructor functions of

both the base class and derived class take arguments?

1. Pass all necessary arguments to the derived class’s constructor.

2. Then pass the appropriate arguments along to the base class.

Example: Constructor of baseclass base {

int i;public:

base(int n) {cout << “constructing base \n”; i = n; }~base() { cout << “destructing base \n”; }

};

Example: Constructor of derived

class derived : public base {int j;

public:derived (int n, int m) : base (m) {cout << “constructing derived\n”;j = n; }~derived() { cout << “destructing derived\n”;}

};

Example: main()int main() {

derived o(10,20);return 0;

}constructing baseconstructing deriveddestructing deriveddestructing base

Multiple InheritanceType 1:

base 1

derived 1

derived 2

Multiple Inheritance

• Type 2:

base 1 base 2

derived

Example: Type 2

// Create first base classclass B1 {

int a;public:

B1(int x) { a = x; }int geta() { return a; }

};

Example: Type 2

// Create second base classclass B2 {

int b;public:

B2(int x) { b = x; }int getb() { return b; }

};

// Directly inherit two base classes.class D : public B1, public B2 {int c;public:D(int x, int y, int z) : B1(z), B2(y) {c = x; }void show() {cout << geta() << getb() << c;}} ;

Example: Type 2

Potential Problem

Base is inherited twice by Derived 3!

Base Base

Derived 1 Derived 2

Derived 3

Virtual Base ClassTo resolve this problem, virtual base class can be used.

class base {public:

int i;};

Virtual Base Class// Inherit base as virtualclass D1 : virtual public base {public: int j; };class D2 : virtual public base {public: int k; };

Virtual Base Class

/* Here, D3 inherits both D1 and D2. However, only one copy of base is present */class D3 : public D1, public D2 {public:

int product () { return i * j * k; }};

Pointers to Derived ClassesA pointer declared as a pointer to

base class can also be used to point to any class derived from that base.

However, only those members of the derived object that were inherited from the base can be accessed.

Example

base *p; // base class pointerbase B_obj;derived D_obj;p = &B_obj; // p can point to base object p = &D_obj; // p can also point to derived // object

Virtual FunctionA virtual function is a member

function declared within a base class redefined by a derived class (i.e.

overriding)

It can be used to support run-time polymorphism.

Example

class base {public:

int i;base (int x) { i = x; }virtual void func() {cout << i; }

};

Example

class derived : public base {public:

derived (int x) : base (x) {}// The keyword virtual is not needed.void func() {cout << i * i; }

};

Exampleint main() {base ob(10), *p;derived d_ob(10);

p = &ob;p->func(); // use base’s func()p = &d_ob;p->func(); // use derived’s func()}

Pure Virtual FunctionsA pure virtual function has no definition relative to the base class. Only the function’s prototype is included.General form:

virtual type func-name(paremeter-list) = 0

Example: area

class area {public:

double dim1, dim2; area(double x, double y) {dim1 = x; dim2 = y;}// pure virtual functionvirtual double getarea() = 0;

};

Example: rectangle

class rectangle : public area {public:// function overriding

double getarea() {return dim1 * dim2;

}};

Example: triangle

class triangle : public area {public:// function overriding

double getarea() {return 0.5 * dim1 * dim2;

}};