[OOP - Lec 20,21] Inheritance

Inheritance
Speaker: Muhammad Hammad Waseem
m.hammad.wasim@gmail.com

Inheritance
• Inheritance: The mechanism by which one class can inherit the
properties of another.
• Inheritance: A parent-child relationship between classes
• Inheritance allows sharing of the behavior of the parent class into its
child classes.
• child class can add new behavior or override existing behavior from parent
• It allows a hierarchy of classes to be built, moving from the most
general to the most specific.
• Eg: point -> 3D_point -> sphere…

Base Class, Derived Class
• Base Class
• Terms to describe the parent in the relationship, which shares its functionality
• Also called Superclass, Parent class
• Derived Class
• Terms to describe the child in the relationship, which accepts functionality
from its parent
• Also called Subclass, Child class
• General Syntax:
• class derivedClassName : AccessSpecifier
baseClassName{… … …}

Example: Base Class
class base
{
int x;
public:
void setx(int n)
{ x = n; }
void showx()
{ cout << x << ‘n’ }
};

Example: Derived Class
// Inherit as public
class derived : public base {
int y;
public:
void sety(int n)
{ y = n; }
void showy()
{ cout << y << ‘n’;}
};

Access Specifier: public
• The 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
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: private
• If the access specifier is private:
• public members of base become private members of
derived.
• these members are still accessible by member functions
of derived.

// Inherit as private
class 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!
}

class derived : private base {
int y;
public:
// setx is accessible from within derived
void setxy(int n, int m) { setx(n); y = m; }
// showx is also accessible
void showxy() { showx(); cout<<y<< ‘n’;}
};

Protected Members
• Sometimes 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 members
protected:
// protected members
public:
// public members
};

3 Types of Access Specifiers
• Type 1: Inherit as Private
Base Derived
Private members Inaccessible
Protected members Private members
Public members Private members

• Type 2: Inherit as Protected
Base Derived
Protected members Protected members
Public members Protected members

• Type 3: Inherit as Public
Base Derived
Protected members Protected members
Public members Public members

Constructor and Destructor
• It 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 base
class base {
int i;
public:
base(int n) {
cout << “constructing base n”;
i = n; }
~base() { cout << “destructing base n”; }
};

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

Example: main()
int main() {
derived o(10,20);
return 0;
}
OUTPUT:
constructing base
constructing derived
destructing derived
destructing base

Multiple Inheritance
• Type 1:
Base 1
derived 1
derived 2

base 1 base 2
derived
Multiple Inheritance
• Type 2:

Example: Type 2 (first base class)
// Create first base class
class B1 {
int a;
public:
B1(int x) { a = x; }
int geta() { return a; }
};

Example: Type 2 (second base class)
// Create second base class
class B2 {
int b;
public:
B2(int x) { b = x; }
int getb() { return b; }
};

Example: Type 2 (inherit two base classes)
// 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;}
} ;

Potential Problem
• Base is inherited twice by Derived 3!
Derived 3
Base Base
Derived 1 Derived 2

Virtual Base Class
• To resolve this problem, virtual base class can be used.
class base {
public:
int i;
};

Virtual Base Class
// Inherit base as virtual
class 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 Classes
• A 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 pointer
base 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 Function
• A 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
public:
derived (int x) : base (x) {}
// The keyword virtual is not needed.
void func() {cout << i * i; }
};

Example
int 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 Functions
• A 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 function
virtual 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;
}
};

[OOP - Lec 20,21] Inheritance

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