Inheritance and Composition

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Inheritance and Composition
Unit - 04

• Reusing the code and functionality

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Unit Introduction

• This unit covers composition and inheritance

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Unit Objectives

• After covering this unit you will understand
• Composition
• Inheritance
• Inheritance constructor call sequence
• Multiple inheritance
• Upcasting inheritance
• C casting operators
• Different types of Inheritance
• Inheritance vs. composition

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Composition (Aggregation)

• Creating and using objects of other class within
  a different class
• Also known as embedded object (or sub-object)

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Example Composition

• class X
• private
• int m_Data // data member
• public
• X() // default constructor
• SetX (int k) // member function
• m_Data k
• class Y
• private
• int m_Data

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Example Composition (contd.)

• public
• X x // composition
• Y() //default constructor
• void main()
• Y y // creating object of class Y
• y.x.SetX(20) // Access/sets the embedded
  object

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Inheritance

• Parent class is called the base class
• The class which inherits the base class is called
  the derived class and has all the features of
  the base class, plus its own features

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Example Inheritance

• class X
• int m_Data
• public
• X()
• class Y public X // class Y publicly inherits
  class X
• int m_Data
• public
• Y()

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Constructor and Inheritance

• The constructor and destructor calls are
  automatic for the base class, in case the derived
  class object is created
• The order of base and derived class constructors
  and destructors are
• base constructor
• derived constructor
• derived destructor
• base destructor

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Multiple Inheritance

• There could be two or more base class for a
  derived class
• It can cause ambiguity and is not recommended in
  normal cases
• There are ways to have single inheritance used
  instead of multiple inheritance to achieve the
  goal
• class Z public X, public Y
• // class definition

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Example Multiple Inheritance

• class A
• public
• void SomeFunction(void)
• // function code
• class B
• public
• void SomeFunction(void)
• // function code

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Example Multiple Inheritance (contd.)

• class C public A, public B
• public
• void MyFunction(void)
• SomeFunction() // Ambiguous as the compiler
  // does not know whether to //choose
  SomeFunction() of A or B

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Upcasting and Downcasting

• Casting from derived to base class is known as
  Upcasting (implicit)
• Upcasting lose the derived class properties
• Casting from base to derived class is known as
  Downcasting
• Downcasting makes derived classs properties
  available

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Example Upcasting and Downcasting
class Person private int Age char
Name char Gender public // accessor
methods for member variables class Employee
public Person private char
Department int EmployeeCode public //
accessor methods for member variables
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Example Upcasting and Downcasting (contd.)
void main() Person pPerson new
Employee() // upcast, implicit pPerson-gtGetName(
) // fine pPerson-gtGetDepartment() //
error Employee pEmployee (Employee)pPerson /
/ downcast pEmployee-gtGetDepartment() // fine
now
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C Casting Operators

• Replace the traditional, parenthetic casting
  technique
• Explicit
• Self documenting
• Signal a type conversion
• Less risk of illegal cast and data corruption

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C Casting Operators (contd.)

• static_cast
• Compile-time cast
• Casting classes that provide static polymorphism
  (non-virtual classes)
• dynamic_cast
• Run-time cast, to cast classes that provide
  dynamic polymorphism (virtual classes)
• const_cast
• To remove const-ness
• reinterpret_cast
• For conversion of unrelated types

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Example static_cast
// Note that Base is not a virtual class class
Base class Derived public Base void
f(Base pBase) Derived pDerived
static_castltDerivedgtpBase // Also, enum
fruitapple0, orange1, banana2 int x
2 fruit f1 static_castltfuitgt(x) // converts
an int to enum
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Example dynamic_cast
// Note that Base is a virtual class class
Base virtual void f() class Derived
public Base void f () void f(Base
pBase) //Derived pDerived
static_castltDerivedgtpBase // error // use
dynamic_cast instead Derived pDerived
dynamic_castltDerivedgtpBase // fine
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Example const_cast

• double f(double d) // f accepts reference to
  type double
• void g(const double d)
• // while calling f() the const-ness of d is
  removed
• double val f( const_castltdoublegt(d) )

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Example reinterpret_cast
// a practical example could returns hash code
based on an // address unsigned short Hash(void
p) // reinterpret casting a pointer to
integral value unsigned int val
reinterpret_castltunsigned intgt (p) val val
16 // hash code formula return (unsigned
short)val // cast to the returned type void
main() int a10 for (int i 0 i lt 10
i) cout ltlt Hash(a i) ltlt endl //
using Hash()
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Inheritance Types

• Inheritance has three types
• public
• private
• protected
• The following chart gives the accessibility
  details of the inheritance types

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Inheritance Types (contd.)

• Protected Function in the base class is treated
  as Private Function in the child class (taking
  the example of the value in bold), when inherited
  using the private keyword
• class X public Y, private Z, protected W
• // class definition

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Inheritance vs. Composition

• Composition is generally used when you want the
  features of an existing class inside your new
  class, but not its interface
• Use inheritance for sub-typing, where you want
  your new type to have exactly the same interface
  as the existing type (plus any other member
  functions you want to add)

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Unit Summary

• In this unit you have covered
• Composition
• Inheritance
• Multiple inheritance
• Upcasting inheritance
• C casting operators
• Types of inheritance
• Inheritance vs. composition