Inheritance

Association

Dependency

The 2nd principle of Object Oriented Programming

We use Unified Modeling Language (UML) to shows the relationship between classes & objects graphically

Base class or superclass - the general class

Derived class or subclass - a specific class

A derived class extends its base class;

• Derived class inherits all of the data members & member functions of the base class (with the exception of constructors, destructor, & assignment operators that need to be redefined)
• It can create new data members & member functions

UML diagram for inheritance

Types of inheritance in C++;

• Private inheritance (default type)
• Protected inheritance
• Public inheritance

Use (:) after a base class (e.g. B-base & D-derived class)

Types of inheritance

A private member in the base class becomes an inaccessible (hidden) member in the derived class

A public member in the base class becomes a public member in the derived class

Public inheritance is the most common. e.g.

• Class Student inherits from the class Person

Protected inheritance is rare and virtually never used

Private inheritance does not define a type inheritance; it defines an implementation inheritance

Public members of the base class become private members in the derived class

This property allows inherited implementation (code reuse)

Bitbucket: Inheritance

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Forms of inheritance

• Single inheritance
• Multiple inheritance
• Hierarchical inheritance
• Multilevel inheritance
• Hybrid Inheritance (also known as Virtual Inheritance)

Bitbucket: Multiple inheritance

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Bitbucket: Multilevel inheritance

Home: Multilevel inheritance

Bitbucket: Hierarchical inheritance

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Bitbucket: Hybrid inheritance

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Not all relationships between classes can be defined as inheritance

it models the has-a relationship

An association between two classes shows a relationship. For example, we can define a class named Person and another class named Address

A person lives in an address and the address is occupied by a person. The Address class is not inherited from the Person class; neither the other way

1. Aggregation

• An aggregation is a one-to-may relationship from the aggregator to the aggregatee
• In an aggregation, the lifetime of the aggregatee is independent of the lifetime of the aggregator

Example

2. Composition

• Composition is a special kind of aggregation in which the lifetime of the containee depends on the lifetime of the container
• The distinction between aggregation and composition is normally conceptual; it depends on how the designer thinks about the relationship

Dependency is a weaker relationship than inheritance or association

Dependency models the uses relationship

For Class A & B, dependency happens when

• Class A uses an object of type B as a parameter in a member function
• Class A has a member function that returns an object of type B
• Class A has a member function that has a local variable of type B

Queries about this Lesson, please send them to: jmwaura.uni@gmail.com

*References*

• C++ Programming - An Object-Oriented Approach, 2019 Behrouz, Richard et.al
• Accelerated C++ - Practical Programming by Example, 2000 Andrew, Barbara et.al