1. COP 3330 Object-oriented Programming in C++
Composition
Spring 2019

2. Object as Class Members
Aggregation is a relationship between objects
Embed an object (or a pointer) of one class type as member data of another class type
Composition refers to a stronger form of aggregation
Embedded objects would typically not exist independent of the container object
Is also known as the “has a” relationship
Engine object inside a Car object as member data
52 Card objects as member data of class Deck
Promotes the idea of tool building
Objects of a class can be used as components inside other classes

3. Let’s Build a Timer
Hour Display
Minute Display
Second Display

4. Timer & Display The user of Timer is the main program
The user of Display objects is Timer object
Timer object will call Display functions through its member data objects (hours, minutes, seconds)
Timer Class
Display hours
Display minutes
Display seconds
0..23
0..59
0..59

5. Display Class

6. Timer Class
Three Display components

7. Constructors/Destructors for Embedded Objects
When an object (Timer) is created, its constructor runs and also invokes the constructors of its embedded objects (Display)
If nothing is done, it will invoke the default constructor
Use initialization lists to invoke a constructor with parameters for embedded objects
When an object (Timer) is destroyed, its destructor runs and also invokes the destructors of its embedded objects (Display)
The life span of a Timer object is longer than, and fully covers, those of Display objects

8. Constructors for Embedded Objects
Normal creation of
Display objects
Creation of Display objects in the Timer Constructor
Display hours(24); Display minutes(60); Display seconds(60);
Initialization cannot be performed in the class declaration
Need to use an initialization list
Display has no default constructor
Timer::Timer() : hours(24),
minutes(60),
Seconds(60) { }

9. Questions