1. EECE.3220 Data Structures Instructor: Dr. Michael Geiger Spring 2019
Lecture 14:
Class relationships

2. Announcements/reminders
Program 2 to be posted; due TBD
Will implement dictionary
Each entry contains word, part of speech, definition
Load contents of dictionary from file
Implement string-based commands to interact
Friday, 3/8: deadline for P1 resubmissions
Exams to be returned next week
5/24/2019
Data Structures: Lecture 11

3. Data Structures: Lecture 11
Today’s lecture
Review
Abstract data types
Class basics
More details on classes
Class definitions
Mutators/accessors
Constructors
Composition
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Data Structures: Lecture 11

4. Data Structures: Lecture 11
Review: Classes
Classes: programmer-defined types
Concrete implementation of ADT
Objects: instances of a class
Each class typically contains
Data members: attributes for each object
Each object has own copy of data members
Member functions: Tasks specific to class
Data/functions can be:
Public: accessible anywhere
Private: accessible only in member functions
Members private by default
Private functions also known as helper functions
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Data Structures: Lecture 11

5. Data Structures: Lecture 11
Designing a Class
Public members of class accessible to everyone
Most function members are public
Private members of class accessible only in member functions
Data members almost always private
Some private function members (helper or utility functions)
Class definition in .h file (i.e., Time.h)
Data members
Member function prototypes
Friend function prototypes
Function definitions in .cpp file (i.e., Time.cpp)
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Data Structures: Lecture 11

6. Data Structures: Lecture 11
Class relationships
Typically have multiple objects in program
Different types may interact with one another
Basic interactions: association
One class “uses” another in some way
Example (from text): ATM “executes” a Withdrawal
Classes as data members: “has a”
Two such relationships: aggregation and composition
Aggregation: “parent” contains pointer to “child”
Composition: “parent” contains object of “child” type
Like nested structures
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Data Structures: Lecture 11

7. Data Structures: Lecture 11
Composition example
A rectangle is a shape that has a:
point of origin
width
height
Can implement this concept by defining a class named Rectangle
Methods might include:
Accessing width/height/origin
Setting width/height/origin
Calculating area
.h files on next two slides
Most function definitions self-explanatory
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Data Structures: Lecture 11

8. Data Structures: Lecture 11
Point.h
class Point { public: Point(); // Default constructor Point(double X, double Y); // Parameterized constructor void setX(double newX); // Set X coordinate void setY(double newY); // Set Y coordinate double getX(); // Returns X coordinate double getY(); // Returns Y coordinate void printPoint(ostream &out); // Output Point as // (xCoord, yCoord) private: double xCoord; // X coordinate double yCoord; // Y coordinate };
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Data Structures: Lecture 11

9. Data Structures: Lecture 11
Rectangle.h
class Rectangle { public: Rectangle(); // Default constructor Rectangle(double h, double w, // Parameterized const. double x, double y); double getHeight(); // Return height double getWidth(); // Return width Point getOrigin(); // Return origin void setHeight(double h); // Change height void setWidth(double w); // Change width void setOrigin(Point p); // Change origin double area(); // Return area of rectangle private: double width; double height; Point origin; // Lower left corner };
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Data Structures: Lecture 11

10. Example code: setOrigin()
void Rectangle::setOrigin(double x, double y) { origin.xCoord = x; // Won’t work origin.setY(y); }
Slightly different version than in .h file
Takes two doubles, not Point
Example shows two different ways of accessing elements of Point
Directly changing private data still won’t work
Must use set functions
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Data Structures: Lecture 11

11. Data Structures: Lecture 11
Composition example
Write code for:
Point Rectangle::getOrigin();
void Rectangle::setOrigin(Point p);
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Data Structures: Lecture 11

12. Cleaning up Wednesday’s mess (1/?) …
On Wednesday, I showed you these solutions …
Point Rectangle::getOrigin() {
return origin; }
void Rectangle::setOrigin(Point p) {
origin.setX(p.getX());
origin.setY(p.getY());
… which prompted a good question: why can we copy origin as a return value, but can’t copy an argument to origin?
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Data Structures: Lecture 11

13. Cleaning up Wednesday’s mess (2/?) …
The short answer: I was wrong
In this case, you absolutely could write setOrigin() as follows:
void Rectangle::setOrigin(Point p) {
origin = p; }
So, why did I show you the other version?
5/24/2019
Data Structures: Lecture 11

14. Cleaning up Wednesday’s mess (3/?) …
The long answer: 2 parts
To directly access Point data members in a Rectangle function, you must use Point functions
Say we wanted to setOrigin() to take 2 args:
void Rectangle::setOrigin(double x, double y) {
origin.xCoord = x; // Won’t work
origin.setX(x);
origin.setY(y); }
I forgot: by default, copying one object to another is a defined operation
In some cases, default behavior can be overridden
In other cases, default behavior won’t work
Tried to avoid too much detail to avoid confusion …
… and made the lecture MORE confusing
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Data Structures: Lecture 11

15. Data Structures: Lecture 11
Initialization lists
How would we write Rectangle constructor(s)?
Could use Point set functions
Ideally, we’d like to call Point constructor as well
Create new Point every time we create Rectangle object
Use an initialization list
Explicitly calls constructors for member data
Requires parameterized constructor to be defined
Can be used for predefined types as well
Example:
Rectangle::Rectangle() : height(1), width(1), origin(0,0) {}
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Data Structures: Lecture 11

16. Initialization list example
Write the parameterized constructor for the Rectangle class for which the prototype is:
Rectangle(double h, double w, double x, double y);
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Data Structures: Lecture 11

17. Data Structures: Lecture 11
Example solution
Rectangle::Rectangle(double h, double w, double x, double y) : height(h), width(w), origin(x,y) {}
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Data Structures: Lecture 11

18. Data Structures: Lecture 11
Final notes
Next time
Dynamic allocation
Linked lists
(Hopefully) Program 2 intro
Reminders:
Program 2 to be posted; due TBD
Will implement dictionary
Each entry contains word, part of speech, definition
Load contents of dictionary from file
Implement string-based commands to interact
Exams to be returned next week
5/24/2019
Data Structures: Lecture 11