C++ Class Aggregation

[qingquan-li]

Reference: Book: Starting Out with C++ from Control Structures to Objects, byTony Gaddis, eighth edition

Concept: Aggregation occurs when a class contains an instance of another class.

Example:

Instructor.h

#ifndef INSTRUCTOR
#define INSTRUCTOR
#include <iostream>
#include <string>
using namespace std;

// Instructor class
class Instructor
{
private:
    string lastName;		// Last name
    string firstName;    // First name
    string officeNumber; // Office number
public:
    // The default constructor stores empty strings
    // in the string objects.
    Instructor()
    { set("", "", ""); }

    // Constructor
    Instructor(string lname, string fname, string office)
    { set(lname, fname, office); }

    // set function
    void set(string lname, string fname, string office)
    { lastName = lname;
        firstName = fname;
        officeNumber = office; }

    // print function
    void print() const
    {  cout << "Last name: " << lastName << endl;
        cout << "First name: " << firstName << endl;
        cout << "Office number: " << officeNumber << endl; }
};
#endif

TextBook.h

#ifndef TEXTBOOK
#define TEXTBOOK
#include <iostream>
#include <string>
using namespace std;

// TextBook class
class TextBook
{
private:
    string title;     // Book title
    string author;    // Author name
    string publisher; // Publisher name
public:
    // The default constructor stores empty strings
    // in the string objects.
    TextBook()
    { set("", "", ""); }

    // Constructor
    TextBook(string textTitle, string auth, string pub)
    { set(textTitle, auth, pub); }

    // set function
    void set(string textTitle, string auth, string pub)
    { title = textTitle;
        author = auth;
        publisher = pub; }

    // print function
    void print() const
    {  cout << "Title: " << title << endl;
        cout << "Author: " << author << endl;
        cout << "Publisher: " << publisher << endl; }
};
#endif

Course.h

#ifndef COURSE
#define COURSE
#include <iostream>
#include <string>
#include "Instructor.h"
#include "TextBook.h"
using namespace std;

class Course
{
private:
    string courseName;		// Course name
    Instructor instructor;   // Instructor
    TextBook textbook;       // Textbook
public:
    // Constructor
    Course(string course, string instrLastName,
           string instrFirstName, string instrOffice,
           string textTitle, string author,
           string publisher)
    { // Assign the course name.
        courseName = course;

        // Assign the instructor.
        instructor.set(instrLastName, instrFirstName, instrOffice);

        // Assign the textbook.
        textbook.set(textTitle, author, publisher); }

    // print function
    void print() const
    {  cout << "Course name: " << courseName << endl << endl;
        cout << "Instructor Information:\n";
        instructor.print();
        cout << "\nTextbook Information:\n";
        textbook.print();
        cout << endl; }
};
#endif

The Course class has an Instructor object and a TextBook object as member variables. Those objects are used as attributes of the Course object. Making an instance of one class an attribute of another class is called object aggregation. The word aggregate means "a whole that is made of constituent parts."

main.cpp

// This program demonstrates the Course class.
#include "Course.h"

int main()
{
    // Create a Course object.
    Course myCourse("Intro to Computer Science", // Course name
                    "Kramer", "Shawn", "RH3010",       // Instructor info
                    "Starting Out with C++", "Gaddis", // Textbook title and author
                    "Addison-Wesley");                 // Textbook publisher

    // Display the course info.
    myCourse.print();
    return 0;
}

Output:

$ g++ main.cpp -o main
$ ./main
Course name: Intro to Computer Science

Instructor Information:
Last name: Kramer
First name: Shawn
Office number: RH3010

Textbook Information:
Title: Starting Out with C++
Author: Gaddis
Publisher: Addison-Wesley