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COMPOSITION AND INHERITANCE
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[CHAP. 12
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EXAMPLE 12.2 A Date Class
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class Date { friend istream& operator>>(istream&, Date&); friend ostream& operator<<(ostream&, const Date&); public: Date(int m=0, int d=0, int y=0) : month(m), day(d), year(y) { } void setDate(int m, int d, int y) { month = m; day = d; year = y; } private: int month, day, year; }; istream& operator>>(istream& in, Date& x) { in >> x.month >> x.day >> x.year; return in; } ostream& operator<<(ostream& out, const Date& x) { static char* monthName[13] = {"", "January","February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"}; out << monthName[x.month] << ' ' << x.day << ", " << x.year; return out; } int main() { Date peace(11,11,1918); cout << "World War I ended on " << peace << ".\n"; peace.setDate(8,14,1945); cout << "World War II ended on " << peace << ".\n"; cout << "Enter month, day, and year: "; Date date; cin >> date; cout << "The date is " << date << ".\n"; } World War I ended on November 11, 1918. World War II ended on August 14, 1945. Enter month, day, and year: 7 4 1776 The date is July 4, 1776.
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The test driver tests the default constructor, the setDate() function, the overloaded insertion operator << , and the overloaded extraction operator >>.
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Now we can use the Date class inside the Person class to store a person s date of birth and date of death: EXAMPLE 12.3 Composing the Date Class with the Person Class
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#include "Date.h" class Person { public:
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COMPOSITION AND INHERITANCE
Person(char* n="", int s=0, char* nat="U.S.A.") : name(n), sex(s), nationality(nat) { } void setDOB(int m, int d, int y) { dob.setDate(m, d, y); } void setDOD(int m, int d, int y) { dod.setDate(m, d, y); } void printName() { cout << name; } void printNationality() { cout << nationality; } void printDOB() { cout << dob; } void printDOD() { cout << dod; } private: string name, nationality; Date dob, dod; // date of birth, date of death int sex; // 0 = female, 1 = male }; int main() { Person author("Thomas Jefferson", 1); author.setDOB(4,13,1743); author.setDOD(7,4,1826); cout << "The author of the Declaration of Independence was "; author.printName(); cout << ".\nHe was born on "; author.printDOB(); cout << " and died on "; author.printDOD(); cout << ".\n"; } The author of the Declaration of Independence was Thomas Jefferson. He was born on April 13, 1743 and died on July 4, 1826. Notice again that we have used a member function of one class to define member functions of the composed class: the setDate() function is used to define the setDOB() and setDOD() functions.
Composition is often referred to as a has-a relationship because the objects of the composite class have objects of the composed class as members. Each object of the Person class has a name and a nationality which are string objects. Composition is one way of reusing existing software to create new software. 12.3 INHERITANCE Another way to reuse existing software to create new software is by means of inheritance (also called specialization or derivation). This is often referred to as an is-a relationship because every object of the class being defined is also an object of the inherited class. The common syntax for deriving a class Y from a class X is
class Y : public X { // ... }; Here X is called the base class (or superclass) and Y is called the derived class (or subclass). The keyword public after the colon specifies public inheritance, which means that public members of the base class become public members of the derived class.
COMPOSITION AND INHERITANCE
[CHAP. 12
EXAMPLE 12.4 Deriving a Student Class from the Person Class
Students are people. So it is natural to use the Person class to derive a Student class: #include "Person.h" class Student : public Person { public: Student(char* n, int s=0, char* i="") : Person(n, s), id(i), credits(0) { } void setDOM(int m, int d, int y) { dom.setDate(m, d, y); } void printDOM() { cout << dom; } private: string id; // student identification number Date dom; // date of matriculation int credits; // course credits float gpa; // grade-point average }; The Student class inherits all the public functionality of the Person class, including the Person() constructor which it uses in its constructor to initialize name in the Person class. Note that this is a private member of the Person class, so it could not be accessed directly. Here is a test driver for the Student class: #include "Student.h" int main() { Student x("Ann Jones", 0, "219360061"); x.setDOB(5, 13, 1977); x.setDOM(8, 29, 1995); x.printName(); cout << "\n\t Born: "; x.printDOB(); cout << "\n\tMatriculated: "; x.printDOM(); cout << endl; } Ann Jones Born: May 13, 1977 Matriculated: August 29, 1995
12.4 protected CLASS MEMBERS The Student class in Section 12.3 has a significant problem: it cannot directly access the private data members of its Person superclass: name, nationality, DOB, DOD, and sex. The lack of access on the first four of these is not serious because these can be written and read through the Person class s constructor and public access functions. However, there is no way to write or read a Student s sex. One way to overcome this problem would be to make sex a data member of the Student class. But that is unnatural: sex is an attribute that all Person objects have, not just Students. A better solution is to change the private access specifier to protected in the Person class. That allows access to these data members from derived classes. EXAMPLE 12.5 The Person Class with protected Data Members
These are the same class definitions that were given in the two previous examples except that the private access specifier has been changed to protected, and we have added the access function printSex() to the Student class:
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