Composition: Objects as Members of Classes

Composition Objects as Members of Classes

An AlarmClock object needs to know when it is supposed to sound its alarm, so why not include a Time object as a member of the AlarmClock class? Such a capability is called composition and is sometimes referred to as a has-a relationship. A class can have objects of other classes as members.

Software Engineering Observation 10.5

A common form of software reusability is composition, in which a class has objects of other classes as members.

When an object is created, its constructor is called automatically. Previously, we saw how to pass arguments to the constructor of an object we created in main. This section shows how an object's constructor can pass arguments to member-object constructors, which is accomplished via member initializers. Member objects are constructed in the order in which they are declared in the class definition (not in the order they are listed in the constructor's member initializer list) and before their enclosing class objects (sometimes called host objects) are constructed.

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The program of Figs. 10.1010.14 uses class Date (Figs. 10.1010.11) and class Employee (Figs. 10.1210.13) to demonstrate objects as members of other objects. The definition of class Employee (Fig. 10.12) contains private data members firstName, lastName, birthDate and hireDate. Members birthDate and hireDate are const objects of class Date, which contains private data members month, day and year. The Employee constructor's header (Fig. 10.13, lines 1821) specifies that the constructor receives four parameters (first, last, dateOfBirth and dateOfHire). The first two parameters are used in the constructor's body to initialize the character arrays firstName and lastName. The last two parameters are passed via member initializers to the constructor for class Date. The colon (:) in the header separates the member initializers from the parameter list. The member initializers specify the Employee constructor parameters being passed to the constructors of the member Date objects. Parameter dateOfBirth is passed to object birthDate's constructor (Fig. 10.13, line 20), and parameter dateOfHire is passed to object hireDate's constructor (Fig. 10.13, line 21). Again, member initializers are separated by commas. As you study class Date (Fig. 10.10), notice that the class does not provide a constructor that receives a parameter of type Date. So, how is the member initializer list in class Employee's constructor able to initialize the birthDate and hireDate objects by passing Date object's to their Date constructors? As we mentioned in Chapter 9, the compiler provides each class with a default copy constructor that copies each member of the constructor's argument object into the corresponding member of the object being initialized. Chapter 11 discusses how programmers can define customized copy constructors.

Figure 10.10. Date class definition.

 1 // Fig. 10.10: Date.h
 2 // Date class definition; Member functions defined in Date.cpp
 3 #ifndef DATE_H
 4 #define DATE_H
 5
 6 class Date
 7 {
 8 public:
 9 Date( int = 1, int = 1, int = 1900 ); // default constructor
10 void print() const; // print date in month/day/year format
11 ~Date(); // provided to confirm destruction order
12 private:
13 int month; // 1-12 (January-December)
14 int day; // 1-31 based on month
15 int year; // any year
16
17 // utility function to check if day is proper for month and year
18 int checkDay( int ) const;
19 }; // end class Date
20
21 #endif

Figure 10.11. Date class member-function definitions.

(This item is displayed on pages 536 - 537 in the print version)

 1 // Fig. 10.11: Date.cpp
 2 // Member-function definitions for class Date.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6
 7 #include "Date.h" // include Date class definition
 8
 9 // constructor confirms proper value for month; calls
10 // utility function checkDay to confirm proper value for day
11 Date::Date( int mn, int dy, int yr )
12 {
13 if ( mn > 0 && mn <= 12 ) // validate the month
14 month = mn;
15 else
16 {
17 month = 1; // invalid month set to 1
18 cout << "Invalid month (" << mn << ") set to 1.
";
19 } // end else
20
21 year = yr; // could validate yr
22 day = checkDay( dy ); // validate the day
23
24 // output Date object to show when its constructor is called
25 cout << "Date object constructor for date ";
26 print();
27 cout << endl;
28 } // end Date constructor
29 
30 // print Date object in form month/day/year
31 void Date::print() const
32 {
33 cout << month << '/' << day << '/' << year;
34 } // end function print
35 
36 // output Date object to show when its destructor is called
37 Date::~Date()
38 {
39 cout << "Date object destructor for date ";
40 print();
41 cout << endl;
42 } // end ~Date destructor
43 
44 // utility function to confirm proper day value based on
45 // month and year; handles leap years, too
46 int Date::checkDay( int testDay ) const
47 {
48 static const int daysPerMonth[ 13 ] =
49 { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
50 
51 // determine whether testDay is valid for specified month
52 if ( testDay > 0 && testDay <= daysPerMonth[ month ] )
53 return testDay;
54 
55 // February 29 check for leap year
56 if ( month == 2 && testDay == 29 && ( year % 400 == 0 ||
57 ( year % 4 == 0 && year % 100 != 0 ) ) )
58 return testDay;
59 
60 cout << "Invalid day (" << testDay << ") set to 1.
";
61 return 1; // leave object in consistent state if bad value
62 } // end function checkDay

Figure 10.12. Employee class definition showing composition.

(This item is displayed on page 537 in the print version)

 1 // Fig. 10.12: Employee.h
 2 // Employee class definition.
 3 // Member functions defined in Employee.cpp.
 4 #ifndef EMPLOYEE_H
 5 #define EMPLOYEE_H
 6
 7 #include "Date.h" // include Date class definition
 8
 9 class Employee
10 {
11 public:
12 Employee( const char * const, const char * const,
13 const Date &, const Date & );
14 void print() const;
15 ~Employee(); // provided to confirm destruction order
16 private:
17 char firstName[ 25 ];
18 char lastName[ 25 ];
19 const Date birthDate; // composition: member object
20 const Date hireDate; // composition: member object
21 }; // end class Employee
22
23 #endif

Figure 10.13. Employee class member-function definitions, including constructor with a member initializer list.

(This item is displayed on pages 538 - 539 in the print version)

 1 // Fig. 10.13: Employee.cpp
 2 // Member-function definitions for class Employee.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6
 7 #include  // strlen and strncpy prototypes
 8 using std::strlen;
 9 using std::strncpy;
10
11 #include "Employee.h" // Employee class definition
12 #include "Date.h" // Date class definition
13
14 // constructor uses member initializer list to pass initializer
15 // values to constructors of member objects birthDate and hireDate
16 // [Note: This invokes the so-called "default copy constructor" which the
17 // C++ compiler provides implicitly.]
18 Employee::Employee( const char * const first, const char * const last,
19 const Date &dateOfBirth, const Date &dateOfHire )
20 : birthDate( dateOfBirth ), // initialize birthDate
21 hireDate( dateOfHire ) // initialize hireDate
22 {
23 // copy first into firstName and be sure that it fits
24 int length = strlen( first );
25 length = ( length < 25 ? length : 24 );
26 strncpy( firstName, first, length );
27 firstName[ length ] = '';
28
29 // copy last into lastName and be sure that it fits
30 length = strlen( last );
31 length = ( length < 25 ? length : 24 );
32 strncpy( lastName, last, length );
33 lastName[ length ] = '';
34
35 // output Employee object to show when constructor is called
36 cout << "Employee object constructor: "
37 << firstName << ' ' << lastName << endl;
38 } // end Employee constructor
39
40 // print Employee object
41 void Employee::print() const
42 {
43 cout << lastName << ", " << firstName << " Hired: ";
44 hireDate.print();
45 cout << " Birthday: ";
46 birthDate.print();
47 cout << endl;
48 } // end function print
49
50 // output Employee object to show when its destructor is called
51 Employee::~Employee()
52 {
53 cout << "Employee object destructor: "
54 << lastName << ", " << firstName << endl;
55 } // end ~Employee destructor

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Figure 10.14 creates two Date objects (lines 1112) and passes them as arguments to the constructor of the Employee object created in line 13. Line 16 outputs the Employee object's data. When each Date object is created in lines 1112, the Date constructor defined at lines 1128 of Fig. 10.11 displays a line of output to show that the constructor was called (see the first two lines of the sample output). [Note: Line 13 of Fig. 10.14 causes two additional Date constructor calls that do not appear in the program's output. When each of the Employee's Date member object's is initialized in the Employee constructor's member initializer list, the default copy constructor for class Date is called. This constructor is defined implicitly by the compiler and does not contain any output statements to demonstrate when it is called. We discuss copy constructors and default copy constructors in detail in Chapter 11.]

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Figure 10.14. Member-object initializers.

 1 // Fig. 10.14: fig10_14.cpp
 2 // Demonstrating composition--an object with member objects.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6
 7 #include "Employee.h" // Employee class definition
 8
 9 int main()
10 {
11 Date birth( 7, 24, 1949 );
12 Date hire( 3, 12, 1988 );
13 Employee manager( "Bob", "Blue", birth, hire );
14
15 cout << endl;
16 manager.print();
17
18 cout << "
Test Date constructor with invalid values:
";
19 Date lastDayOff( 14, 35, 1994 ); // invalid month and day
20 cout << endl;
21 return 0;
22 } // end main

 

 Date object constructor for date 7/24/1949
 Date object constructor for date 3/12/1988
 Employee object constructor: Bob Blue

 Blue, Bob Hired: 3/12/1988 Birthday: 7/24/1949

 Test Date constructor with invalid values:
 Invalid month (14) set to 1.
 Invalid day (35) set to 1.
 Date object constructor for date 1/1/1994

 Date object destructor for date 1/1/1994
 Employee object destructor: Blue, Bob
 Date object destructor for date 3/12/1988
 Date object destructor for date 7/24/1949
 Date object destructor for date 3/12/1988
 Date object destructor for date 7/24/1949

 

Class Date and class Employee each include a destructor (lines 3742 of Fig. 10.11 and lines 5155 of Fig. 10.13, respectively) that prints a message when an object of its class is destroyed. This enables us to confirm in the program output that objects are constructed from the inside out and destroyed in the reverse order from the outside in (i.e., the Date member objects are destroyed after the Employee object that contains them). Notice the last four lines in the output of Fig. 10.14. The last two lines are the outputs of the Date destructor running on Date objects hire (line 12) and birth (line 11), respectively. These outputs confirm that the three objects created in main are destructed in the reverse of the order in which they were constructed. (The Employee destructor output is five lines from the bottom.) The fourth and third lines from the bottom of the output window show the destructors running for the Employee's member objects hireDate (Fig. 10.12, line 20) and birthDate (Fig. 10.12, line 19). These outputs confirm that the Employee object is destructed from the outside ini.e., the Employee destructor runs first (output shown five lines from the bottom of the output window), then the member objects are destructed in the reverse order from which they were constructed. Again, the outputs in Fig. 10.14 did not show the constructors running for these objects, because these were the default copy constructors provided by the C++ compiler.

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A member object does not need to be initialized explicitly through a member initializer. If a member initializer is not provided, the member object's default constructor will be called implicitly. Values, if any, established by the default constructor can be overridden by set functions. However, for complex initialization, this approach may require significant additional work and time.

Common Programming Error 10.6

A compilation error occurs if a member object is not initialized with a member initializer and the member object's class does not provide a default constructor (i.e., the member object's class defines one or more constructors, but none is a default constructor).

Performance Tip 10.2

Initialize member objects explicitly through member initializers. This eliminates the overhead of "doubly initializing" member objectsonce when the member object's default constructor is called and again when set functions are called in the constructor body (or later) to initialize the member object.

Software Engineering Observation 10.6

If a class member is an object of another class, making that member object public does not violate the encapsulation and hiding of that member object's private members. However, it does violate the encapsulation and hiding of the containing class's implementation, so member objects of class types should still be private, like all other data members.

In line 26 of Fig. 10.11, notice the call to Date member function print. Many member functions of classes in C++ require no arguments. This is because each member function contains an implicit handle (in the form of a pointer) to the object on which it operates. We discuss the implicit pointer, which is represented by keyword this, in Section 10.5.

Class Employee uses two 25-character arrays (Fig. 10.12, lines 1718) to represent the first name and last name of the Employee. These arrays may waste space for names shorter than 24 characters. (Remember, one character in each array is for the terminating null character, '', of the string.) Also, names longer than 24 characters must be truncated to fit in these fixed-size character arrays. Section 10.7 presents another version of class Employee that dynamically creates the exact amount of space required to hold the first and the last name.

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Note that the simplest way to represent an Employee's first and last name using the exact amount of space required is to use two string objects (C++ Standard Library class string was introduced in Chapter 3). If we did this, the Employee constructor would appear as follows

 Employee::Employee( const string &first, const string &last,
 const Date &dateOfBirth, const Date &dateOfHire )
 : firstName( first), // initialize firstName
 lastName( last ), // initialize lastName
 birthDate( dateOfBirth ), // initialize birthDate
 hireDate( dateOfHire ) // initialize hireDate
 {
 // output Employee object to show when constructor is called
 cout << "Employee object constructor: "
 << firstName << ' ' << lastName << endl;
 } // end Employee constructor

Notice that data members firstName and lastName (now string objects) are initialized through member initializers. The Employee classes presented in Chapters 1213 use string objects in this fashion. In this chapter, we use pointer-based strings to provide the reader with additional exposure to pointer manipulation.