April 5, 2008
WikiDefining class GradeBook with a member function, creating a GradeBook object and calling its member function.
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Defining class GradeBook with a member function, creating a GradeBook object and calling its member function.
1
2 // Define class GradeBook with a member function displayMessage;
3 // Create a GradeBook object and call its displayMessage function.
4 #include <iostream>
5 using std::cout;
6 using std::endl;
7
8 // GradeBook class definition
9 class GradeBook
10 {
11 public:
12 // function that displays a welcome message to the GradeBook user
13 void displayMessage()
14 {
15 cout << "Welcome to the Grade Book!" << endl;
16 } // end function displayMessage
17 }; // end class GradeBook
18
19 // function main begins program execution
20 int main()
21 {
22 GradeBook myGradeBook; // create a GradeBook object named myGradeBook
23 myGradeBook.displayMessage(); // call object's displayMessage function
24 return 0; // indicate successful termination
25 } // end main
- Methods of a class are called member functions. \
- Line 11 contains the access-specifier label public:. The keyword public is called an access specifier. \
Defining a Member Function with a Parameter
1
2 // Define class GradeBook with a member function that takes a parameter;
3 // Create a GradeBook object and call its displayMessage function.
4 #include <iostream>
5 using std::cout;
6 using std::cin;
7 using std::endl;
8
9 #include <string> // program uses C++ standard string class
10 using std::string;
11 using std::getline;
12
13 // GradeBook class definition
14 class GradeBook
15 {
16 public:
17 // function that displays a welcome message to the GradeBook user
18 void displayMessage( string courseName )
19 {
20 cout << "Welcome to the grade book for\n" << courseName << "!"
21 << endl;
22 } // end function displayMessage
23 }; // end class GradeBook
24
25 // function main begins program execution
26 int main()
27 {
28 string nameOfCourse; // string of characters to store the course name
29 GradeBook myGradeBook; // create a GradeBook object named myGradeBook
30
31 // prompt for and input course name
32 cout << "Please enter the course name:" << endl;
33 getline( cin, nameOfCourse ); // read a course name with blanks
34 cout << endl; // output a blank line
35
36 // call myGradeBook's displayMessage function
37 // and pass nameOfCourse as an argument
38 myGradeBook.displayMessage( nameOfCourse );
39 return 0; // indicate successful termination
40 } // end main
- Line 28 creates a variable of type string called nameOfCourse that will be used to store the course name entered by the user. A string is actually an object of the C++ Standard Library class string. This class is defined in header file
, and the name string, like cout, belongs to namespace std. To enable line 28 to compile, line 9 includes the header file. Note that the using declaration in line 10 allows us to simply write string in line 28 rather than std::string. \ - Line 33 reads the name from the user and assigns it to the nameOfCourse variable, using the library function getline to perform the input.\
- When cin is used with the stream extraction operator, it reads characters until the first white-space character is reached. In this example, we’d like the user to type the complete course name and press Enter to submit it to the program, and we’d like to store the entire course name in the string variable nameOfCourse. \
- The function call getline( cin, nameOfCourse ) in line 33 reads characters (including the space characters that separate the words in the input) from the standard input stream object cin (i.e., the keyboard) until the newline character is encountered, places the characters in the string variable nameOfCourse and discards the newline character. Note that when you press Enter while typing program input, a newline is inserted in the input stream. Also note that the
header file must be included in the program to use function getline and that the name getline belongs to namespace std. \
Data Members, set Functions and get Functions
1
2 // Define class GradeBook that contains a courseName data member
3 // and member functions to set and get its value;
4 // Create and manipulate a GradeBook object with these functions.
5 #include <iostream>
6 using std::cout;
7 using std::cin;
8 using std::endl;
9
10 #include <string> // program uses C++ standard string class
11 using std::string;
12 using std::getline;
13
14 // GradeBook class definition
15 class GradeBook
16 {
17 public:
18 // function that sets the course name
19 void setCourseName( string name )
20 {
21 courseName = name; // store the course name in the object
22 } // end function setCourseName
23
24 // function that gets the course name
25 string getCourseName()
26 {
27 return courseName; // return the object's courseName
28 } // end function getCourseName
29
30 // function that displays a welcome message
31 void displayMessage()
32 {
33 // this statement calls getCourseName to get the
34 // name of the course this GradeBook represents
35 cout << "Welcome to the grade book for\n" << getCourseName() << "!"
36 << endl;
37 } // end function displayMessage
38 private:
39 string courseName; // course name for this GradeBook
40 }; // end class GradeBook
41
42 // function main begins program execution
43 int main()
44 {
45 string nameOfCourse; // string of characters to store the course name
46 GradeBook myGradeBook; // create a GradeBook object named myGradeBook
47
48 // display initial value of courseName
49 cout << "Initial course name is: " << myGradeBook.getCourseName()
50 << endl;
51
52 // prompt for, input and set course name
53 cout << "\nPlease enter the course name:" << endl;
54 getline( cin, nameOfCourse ); // read a course name with blanks
55 myGradeBook.setCourseName( nameOfCourse ); // set the course name
56
57 cout << endl; // outputs a blank line
58 myGradeBook.displayMessage(); // display message with new course name
59 return 0; // indicate successful termination
60 } // end main
- Declaring data members with access specifier private is known as data hiding. \
- Note that set functions are also sometimes called mutators (because they mutate, or change, values), and get functions are also sometimes called accessors (because they access values). \
Initializing Objects with Constructors
What if you want to provide a course name when you create a GradeBook object? Each class you declare can provide a constructor that can be used to initialize an object of the class when the object is created. A constructor is a special member function that must be defined with the same name as the class, so that the compiler can distinguish it from the class’s other member functions. An important difference between constructors and other functions is that constructors cannot return values, so they cannot specify a return type (not even void). Normally, constructors are declared public.\
1 // Fig. 3.7: fig03_07.cpp
2 // Instantiating multiple objects of the GradeBook class and using
3 // the GradeBook constructor to specify the course name
4 // when each GradeBook object is created.
5 #include <iostream>
6 using std::cout;
7 using std::endl;
8
9 #include <string> // program uses C++ standard string class
10 using std::string;
11
12 // GradeBook class definition
13 class GradeBook
14 {
15 public:
16 // constructor initializes courseName with string supplied as argument
17 GradeBook( string name )
18 {
19 setCourseName( name ); // call set function to initialize courseName
20 } // end GradeBook constructor
21
22 // function to set the course name
23 void setCourseName( string name )
24 {
25 courseName = name; // store the course name in the object
26 } // end function setCourseName
27
28 // function to get the course name
29 string getCourseName()
30 {
31 return courseName; // return object's courseName
32 } // end function getCourseName
33
34 // display a welcome message to the GradeBook user
35 void displayMessage()
36 {
37 // call getCourseName to get the courseName
38 cout << "Welcome to the grade book for\n" << getCourseName()
39 << "!" << endl;
40 } // end function displayMessage
41 private:
42 string courseName; // course name for this GradeBook
43 }; // end class GradeBook
44
45 // function main begins program execution
46 int main()
47 {
48 // create two GradeBook objects
49 GradeBook gradeBook1( "CS101 Introduction to C++ Programming" );
50 GradeBook gradeBook2( "CS102 Data Structures in C++" );
51
52 // display initial value of courseName for each GradeBook
53 cout << "gradeBook1 created for course: " << gradeBook1.getCourseName()
54 << "\ngradeBook2 created for course: " << gradeBook2.getCourseName()
55 << endl;
56 return 0; // indicate successful termination
57 } // end main
Any constructor that takes no arguments is called a default constructor. A class gets a default constructor in one of two ways: \
- The compiler implicitly creates a default constructor in a class that does not define a constructor. Such a default constructor does not initialize the class’s data members, but does call the default constructor for each data member that is an object of another class. \
- The programmer explicitly defines a constructor that takes no arguments. Such a default constructor will perform the initialization specified by the programmer and will call the default constructor for each data member that is an object of another class. \
Placing a Class in a Separate File for Reusability
Each of the previous examples in the chapter consists of a single .cpp file, also known as a source-code file, that contains a GradeBook class definition and a main function. When building an object-oriented C++ program, it is customary to define reusable source code (such as a class) in a file that by convention has a .h filename extension known as a header file. Programs use #include preprocessor directives to include header files and take advantage of reusable software components, such as type string provided in the C++ Standard Library and user-defined types like class GradeBook.\
GradeBook class definition
1 // GradeBook.h
2 // GradeBook class definition in a separate file from main.
3 #include <iostream>
4 using std::cout;
5 using std::endl;
6
7 #include <string> // class GradeBook uses C++ standard string class
8 using std::string;
9
10 // GradeBook class definition
11 class GradeBook
12 {
13 public:
14 // constructor initializes courseName with string supplied as argument
15 GradeBook( string name )
16 {
17 setCourseName( name ); // call set function to initialize courseName
18 } // end GradeBook constructor
19
20 // function to set the course name
21 void setCourseName( string name )
22 {
23 courseName = name; // store the course name in the object
24 } // end function setCourseName
25
26 // function to get the course name
27 string getCourseName()
28 {
29 return courseName; // return object's courseName
30 } // end function getCourseName
31
32 // display a welcome message to the GradeBook user
33 void displayMessage()
34 {
35 // call getCourseName to get the courseName
36 cout << "Welcome to the grade book for\n" << getCourseName()
37 << "!" << endl;
38 } // end function displayMessage
39 private:
40 string courseName; // course name for this GradeBook
41 }; // end class GradeBook
- See that in the constructor declaration we call setCourseName function, the reason for this is simple. We can put some kind of validation into this function and don’t worry about it in the constructor. Modular code.
Including class GradeBook from file GradeBook.h for use in main
1 // fig03_10.cpp
2 // Including class GradeBook from file GradeBook.h for use in main.
3 #include <iostream>
4 using std::cout;
5 using std::endl;
6
7 #include "GradeBook.h" // include definition of class GradeBook
8
9 // function main begins program execution
10 int main()
11 {
12 // create two GradeBook objects
13 GradeBook gradeBook1( "CS101 Introduction to C++ Programming" );
14 GradeBook gradeBook2( "CS102 Data Structures in C++" );
15
16 // display initial value of courseName for each GradeBook
17 cout << "gradeBook1 created for course: " << gradeBook1.getCourseName()
18 << "\ngradeBook2 created for course: " << gradeBook2.getCourseName()
19 << endl;
20 return 0; // indicate successful termination
21 } // end main
- To ensure that the preprocessor can locate header files correctly, #include preprocessor directives should place the names of user-defined header files in quotes (e.g., “GradeBook.h”) and place the names of C++ Standard Library header files in angle brackets (e.g.,
).
Separating Interface from Implementation
- The interface of a class describes what services a class’s clients can use and how to request those services, but not how the class carries out the services. A class’s interface consists of the class’s public member functions (also known as the class’s public services). For example, class GradeBook’s interface contains a constructor and member functions setCourseName, getCourseName and displayMessage. GradeBook’s clients (e.g., main) use these functions to request the class’s services. As you will soon see, you can specify a class’s interface by writing a class definition that lists only the member function names, return types and parameter types. \
- In our prior examples, each class definition contained the complete definitions of the class’s public member functions and the declarations of its private data members. However, it is better software engineering to define member functions outside the class definition, so that their implementation details can be hidden from the client code. \
- The program separates class GradeBook’s interface from its implementation by splitting the class definition into two files. the header file GradeBook.h in which class GradeBook is defined, and the source-code file GradeBook.cpp in which GradeBook’s member functions are defined. By convention, member-function definitions are placed in a source-code file of the same base name (e.g., GradeBook) as the class’s header file but with a .cpp filename extension. \
- A function prototype is a declaration of a function that tells the compiler the function’s name, its return type and the types of its parameters. Note that the header file still specifies the class’s private data member as well. \
- These function prototypes are the same as the corresponding function headers in the previous example, except that the parameter names (which are optional in prototypes) are not included and each function prototype must end with a semicolon. \
GradeBook class definition containing function prototypes that specify the interface of the class.
1 // GradeBook.h
2 // GradeBook class definition. This file presents GradeBook's public
3 // interface without revealing the implementations of GradeBook's member
4 // functions, which are defined in GradeBook.cpp.
5 #include <string> // class GradeBook uses C++ standard string class
6 using std::string;
7
8 // GradeBook class definition
9 class GradeBook
10 {
11 public:
12 GradeBook( string ); // constructor that initializes courseName
13 void setCourseName( string ); // function that sets the course name
14 string getCourseName(); // function that gets the course name
15 void displayMessage(); // function that displays a welcome message
16 private:
17 string courseName; // course name for this GradeBook
18 }; // end class GradeBook
Defining Member Functions in a Separate Source-Code File
- Notice that each member function name in the function headers (lines 11, 17, 23 and 29) is preceded by the class name and ::, which is known as the binary scope resolution operator. \
- This “ties” each member function to the (now separate) GradeBook class definition, which declares the class’s member functions and data members. Without “GradeBook::” preceding each function name, these functions would not be recognized by the compiler as member functions of class GradeBookthe compiler would consider them “free” or “loose” functions, like main. Such functions cannot access GradeBook’s private data or call the class’s member functions, without specifying an object. So, the compiler would not be able to compile these functions. \
1 // GradeBook.cpp
2 // GradeBook member-function definitions. This file contains
3 // implementations of the member functions prototyped in GradeBook.h.
4 #include <iostream>
5 using std::cout;
6 using std::endl;
7
8 #include "GradeBook.h" // include definition of class GradeBook
9
10 // constructor initializes courseName with string supplied as argument
11 GradeBook::GradeBook( string name )
12 {
13 setCourseName( name ); // call set function to initialize courseName
14 } // end GradeBook constructor
15
16 // function to set the course name
17 void GradeBook::setCourseName( string name )
18 {
19 courseName = name; // store the course name in the object
20 } // end function setCourseName
21
22 // function to get the course name
23 string GradeBook::getCourseName()
24 {
25 return courseName; // return object's courseName
26 } // end function getCourseName
27
28 // display a welcome message to the GradeBook user
29 void GradeBook::displayMessage()
30 {
31 // call getCourseName to get the courseName
32 cout << "Welcome to the grade book for\n" << getCourseName()
33 << "!" << endl;
34 } // end function displayMessage
GradeBook class demonstration after separating its interface from its implementation.
1 // fig03_13.cpp
2 // GradeBook class demonstration after separating
3 // its interface from its implementation.
4 #include <iostream>
5 using std::cout;
6 using std::endl;
7
8 #include "GradeBook.h" // include definition of class GradeBook
9
10 // function main begins program execution
11 int main()
12 {
13 // create two GradeBook objects
14 GradeBook gradeBook1( "CS101 Introduction to C++ Programming" );
15 GradeBook gradeBook2( "CS102 Data Structures in C++" );
16
17 // display initial value of courseName for each GradeBook
18 cout << "gradeBook1 created for course: " << gradeBook1.getCourseName()
19 << "\ngradeBook2 created for course: " << gradeBook2.getCourseName()
20 << endl;
21 return 0; // indicate successful termination
22 } // end main
- Before executing this program, the source-code files in previous two snippets must both be compiled, then linked together that is, the member-function calls in the client code need to be tied to the implementations of the class’s member functionsa job performed by the linker.\
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