Day 6

• Day 6
• Basic Classes
• Creating New Types
• Why Create a New Type?
• Classes and Members
• Declaring a Class
• A Word on Naming Conventions
• Defining an Object
• Classes Versus Objects
• Accessing Class Members
• Assign to Objects, Not to Classes
• If You Dont Declare It, Your Class Wont Have It
• Private Versus Public
• Listing 6.1. Accessing the public members
• of a simple class.
• Make Member Data Private
• Listing 6.2. A class with accessor methods.
• Privacy Versus Security
• The class keyword
• Implementing Class Methods
• Listing 6.3. Implementing
• the methods of a simple class.
• Constructors and Destructors
• Default Constructors and Destructors
• Listing 6.4. Using constructors and destructors.
• const Member Functions
• Interface Versus Implementation
• Listing 6.5. A demonstration of violations of the interface.
• Why Use the Compiler to Catch Errors?
• Where to Put Class Declarations and Method Definitions
• Inline Implementation
• Listing 6.6. Cat class declaration in CAT.HPP.
• Listing 6.7. Cat implementation in CAT.CPP.
• Classes with Other Classes as Member Data
• Listing 6.8. Declaring a complete class.
• Listing 6.9. RECT.CPP.
• Structures
• Why Two Keywords Do the Same Thing
• Summary
• Q&A
• Workshop
• Quiz
• Exercises

• What classes and objects are.
• How to define a new class and create objects of that class.
• What member functions and member data are.
• What constructors are and how to use them.

Just by declaring these variables to be unsigned short integers, you know that it is possible to add Height to Width and to assign that number to another number.

The type of these variables tells you:

• Their size in memory.
• What information they can hold.
• What actions can be performed on them.

One way to think about a car is as a collection of wheels, doors, seats, windows, and so forth. Another way is to think about what a car can do: It can move, speed up, slow down, stop, park, and so on. A class enables you to encapsulate, or bundle, these various parts and various functions into one collection, which is called an object.

Encapsulating everything you know about a car into one class has a number of advantages for a programmer. Everything is in one place, which makes it easy to refer to, copy, and manipulate the data. Likewise, clients of your class--that is, the parts of the program that use your class--can use your object without worry about what is in it or how it works.

A class can consist of any combination of the variable types and also other class types. The variables in the class are referred to as the member variables or data members. A Car class might have member variables representing the seats, radio type, tires, and so forth.

---

New Term: Member variables , also known as data members , are the variables in your class. Member variables are part of your class, just like the wheels and engine are part of your car. 

---

---

New Term: Member functions , also known as methods , are the functions in your class. Member functions are as much a part of your class as the member variables. They determine what the objects of your class can do. 

---

class Cat
{
unsigned int  itsAge;
unsigned int  itsWeight;
Meow();
};

C++ is case-sensitive, and all class names should follow the same pattern. That way you never have to check how to spell your class name; was it Rectangle, rectangle, or RECTANGLE? Some programmers like to prefix every class name with a particular letter--for example, cCat or cPerson--whereas others put the name in all uppercase or all lowercase. The convention that I use is to name all classes with initial-capitalization, as in Cat and Person.

Similarly, many programmers begin all functions with capital letters and all variables with lowercase. Words are usually separated with an underbar--as in Chase_Mice--or by capitalizing each word--for example, ChaseMice or DrawCircle.

The important idea is that you should pick one style and stay with it through each program. Over time, your style will evolve to include not only naming conventions, but also indentation, alignment of braces, and commenting style.

---

NOTE: It's common for development companies to have house standards for many style issues. This ensures that all developers can easily read one another's code. 

---

unsigned int GrossWeight;         // define an unsigned integer
Cat Frisky;                       // define a Cat

---

New Term: An object is an individual instance of a class. 

---

Frisky.Weight = 50;

Frisky.Meow();

int = 5;               // wrong

int  x;                // define x to be an int
x = 5;                 // set x's value to 5

Cat.age=5;             // wrong
???

Cat Frisky;            // just like  int x;
Frisky.age = 5;        // just like  x = 5;

If you wrote

Cat  Frisky;           // make a Cat named Frisky
Frisky.Bark()          // tell Frisky to bark

---

DO use the keyword class to declare a class. DON'T confuse a declaration with a definition. A declaration says what a class is. A definition sets aside memory for an object. DON'T confuse a class with an object. DON'T assign values to a class. Assign values to the data members of an object. DO use the dot operator (.) to access class members and functions. 

---

All members of a class--data and methods--are private by default. Private members can be accessed only within methods of the class itself. Public members can be accessed through any object of the class. This distinction is both important and confusing. To make it a bit clearer, consider an example from earlier in this chapter:

class Cat
{
unsigned int  itsAge;
unsigned int  itsWeight;
Meow();
};

However, if you write

Cat  Boots;
Boots.itsAge=5;        // error! can't access private data!

This is a source of endless confusion to new C++ programmers. I can almost hear you yelling, "Hey! I just said Boots is a cat. Why can't Boots access his own age?" The answer is that Boots can, but you can't. Boots, in his own methods, can access all his parts--public and private. Even though you've created a Cat, that doesn't mean that you can see or change the parts of it that are private.

The way to use Cat so that you can access the data members is

class Cat
{
public:
unsigned int  itsAge;
unsigned int  itsWeight;
Meow();
};

Listing 6.1 shows the declaration of a Cat class with public member variables.

Listing 6.1. Accessing the public members of a simple class.

1:   // Demonstrates declaration of a class and
2:   // definition of an object of the class,
3:
4:   #include <iostream.h>   // for cout
5:
6:   class Cat                // declare the class object
7:   {
8:    public:                 // members which follow are public
9:      int itsAge;
10:     int itsWeight;
11:  };
12:
13:
14:  void main()
15:  {
16:     Cat Frisky;
17:     Frisky.itsAge = 5;    // assign to the member variable
18:     cout << "Frisky is a cat who is " ;
19:     cout << Frisky.itsAge << " years old.\n";
20: 
Output: Frisky is a cat who is 5 years old.

Lines 9 and 10 contain the declarations of the class members itsAge and itsWeight.

Line 14 begins the main function of the program. Frisky is defined in line 16 as an instance of a Cat--that is, as a Cat object. Frisky's age is set in line 17 to 5. In lines 18 and 19, the itsAge member variable is used to print out a message about Frisky.

---

NOTE: Try commenting out line 8 and try to recompile. You will receive an error on line 17 because itsAge will no longer have public access. The default for classes is private access. 

---

---

New Term: A public accessor method is a class member function used either to read the value of a private class member variable or to set its value. 

---

Accessor functions enable you to separate the details of how the data is stored from how it is used. This enables you to change how the data is stored without having to rewrite functions that use the data.

If a function that needs to know a Cat's age accesses itsAge directly, that function would need to be rewritten if you, as the author of the Cat class, decided to change how that data is stored. By having the function call GetAge(), your Cat class can easily return the right value no matter how you arrive at the age. The calling function doesn't need to know whether you are storing it as an unsigned integer or a long, or whether you are computing it as needed.

This technique makes your program easier to maintain. It gives your code a longer life because design changes don't make your program obsolete.

Listing 6.2 shows the Cat class modified to include private member data and public accessor methods. Note that this is not an executable listing.

Listing 6.2. A class with accessor methods.

1:       // Cat class declaration
2:       // Data members are private, public accessor methods
3:       // mediate setting and getting the values of the private data
4:
5:  class Cat
6:  {
7:  public:
8:       // public accessors
9:     unsigned int GetAge();
10:    void SetAge(unsigned int Age);
11:
12:    unsigned int GetWeight();
13:    void SetWeight(unsigned int Weight);
14:
15:      // public member functions
16:    Meow();
17:
18:      // private member data
19: private:
20:    unsigned int  itsAge;
21:    unsigned int  itsWeight;
22:
23: };

The member variables themselves are declared in lines 20 and 21.

To set Frisky's age, you would pass the value to the SetAge() method, as in

Cat  Frisky;
Frisky.SetAge(5);    // set Frisky's age using the public accessor

class class_name
{
// access control keywords here
// class variables and methods declared here
};

class Cat
{
public:
unsigned int Age;
unsigned int Weight;
void Meow();
};

Cat  Frisky;
Frisky.Age = 8;
Frisky.Weight = 18;
Frisky.Meow();

Example

class Car
{
public:                            // the next five are public

void Start();
void Accelerate();
void Brake();
void SetYear(int year);
int GetYear();

private:                           // the rest is private

int Year;
Char Model [255];
};                                 // end of class declaration

Car OldFaithful;                   // make an instance of car
int bought;                        // a local variable of type int
OldFaithful.SetYear(84) ;          // assign 84 to the year
bought = OldFaithful.GetYear();    // set bought to 84
OldFaithful.Start();               // call the start method

---

DO declare member variables private. DO use public accessor methods. DON'T try to use private member variables from outside the class. DO access private member variables from within class member functions. 

---

A member function definition begins with the name of the class, followed by two colons, the name of the function, and its parameters. Listing 6.3 shows the complete declaration of a simple Cat class and the implementation of its accessor function and one general class member function.

Listing 6.3. Implementing the methods of a simple class.

1:   // Demonstrates declaration of a class and
2:   // definition of class methods,
3:
4:   #include <iostream.h>      // for cout
5:
6:   class Cat                   // begin declaration of the class
7:   {
8:     public:                   // begin public section
9:       int GetAge();           // accessor function
10:      void SetAge (int age);  // accessor function
11:      void Meow();            // general function
12:    private:                  // begin private section
13:      int itsAge;             // member variable
14:  };
15:
16:  // GetAge, Public accessor function
17:  // returns value of itsAge member
18:  int Cat::GetAge()
19:  {
20:     return itsAge;
21:  }
22:
23:  // definition of SetAge, public
24:  // accessor function
25:  // returns sets itsAge member
26:  void Cat::SetAge(int age)
27:  {
28:     // set member variable its age to
29:     // value passed in by parameter age
30:     itsAge = age;
31:  }
32:
33:  // definition of Meow method
34:  // returns: void
35:  // parameters: None
36:  // action: Prints "meow" to screen
37:  void Cat::Meow()
38:  {
39:     cout << "Meow.\n";
40:  }
41:
42:  // create a cat, set its age, have it
43:  // meow, tell us its age, then meow again.
44:  int main()
45:  {
46:     Cat Frisky;
47:     Frisky.SetAge(5);
48:     Frisky.Meow();
49:     cout << "Frisky is a cat who is " ;
50:     cout << Frisky.GetAge() << " years old.\n";
51:     Frisky.Meow();
52;      return 0;
53: }
Output: Meow.
Frisky is a cat who is 5 years old.
Meow.

Line 12 begins the private section, which includes only the declaration in line 13 of the private member variable itsAge. The class declaration ends with a closing brace and semicolon in line 14.

Lines 18-21 contain the definition of the member function GetAge(). This method takes no parameters; it returns an integer. Note that class methods include the class name followed by two colons and the function name (Line 18). This syntax tells the compiler that the GetAge() function that you are defining here is the one that you declared in the Cat class. With the exception of this header line, the GetAge() function is created like any other function.

The GetAge() function takes only one line; it returns the value in itsAge. Note that the main() function cannot access itsAge because itsAge is private to the Cat class. The main() function has access to the public method GetAge(). Because GetAge() is a member function of the Cat class, it has full access to the itsAge variable. This access enables GetAge() to return the value of itsAge to main().

Line 26 contains the definition of the SetAge() member function. It takes an integer parameter and sets the value of itsAge to the value of that parameter in line 30. Because it is a member of the Cat class, SetAge() has direct access to the member variable itsAge.

Line 37 begins the definition, or implementation, of the Meow() method of the Cat class. It is a one-line function that prints the word Meow to the screen, followed by a new line. Remember that the \n character prints a new line to the screen.

Line 44 begins the body of the program with the familiar main() function. In this case, it takes no arguments and returns void. In line 46, main() declares a Cat named Frisky. In line 47, the value 5 is assigned to the itsAge member variable by way of the SetAge() accessor method. Note that the method is called by using the class name (Frisky) followed by the member operator (.) and the method name (SetAge()). In this same way, you can call any of the other methods in a class.

Line 48 calls the Meow() member function, and line 49 prints a message using the GetAge() accessor. Line 51 calls Meow() again.

int Weight;            // define a variable
...                    // other code here
Weight = 7;            // assign it a value

int Weight = 7;        // define and initialize to 7

How do you initialize the member data of a class? Classes have a special member function called a constructor. The constructor can take parameters as needed, but it cannot have a return value--not even void. The constructor is a class method with the same name as the class itself.

Whenever you declare a constructor, you'll also want to declare a destructor. Just as constructors create and initialize objects of your class, destructors clean up after your object and free any memory you might have allocated. A destructor always has the name of the class, preceded by a tilde (~). Destructors take no arguments and have no return value. Therefore, the Cat declaration includes

~Cat();

What good is a constructor that does nothing? In part, it is a matter of form. All objects must be constructed and destructed, and these do-nothing functions are called at the right time. However, to declare an object without passing in parameters, such as

Cat Rags;           // Rags gets no parameters

Cat();

Cat Frisky (5,7);

Cat Frisky (3);

Cat Frisky ;

Cat Frisky;

As a matter of form, if you declare a constructor, be sure to declare a destructor, even if your destructor does nothing. Although it is true that the default destructor would work correctly, it doesn't hurt to declare your own. It makes your code clearer.

Listing 6.4 rewrites the Cat class to use a constructor to initialize the Cat object, setting its age to whatever initial age you provide, and it demonstrates where the destructor is called.

Listing 6.4. Using constructors and destructors.

1:   // Demonstrates declaration of a constructors and
2:   // destructor for the Cat class
3:
4:   #include <iostream.h>      // for cout
5:
6:   class Cat                   // begin declaration of the class
7:   {
8:    public:                    // begin public section
9:      Cat(int initialAge);     // constructor
10:     ~Cat();                  // destructor
11:     int GetAge();            // accessor function
12:     void SetAge(int age);    // accessor function
13:     void Meow();
14:   private:                   // begin private section
15:     int itsAge;              // member variable
16:  };
17:
18:  // constructor of Cat,
19:  Cat::Cat(int initialAge)
20:  {
21:     itsAge = initialAge;
22:  }
23:
24:  Cat::~Cat()                 // destructor, takes no action
25:  {
26:  }
27:
28:  // GetAge, Public accessor function
29:  // returns value of itsAge member
30:  int Cat::GetAge()
31:  {
32:     return itsAge;
33:  }
34:
35:  // Definition of SetAge, public
36:  // accessor function
37:
38:  void Cat::SetAge(int age)
39:  {
40:     // set member variable its age to
41:     // value passed in by parameter age
42:     itsAge = age;
43:  }
44:
45:  // definition of Meow method
46:  // returns: void
47:  // parameters: None
48:  // action: Prints "meow" to screen
49:  void Cat::Meow()
50:  {
51:     cout << "Meow.\n";
52:  }
53:
54:  // create a cat, set its age, have it
55   // meow, tell us its age, then meow again.
56:  int main()
57:  {
58:    Cat Frisky(5);
59:    Frisky.Meow();
60:    cout << "Frisky is a cat who is " ;
61:    cout << Frisky.GetAge() << " years old.\n";
62:    Frisky.Meow();
63:    Frisky.SetAge(7);
64:    cout << "Now Frisky is " ;
65:    cout << Frisky.GetAge() << " years old.\n";
66;     return 0;
67: }

Output: Meow.
Frisky is a cat who is 5 years old.
Meow.
Now Frisky is 7 years old.

Lines 24-26 show the implementation of the destructor ~Cat(). This function does nothing, but you must include the definition of the function if you declare it in the class declaration.

Line 58 contains the definition of a Cat object, Frisky. The value 5 is passed in to Frisky's constructor. There is no need to call SetAge(), because Frisky was created with the value 5 in its member variable itsAge, as shown in line 61. In line 63, Frisky's itsAge variable is reassigned to 7. Line 65 prints the new value.

---

DO use constructors to initialize your objects. DON'T give constructors or destructors a return value. DON'T give destructors parameters. 

---

void SomeFunction() const;

void SetAge(int anAge);
int GetAge();

void SetAge(int anAge);
int GetAge() const;

---

NOTE: Use const whenever possible. Declare member functions to be const whenever they should not change the object. This lets the compiler help you find errors; it's faster and less expensive than doing it yourself. 

---

For example, in the Cat class declaration, you create a contract that every Cat will have a member variable itsAge that can be initialized in its constructor, assigned to by its SetAge() accessor function, and read by its GetAge() accessor. You also promise that every Cat will know how to Meow().

If you make GetAge() a const function--as you should--the contract also promises that GetAge() won't change the Cat on which it is called.

C++ is strongly typed, which means that the compiler enforces these contracts by giving you a compiler error when you violate them. Listing 6.5 demonstrates a program that doesn't compile because of violations of these contracts.

---

WARNING: Listing 6.5 does not compile! 

---

1:   // Demonstrates compiler errors
2:
3:
4:   #include <iostream.h>          // for cout
5:
6:  class Cat
7:  {
8:   public:
9:      Cat(int initialAge);
10:     ~Cat();
11:     int GetAge() const;          // const accessor function
12:     void SetAge (int age);
13:     void Meow();
14:  private:
15:    int itsAge;
16: };
17:
18:    // constructor of Cat,
19:    Cat::Cat(int initialAge)
20:    {
21:       itsAge = initialAge;
21:       cout << "Cat Constructor\n";
22:    }
23:
24:    Cat::~Cat()                   // destructor, takes no action
25:    {
26:       cout << "Cat Destructor\n";
27:    }
28:   // GetAge, const function
29:   // but we violate const!
30:   int Cat::GetAge() const
31:   {
32:      return (itsAge++);         // violates const!
33:   }
34:
35:   // definition of SetAge, public
36:   // accessor function
37:
38:   void Cat::SetAge(int age)
39:   {
40:      // set member variable its age to
41:      // value passed in by parameter age
42:      itsAge = age;
43:   }
44:
45:  // definition of Meow method
46:  // returns: void
47:  // parameters: None
48:  // action: Prints "meow" to screen
49:  void Cat::Meow()
50:  {
51:     cout << "Meow.\n";
52:  }
53:
54:  // demonstrate various violations of the
55   // interface, and resulting compiler errors
56:  int main()
57:  {
58:     Cat Frisky;                 // doesn't match declaration
59:     Frisky.Meow();
60:     Frisky.Bark();              // No, silly, cat's can't bark.
61:     Frisky.itsAge = 7;          // itsAge is private
62:      return 0;
63: }

Line 11 declares GetAge() to be a const accessor function--as it should be. In the body of GetAge(), however, in line 32, the member variable itsAge is incremented. Because this method is declared to be const, it must not change the value of itsAge. Therefore, it is flagged as an error when the program is compiled.

In line 13, Meow() is not declared const. Although this is not an error, it is bad programming practice. A better design takes into account that this method doesn't change the member variables of Cat. Therefore, Meow() should be const.

Line 58 shows the definition of a Cat object, Frisky. Cats now have a constructor, which takes an integer as a parameter. This means that you must pass in a parameter. Because there is no parameter in line 58, it is flagged as an error.

Line 60 shows a call to a class method, Bark(). Bark() was never declared. Therefore, it is illegal.

Line 61 shows itsAge being assigned the value 7. Because itsAge is a private data member, it is flagged as an error when the program is compiled.

The definition must be in a file that the compiler can find. Most C++ compilers want that file to end with .C or .CPP. This book uses .CPP, but check your compiler to see what it prefers.

---

NOTE: Many compilers assume that files ending with .C are C programs, and that C++ program files end with .CPP. You can use any extension, but .CPP will minimize confusion. 

---

For example, you put the declaration of the Cat class into a file named CAT.HPP, and you put the definition of the class methods into a file called CAT.CPP. You then attach the header file to the .CPP file by putting the following code at the top of CAT.CPP:

#include Cat.hpp

---

NOTE: The declaration of a class tells the compiler what the class is, what data it holds, and what functions it has. The declaration of the class is called its interface because it tells the user how to interact with the class. The interface is usually stored in an .HPP file, which is referred to as a header file. The function definition tells the compiler how the function works. The function definition is called the implementation of the class method, and it is kept in a .CPP file. The implementation details of the class are of concern only to the author of the class. Clients of the class--that is, the parts of the program that use the class--don't need to know, and don't care, how the functions are implemented. 

---

inline int Cat::GetWeight()
{
return itsWeight;         // return the Weight data member
}

class Cat
{
public:
int GetWeight() { return itsWeight; }    // inline
void SetWeight(int aWeight);
};

class Cat
{
public:
int GetWeight()
{
return itsWeight;
}                           // inline
void SetWeight(int aWeight);
};

Listing 6.6. Cat class declaration in CAT.HPP

1:  #include <iostream.h>
2:  class Cat
3:   {
4:  public:
5:   Cat (int initialAge);
6:   ~Cat();
7:    int GetAge() { return itsAge;}             // inline!
8:    void SetAge (int age) { itsAge = age;}     // inline!
9:    void Meow() { cout << "Meow.\n";}             // inline!
10:  private:
11: int itsAge;
12: };

1:   // Demonstrates inline functions
2:   // and inclusion of header files
3:
4:   #include "cat.hpp"  // be sure to include the header files!
5:
6:
7:   Cat::Cat(int initialAge)   //constructor
8:   {
9:      itsAge = initialAge;
10:  }
11:
12:  Cat::~Cat()             //destructor, takes no action
13:  {
14:  }
15:
16:  // Create a cat, set its age, have it
17:  // meow, tell us its age, then meow again.
18:  int main()
19:  {
20:     Cat Frisky(5);
21:     Frisky.Meow();
22:     cout << "Frisky is a cat who is " ;
23:     cout << Frisky.GetAge() << " years old.\n";
24:     Frisky.Meow();
25:     Frisky.SetAge(7);
26:     cout << "Now Frisky is " ;
27:     cout << Frisky.GetAge() << " years old.\n";
28:      return 0;
29: }

Output: Meow.
Frisky is a cat who is 5 years old.
Meow.
Now Frisky is 7 years old.

Line 4 of Listing 6.7 shows #include "cat.hpp", which brings in the listings from CAT.HPP. By including cat.hpp, you have told the precompiler to read cat.hpp into the file as if it had been typed there, starting on line 5.

This technique allows you to put your declarations into a different file from your implementation, yet have that declaration available when the compiler needs it. This is a very common technique in C++ programming. Typically, class declarations are in an .HPP file that is then #included into the associated CPP file.

Lines 18-29 repeat the main function from Listing 6.4. This shows that making these functions inline doesn't change their performance.

Consider a second example. A rectangle is composed of lines. A line is defined by two points. A point is defined by an x-coordinate and a y-coordinate. Listing 6.8 shows a complete declaration of a Rectangle class, as might appear in RECTANGLE.HPP. Because a rectangle is defined as four lines connecting four points and each point refers to a coordinate on a graph, we first declare a Point class, to hold the x,y coordinates of each point. Listing 6.9 shows a complete declaration of both classes.

Listing 6.8. Declaring a complete class.

1:   // Begin Rect.hpp
2:   #include <iostream.h>
3:   class Point     // holds x,y coordinates
4:   {
5:      // no constructor, use default
6:      public:
7:         void SetX(int x) { itsX = x; }
8:         void SetY(int y) { itsY = y; }
9:         int GetX()const { return itsX;}
10:        int GetY()const { return itsY;}
11:     private:
12:        int itsX;
13:        int itsY;
14:  };    // end of Point class declaration
15:
16:
17:  class  Rectangle
18:  {
19:     public:
20:        Rectangle (int top, int left, int bottom, int right);
21:        ~Rectangle () {}
22:
23:        int GetTop() const { return itsTop; }
24:        int GetLeft() const { return itsLeft; }
25:        int GetBottom() const { return itsBottom; }
26:        int GetRight() const { return itsRight; }
27:
28:        Point  GetUpperLeft() const { return itsUpperLeft; }
29:        Point  GetLowerLeft() const { return itsLowerLeft; }
30:        Point  GetUpperRight() const { return itsUpperRight; }
31:        Point  GetLowerRight() const { return itsLowerRight; }
32:
33:        void SetUpperLeft(Point Location)  {itsUpperLeft = Location;}
34:        void SetLowerLeft(Point Location)  {itsLowerLeft = Location;}
35:        void SetUpperRight(Point Location)  {itsUpperRight = Location;}
36:        void SetLowerRight(Point Location)  {itsLowerRight = Location;}
37:
38:        void SetTop(int top) { itsTop = top; }
39:        void SetLeft (int left) { itsLeft = left; }
40:        void SetBottom (int bottom) { itsBottom = bottom; }
41:        void SetRight (int right) { itsRight = right; }
42:
43:        int GetArea() const;
44:
45:     private:
46:        Point  itsUpperLeft;
47:        Point  itsUpperRight;
48:        Point  itsLowerLeft;
49:        Point  itsLowerRight;
50:        int    itsTop;
51:        int    itsLeft;
52:        int    itsBottom;
53:        int    itsRight;
54:  };
55: // end Rect.hpp

1:   // Begin rect.cpp
2:   #include "rect.hpp"
3:   Rectangle::Rectangle(int top, int left, int bottom, int right)
4:   {
5:         itsTop = top;
6:         itsLeft = left;
7:         itsBottom = bottom;
8:         itsRight = right;
9:
10:        itsUpperLeft.SetX(left);
11:        itsUpperLeft.SetY(top);
12:
13:        itsUpperRight.SetX(right);
14:        itsUpperRight.SetY(top);
15:
16:        itsLowerLeft.SetX(left);
17:        itsLowerLeft.SetY(bottom);
18:
19:        itsLowerRight.SetX(right);
20:        itsLowerRight.SetY(bottom);
21:  }
22:
23:
24:  // compute area of the rectangle by finding corners,
25:  // establish width and height and then multiply
26:  int Rectangle::GetArea() const
27:  {
28:        int Width = itsRight-itsLeft;
29:        int Height = itsTop - itsBottom;
30:        return (Width * Height);
31:  }
32:
33:  int main()
34:  {
35:        //initialize a local Rectangle variable
36:        Rectangle MyRectangle (100, 20, 50, 80 );
37:
38:        int Area = MyRectangle.GetArea();
39:
40:        cout << "Area: " << Area << "\n";
41:        cout << "Upper Left X Coordinate: ";
42:        cout << MyRectangle.GetUpperLeft().GetX();
43:      return 0;
44: }
Output: Area: 3000
Upper Left X Coordinate: 20

The Point class uses inline accessor functions to get and set the X and Y points declared on lines 7-10. Points use the default constructor and destructor. Therefore, you must set their coordinates explicitly.

Line 17 begins the declaration of a Rectangle class. A Rectangle consists of four points that represent the corners of the Rectangle.

The constructor for the Rectangle (line 20) takes four integers, known as top, left, bottom, and right. The four parameters to the constructor are copied into four member variables (Listing 6.9) and then the four Points are established.

In addition to the usual accessor functions, Rectangle has a function GetArea() declared in line 43. Instead of storing the area as a variable, the GetArea() function computes the area on lines 28-29 of Listing 6.9. To do this, it computes the width and the height of the rectangle, and then it multiplies these two values.

Getting the x-coordinate of the upper-left corner of the rectangle requires that you access the UpperLeft point, and ask that point for its X value. Because GetUpperLeft()is ()a method of Rectangle, it can directly access the private data of Rectangle, including itsUpperLeft. Because itsUpperLeft is a Point and Point's itsX value is private, GetUpperLeft() cannot directly access this data. Rather, it must use the public accessor function GetX() to obtain that value.

Line 33 of Listing 6.9 is the beginning of the body of the actual program. Until line 36, no memory has been allocated, and nothing has really happened. The only thing you've done is tell the compiler how to make a point and how to make a rectangle, in case one is ever needed.

In line 36, you define a Rectangle by passing in values for Top, Left, Bottom, and Right.

In line 38, you make a local variable, Area, of type int. This variable holds the area of the Rectangle that you've created. You initialize Area with the value returned by Rectangle's GetArea() function.

A client of Rectangle could create a Rectangle object and get its area without ever looking at the implementation of GetArea().

RECT.HPP is shown in Listing 6.8. Just by looking at the header file, which contains the declaration of the Rectangle class, the programmer knows that GetArea() returns an int. How GetArea() does its magic is not of concern to the user of class Rectangle. In fact, the author of Rectangle could change GetArea() without affecting the programs that use the Rectangle class.

Try re-entering Listing 6.8 with these changes:

• In line 3, change class Point to struct Point.
• In line 17, change class Rectangle to struct Rectangle.

---

DO put your class declaration in an HPP file and your member functions in a CPP file. DO use const whenever you can. DO understand classes before you move on. 

---

A class has data members, which are variables of various types, including other classes. A class also includes member functions--also known as methods. You use these member functions to manipulate the member data and to perform other services.

Class members, both data and functions, can be public or private. Public members are accessible to any part of your program. Private members are accessible only to the member functions of the class.

It is good programming practice to isolate the interface, or declaration, of the class in a header file. You usually do this in a file with an .HPP extension. The implementation of the class methods is written in a file with a .CPP extension.

Class constructors initialize objects. Class destructors destroy objects and are often used to free memory allocated by methods of the class.

Q. How big is a class object?

A. A class object's size in memory is determined by the sum of the sizes of its member variables. Class methods don't take up room as part of the memory set aside for the object.
Some compilers align variables in memory in such a way that two-byte variables actually consume somewhat more than two bytes. Check your compiler manual to be sure, but at this point there is no reason to be concerned with these details.

Q. If I declare a class Cat with a private member itsAge and then define two Cat objects, Frisky and Boots, can Boots access Frisky's itsAge member variable?

A. No. While private data is available to the member functions of a class, different instances of the class cannot access each other's data. In other words, Frisky's member functions can access Frisky's data, but not Boots'. In fact, Frisky is a completely independent cat from Boots, and that is just as it should be.

Q. Why shouldn't I make all the member data public?

A. Making member data private enables the client of the class to use the data without worrying about how it is stored or computed. For example, if the Cat class has a method GetAge(), clients of the Cat class can ask for the cat's age without knowing or caring if the cat stores its age in a member variable, or computes its age on the fly.

Q. If using a const function to change the class causes a compiler error, why shouldn't I just leave out the word const and be sure to avoid errors?

A. If your member function logically shouldn't change the class, using the keyword const is a good way to enlist the compiler in helping you find silly mistakes. For example, GetAge() might have no reason to change the Cat class, but your implementation has this line:

if (itsAge = 100) cout << "Hey! You're 100 years old\n";

Declaring GetAge() to be const causes this code to be flagged as an error. You meant to check whether itsAge is equal to 100, but instead you inadvertently assigned 100 to itsAge. Because this assignment changes the class--and you said this method would not change the class--the compiler is able to find the error.

This kind of mistake can be hard to find just by scanning the code. The eye often sees only what it expects to see. More importantly, the program might appear to run correctly, but itsAge has now been set to a bogus number. This will cause problems sooner or later.

Q. Is there ever a reason to use a structure in a C++ program?

A. Many C++ programmers reserve the struct keyword for classes that have no functions. This is a throwback to the old C structures, which could not have functions. Frankly, I find it confusing and poor programming practice. Today's methodless structure might need methods tomorrow. Then you'll be forced either to change the type to class or to break your rule and end up with a structure with methods.

1. What is the dot operator, and what is it used for?

2. Which sets aside memory--declaration or definition?

3. Is the declaration of a class its interface or its implementation?

4. What is the difference between public and private data members?

5. Can member functions be private?

6. Can member data be public?

7. If you declare two Cat objects, can they have different values in their itsAge member data?

8. Do class declarations end with a semicolon? Do class method definitions?

9. What would the header for a Cat function, Meow, that takes no parameters and returns void look like?

10. What function is called to initialize a class?

1. Write the code that declares a class called Employee with these data members: age, yearsOfService, and Salary.

2. Rewrite the Employee class to make the data members private, and provide public accessor methods to get and set each of the data members.

3. Write a program with the Employee class that makes two Employees; sets their age, YearsOfService, and Salary; and prints their values.

4. Continuing from Exercise 3, provide a method of Employee that reports how many thousands of dollars the employee earns, rounded to the nearest 1,000.

5. Change the Employee class so that you can initialize age, YearsOfService, and Salary when you create the employee.

6. BUG BUSTERS: What is wrong with the following declaration?

class Square
{
public:
    int Side;
}

7. BUG BUSTERS: Why isn't the following class declaration very useful?

class Cat
{
    int GetAge()const;
private:
    int itsAge;
};

8. BUG BUSTERS: What three bugs in this code will the compiler find?

class  TV
{
public:
    void SetStation(int Station);
    int GetStation() const;
private:
    int itsStation;
};
main()
{
    TV myTV;
    myTV.itsStation = 9;
    TV.SetStation(10);
    TV myOtherTv(2);
}