Classes cpp intro thomson bayan college

Object-Oriented Programming
Using C++
Third Edition

Chapter 7
Using Classes

Creating Classes
• A class is a category of objects; it is a new data
type
– Classes provide a description of an object
– Classes provide a convenient way to group related
data and the functions that use the data
– When you create an object from the class, you
automatically create all the related fields
– You think about them and manipulate them as real-
life classes and objects
• Abstract data type (ADT): a type that you define

Object-Oriented Programming Using C++, Third Edition 2

Creating Classes (continued)

Student aSophomore;
aSophomore.idNum = 7645; Error! By default, all members of a
cout<<aSophomore.idNum; class are private


Creating Classes (continued)

Access modifier


Encapsulating Class Components
• To encapsulate components is to contain them
– Encapsulation is an example of a black box
• An interface intercedes between you and the inner
workings of an object


Designing Classes
• If you need a class for students, you should ask:
– What shall we call it?
– What are its attributes?
– What methods are needed by Student?
– Any other methods?
• In most cases, you declare both fields and functions
– You declare a field using a data type and an identifier
– You declare a function by writing its prototype, which
serves as the interface to the function


Designing Classes

• To instantiate an object is to declare or create it
Student aSophomore;
aSophomore.displayStudentData();
• A function that uses your class is a class client


Implementing Class Functions
• When you construct a class, you create two parts:
– Declaration section: contains the class name,
variables (attributes), and function prototypes
– Implementation section: contains the functions
• Use both the class name and the scope resolution
operator (::) when you implement a class function


Implementing Class Functions
(continued)


Using Public Functions to Alter Private
Data


Using Public Functions to Alter Private
Data (continued)

…


Using Private Functions and Public Data


…
…


Considering Scope when Defining
Member Functions


Considering Scope when Defining
Member Functions (continued)


Using Static Class Members
• When a class field is static, only one memory
location is allocated
– All members of the class share a single storage
location for a static data member of that same class
• When you create a non-static variable within a
function, a new variable is created every time you
call that function
• When you create a static variable, the variable
maintains its memory address and previous value
for the life of the program


Defining Static Data Members

Since it is not const,
anyone can modify it


Defining Static Data Members
(continued)

• Static variables are sometimes called class
variables, class fields, or class-wide fields
because they don’t belong to a specific object; they
belong to the class


Using Static Functions
• A static function can be used without a
declared object
• Non-static functions can access static variables
(provided there is an object)
• Static functions cannot access non-static variables


Using Static Functions (continued)


Understanding the this Pointer

…
…
…


(continued)


(continued)
• The this pointer holds the memory address of the
current object that is using the function
• The this pointer is automatically supplied when
you call a non-static member function of a class
– For example, clerk.displayValues();
– Is actually displayValues(&clerk);
• The actual argument list used by the compiler for
displayValues() is displayValues(Employee *this)
• The this pointer is a constant pointer


Using the this Pointer Explicitly


Using the Pointer-to-Member Operator


Understanding Polymorphism
• Polymorphism is the object-oriented program
feature that allows the same operation to be carried
out differently depending on the object
• For example,
– clerk.displayValues();
– shirt.displayValues();
– XYZCompany.displayValues();


Understanding Polymorphism (continued)


You Do It: Creating and Using a Class
class CollegeCourse
{
private:
string department;
int courseNum;
int seats;
public:
void setDepartmentAndCourse(string, int);
void setSeats(int);
void displayCourseData();
};


Using a static Field
class Letter
{
private:
string title;
string recipient;
static int count;
public:
void setRecipient(string, string);
void displayGreeting();
static void displayCount();
};


Understanding How static and
Non-static Fields are Stored


Summary
• A class is a category of objects
• When you create a class, you hide, or encapsulate,
the individual components
• When you construct a class, you create the
declaration section and the implementation section
• When you create a class, usually you want to make
data items private, and to make functions public
• The scope resolution operator (::) identifies a member
function as being in scope within a class


Summary (continued)
• Each class object gets its own block of memory for
its data members
• You can access a static, class-wide field using a
static function
• One copy of each class member function is stored
no matter how many objects exist
• Within any member function, you can explicitly use
the this pointer to access the object’s data fields
• Polymorphism allows the same operation to be
carried out differently depending on the object


Classes cpp intro thomson bayan college

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