C - Program - 2e - Chapter07.pdf - po ang - Januszek66

Classes and Objects

CHAPTER OBJECTIVES

Introduce the principles and definitions used in object-oriented

programming.

KEY TERMS AND CONCEPTS

access specifiers

array of objects

class

class constructor

class declaration

class definition

class destructor function

class members

instance of a class

method

object

obtaining computer system time

operator function

overloaded constructors

overloaded operators

private

public

Illustrate how classes incorporate data and functions.

Explain the relationship between the class declaration and

objects.

Demonstrate how a class declaration is written.

Show how the “world” may access public class members.

Present several simple program examples that illustrate the

mechanics of working with objects.

Describe the purpose of class constructor and destructor func-

tions, and illustrate how these functions are used in a program.

Illustrate how pointers and references to objects work.

KEYWORDS AND OPERATORS

class

operator

private

public

scoping operator ::

Who’s Job Is It?

H ave you ever worked for a company where everyone had a specific job

title and responsibilities? If you were employed in the airline industry, you

know the various jobs include pilots, flight crew, mechanics, baggage

handlers, and reservation-staff, just to name a few. Perhaps you’ve spent time

working in a restaurant where the various job titles include managers, cooks,

wait-staff, host or hostess, bus-boys and girls. Now, if you were a waiter, what did

you do? You took customers’ orders, picked up, and served their food. You did-

n’t cook the food, nor did you order food from the supplier. If you were a bag-

gage handler for an airline, you didn’t fly the plane nor help customers make

reservations.

Are you asking yourself why we are off track talking about restaurants and

airlines when we should be learning how to write our own classes? Before I an-

swer that, let’s suppose you want to start your own business, say a bike messen-

ger service in the downtown area of a large city. You have to identify the various

jobs at your shop. What types of workers do you need? (Office manager, sched-

uler, messengers?) Then for each job, you need to identify the knowledge re-

quired to perform that job and its assigned tasks. As a programmer, writing

object-oriented programs, you have to examine all of the program goals and

identify the type of “workers” you need. Asking yourself what classes you need

and what each class is supposed to do is an important part of designing your

program. This is the same type of exercise that the new business owner must do

when identifying jobs and tasks. You decide what tasks are appropriate for your

new classes and what things each class will “need to know” in order to execute

specific tasks. Don’t forget, there may be C++ classes available for you to use

too. Isn’t that nice?

“Who’s job is it?” is an important question when dealing with classes and

objects. Keep that thought in mind as we move along here, and it will serve you

well as you design your own classes.

369

370

Chapter 7

❚

Classes and Objects

7.1

What Do We Know About Classes

and Objects?

Let’s review what we know about C++ classes before we jump into writing our

own. What classes have we used so far? Here is a short outline:

string

for handling text data

vector

for lists or groups of data, such as a list of strings or numbers

stringstream

for creating a formatted string

queue

models a line, (first in, first out) such as a bank line or a

play list for songs

ifstream

open and read data from a data file

ofstream

open and write data to a data file

For each of these classes, we include the appropriate library at the top of the file.

Inside a function we make an object that works for us. To use a class function, we

ask the object to perform a task by writing the object and a dot operator to access

the class’ function. The class is the job description for the object—it contains the

task list for the object.

The string class is handy for text data. We assign text into a string. The string

class’ “job description” gives us (among other things) the ability to assign, deter-

mine length, and find a substring in the string.

string sText;

//make a string object named sText

sText = "C++ classes are fun!";

//use = to assign

int length = sText.size();

//size() gives us length

int where = sText.find("fun");

//find() searches for us

We used the vector class in Chapter 3’s Program 3-18 when we searched for a

woman’s name. We stored the name and origin data in two separate vectors. To use

a vector object, we tell the object what type of data it will contain. We add items to

the vector using the push_back() function. We ask the vector how many items are

in the list with the size() function. The at() returns the item given an integer

index. Here are a few lines of code using a vector object:

vector <string> vNames;

//make a vector object, vNames

vNames.push_back("Mary");

//load names using push_back()

vNames.push_back("Carol");

vNames.push_back("Elizabeth");

int number = vNames.size();

//size() tells us how many names are

stored

cout << vNames.at(0);

//at() returns the first name

Our C++ Bank program in Chapter 5 (Program 5-8) incorporated a queue object.

Do you remember how to make and use a queue? We include the library, make an

object, then have the object perform tasks for us. The task list is defined in the

class. (Same song, different verse, right?) Here we make a bankLine queue object

371

Section 7.2

Writing Our Own Classes

❚

for customers (string objects), put three in the line, then serve and remove the

first one.

queue <string> bankLine;

//make a queue object bankLine

bankLine.push("Richard");

//add to the end of the line

using push()

bankLine.push("Roberto");

bankLine.push("Kristy");

cout << "Serving " << bankLine.front(); //front() returns the first name

bankLine.pop();

//remove the first element from

the line

If you haven’t discovered Appendix G, “Partial C++ Class Reference,” now is

the perfect time for you to look at this appendix. Here you can scan through a par-

tial listing of the string, vector, and queue class functions—in essence, these func-

tions show you their job descriptions. The designers of these C++ classes thought

about the class tasks when they wrote the class specifications. We’ll be doing the

same in this chapter.

7.2

Writing Our Own Classes

Hand-Waving Example: Our Date Class

No time like the present to design our own class. Let’s talk through a simple exam-

ple instead of getting bogged down with object-oriented principles, terminology,

and C++ syntax. The time to worry about semi-colons and braces will come soon

enough. For now, let’s pick an easy class and walk through its design. In this discus-

sion, we’ll layout the features for a Date class. After we design it, we’ll write it in

C++ code, and see how it works. It will then be ready for us to use throughout these

last two chapters.

What is the date today? If you’re like most of us, you’ll have to stop a minute

and think—unless it is an important date like your birthday! Now, did you think of

today’s date in terms of a month name, day, and year, or as three integers? The

exact form doesn’t matter—what does matter is that you know that the date data is

represented using three integers, or two integers and a string. A date can be de-

scribed using many formats. Here are a few ways to write the same date:

March, 14, 2007

Mar. 14, 2007

14-Mar-2007

14/3/2007

3/14/2007

03/14/07

14-March-2007

14/3/07

Regardless of the format, we have four pieces of data with the date, i.e., the month,

day, year (integers), and month name (string). What other information might you

need when talking about a date? You may need to know if your date is a leap year.

The year 2007 is not a leap year. A useful date value to know is what day of the year it

is. January 1 is the first day of the year. February 1 is the 32 nd day of the year. Non-leap

years contain 365 days. Leap years contain an extra day: February, the 29 th .Therefore,

in a non-leap year March 14 is the 73 rd day of the year (obtained by adding

). One last feature of our customized Date class is that it will have a

31 + 28 + 14

372

Chapter 7

❚

Classes and Objects

You, the programmer

Date object

contains information

for one Date.

Date object

month =3

day = 14

year = 2007

day of the year = 73

leap year = false

description = Jason's Birthday

Date data

What can you ask the

Date object?

What month are you?

What day are you?

What year are you?

What day of year are you?

Give me a formatted

string discribing

yourself

Are you a leap year?

Figure 7-1

Information available from our date object.

string variable to hold a description so that it “knows” what the date represents—such

as “Jason’s Birthday”, “first day of school”, or “driver’s license expiration”.

When you write a class, you must think about the type of tasks (jobs) the

class objects will do for you. Our Date class holds date information for us, so what

would you like your Date object to do for you? Obviously, our Date object tell us

the month, day, year, and day of the year. It would be nice if it gave us a formatted

string of itself—such as “Jason’s Birthday: March 14, 2007”, as well as be able to

tell us if it is a leap year. Look at Figure 7-1 and see all the questions that our pro-

grammer can ask our Date object.

If you look at our Date object in Figure 7-1, it contains data for Jason’s Birth-

day. How did the data get into the Date object? There are a few ways to set the

Date object’s data. Let’s look at one way here. We need to be able to tell the Date

object what date it is holding and its description. Let’s allow our programmer to

pass three integers and one string into the Date object. This means we can change

the date information in our object anytime we wish. Let’s set our Date object data

to 7/30/2004, with “Ryan’s Birthday”. Figure 7-2 shows this. (Wait! Who’s job is it to

determine the day of the year? What about that pesky leap year?)

As class designers we must think about what happens when our object is con-

structed. We have to be sure that the class data in the object are initialized to

C - Program - 2e - Chapter07.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: