Using Classes

In this chapter you’ve seen how to define a class and its class methods. The next step is to produce a program that creates and uses objects of a class. The C++ goal is to make using classes as similar as possible to using the basic, built-in types, such as int and char. You can create a class object by declaring a class variable or using new to allocate an object of a class type. You can pass objects as arguments, return them as function return values, and assign one object to another. C++ provides facilities for initializing objects, teaching cin and cout to recognize objects, and even providing automatic type conversions between objects of similar classes. It will be a while before you can do all those things, but let’s start now with the simpler properties. Indeed, you’ve already seen how to declare a class object and call a member function. Listing 10.3 provides a program to use the interface and implementation files. It creates a Stock object named fluffy_the_cat. The program is simple, but it tests the features built in to the class. To compile the complete program, use the techniques for multifile programs described in Chapter 1, “Getting Started with C++,” and in Chapter 9. In particular, compile it with stock00.cpp and have stock00.h present in the same directory or folder.

Listing 10.3. usestok0.cpp

// usestck0.cpp -- the client program
// compile with stock00.cpp
#include <iostream>
#include "stock00.h"
int main()
{
    Stock fluffy_the_cat;
    fluffy_the_cat.acquire("NanoSmart", 20, 12.50);
    fluffy_the_cat.show();
    fluffy_the_cat.buy(15, 18.125);
    fluffy_the_cat.show();
    fluffy_the_cat.sell(400, 20.00);
    fluffy_the_cat.show();
    fluffy_the_cat.buy(300000,40.125);
    fluffy_the_cat.show();
    fluffy_the_cat.sell(300000,0.125);
    fluffy_the_cat.show();
    return 0;
}

Here is the output of the program in Listing 10.3:

Company: NanoSmart  Shares: 20
  Share Price: $12.5  Total Worth: $250
Company: NanoSmart  Shares: 35
  Share Price: $18.125  Total Worth: $634.375
You can't sell more than you have! Transaction is aborted.
Company: NanoSmart  Shares: 35
  Share Price: $18.125  Total Worth: $634.375
Company: NanoSmart  Shares: 300035
  Share Price: $40.125  Total Worth: $1.20389e+007
Company: NanoSmart  Shares: 35
  Share Price: $0.125  Total Worth: $4.375

Note that main() is just a vehicle for testing the design of the Stock class. When the Stock class works as you want it to, you can use it as a user-defined type in other programs. The critical point in using the new type is to understand what the member functions do; you shouldn’t have to think about the implementation details. See the following sidebar, “The Client/Server Model.”