Data Structures and Other Objects Using C++ (Chapter 2) 学习笔记四

Chapter 2 Abstract Data Types and C++ Classes

2.1 CLASSES AND MEMBERS

A class is a new kind of data type. Each class that you define is a collection of data, and has the ability to include member functions. Member functions are incorporated into
the class's definition and are designed specifically to manipulate the class.
PROGRAMMING EXAMPLE: The Throttle Class

class throttle
{
public:
// MODIFICATION MEMBER FUNCTIONS
void shut_off ();
void shift (int amount);
// CONSTANT MEMBER FUNCTIONS
double flow() const;
bool is_on() const;
private:
int position;
};

A class has some sections as follows :

The class head. The head of the definition consists of the C++ keyword class, followed by the name of the new class.
The member list.
The public section.
modification functions.
constant member functions:
may examine the status of its object, but it is forbidden from changing the object.
The private section.

Using a Class

declare objects of a class:
throttle my_throttle;
throttle control;
Every throttle object contains the private member variable position, but there is no way for a program to access this component （组件）directly, because it is a private member. The only
way that a program can use its throttle objects is by using the public member functions. such as: control.shut_off(); control.shift(3);
calling a member function always involves the following four steps:

Start with the name of the object that you are manipulating.
After the object name, place a single period.
Next, write the name of the member function.
Finally, list the arguments for the function call.

A Small Demonstration Program for the Throttle Class

// FILE: demol.cxx
// This small demonstration shows how the throttle class is used.
#include <iostream>
#include <cstdlib>
using namespace std;

class throttle
{
public:
// MODIFICATION MEMBER FUNCTIONS
void shut_off ();
void shift (int amount);
// CONSTANT MEMBER FUNCTIONS
double flow() const;
bool is_on () const;
private:
　　　int position;
};

int main()
{
throttle sample;
int user_input;

// Set the sample throttle to a position indicated by the user.
cout << "I have a throttle with 6 positions." << endl;
cout << "Where would you like to set the throttle? " << endl;
cout << "Please type a number from 0 to 6: ";
cin >> user_input;
sample.shut_off();
sample.shift (user_input);

// shift the throttle down to zero, printing the flow along the way.
while (sample.is_on())
{
cout << "The flow is now " << sample.flow()  << endl;
sample.shift(-1);
}
cout << "The folw is now off" << endl;
return EXIT_SUCCESS;
}

Implementing Member Functions

void throttle::shut_off()
// precondition: None
// Postcondition: The throttle has been turned off.
{
position = 0;
}

void throttle::shift (int amount)
// Precondition: shut_off has been called at least once to initialize the throttle.
// Postcondition: The throttle's position has been moved by amount
{
position += amount;

if (position < 0)
position = 0;
else if (position > 6)
position = 6;
}

double throttle::flow() const
// Precondition: shut_off has been called at least once to initialize the throttle.
// Postcondition: The value returned is the current flow as a proportion of the max flow.
{
return position / 6.0;
}

bool throttle::is_on() const
// Precondition: shut_off has been called at least once to initialize the throttle.
// Postcondition: If the throttle's flow is above 0,
// then the function returns true; otherwise, it returns false.
{
return (flow() > 0);
}