类的声明，定义及使用

类（Class）是一种数据类型，即符合该类型的数据都可以归到你定义的类中。所以要定义一个类，就必须弄清楚其特点及其作用。计算机软件各种功能不再是简单的四则运算能实现的了，因此，需要创建区别与int、char、bool等数据类型的新数据类型——类，来实现更复杂的运算。

当然，不使用类也能完成复杂的运算，面向对象的概念出现之前就是靠工程师们强大的逻辑思维来实现的。但是当面向对象的编程思想出现后，类出现后，内核的技术人员将各个相同属性的数据封装成类，形成一个个对象，令上层的开发人员能直接通过对各对象进行调度来开发软件。

这就好比集成电路，大量门电路集成（封装）为加法器，触发器等逻辑电路。而各种逻辑电路又集成（封装）为放大器，比较器等单元电路。而大量单元电路集成（封装）为CPU，RAM，ROM。上层的设计师不了解内部构造，也不能修改封装好的对象，但不影响其使用。反而加快了开发效率。这就是面向对象的编程思想，这就是类的作用。

本次实验使用Visual Studio 2012，创建一个货币类，并且使用它。
例子来源：《数据结构、算法与应用 C++语言描述》机械工业出版社

创建项目及相应的头文件，源程序文件

TheClass.h

/**************************************************************** *Name: TheClass.h *Content: The definition of currency and illegalParameterValue *Instructions: Used to define some classes *Version: V1.0 *Author: Caddress *Data: 20160226 ***************************************************************/
#pragma once

#include<stdio.h>
#include<iostream>
#include<string>

using namespace std;

//Define an enumerated type
enum signType{plus, minus};

/********************************************************** *Class: currency *Members: amount *Instructions: Class about currency *Data Version Author Content *---------------------------------------------------------- *20160226 V1.0 Caddress Declaration ***********************************************************/
class currency
{
    public:
        //The constructor definition
        currency(signType theSign = plus,
        unsigned long theDollars = 0,
        unsigned int theCent = 0);

        //The destructor
        ~currency() {}

        //Two member function for assignment
        void setValue(signType, unsigned long, unsigned int);
        void setValue(double);

        //Get the currency symbol
        signType getSign() const
        {if (amount < 0) return minus;
        else return plus;}

        //Get the dollar value
        unsigned long getDollars() const
        {if (amount < 0) return (-amount)/100;
        else return amount / 100;}

        //Get the cent value
        unsigned int getCents() const
        {if (amount < 0) return -amount - getDollars() * 100;
        else return amount - getDollars() * 100;}

        //Reloading the "+" operator
        currency operator+(const currency&) const;

        //Reloading the "+=" operator
        currency operator+=(const currency& x)
            {amount += x.amount; return *this;}

        //For the output in the specified format
        void output(ostream&) const;

    private:
        long amount;
};

/********************************************************** *Class: illegalParameterValue *Members: message *Instructions: Class about Anomaly detection *Data Version Author Content *---------------------------------------------------------- *20160226 V1.0 Caddress Declaration ***********************************************************/
class illegalParameterValue
{
    public:
        illegalParameterValue():
            message("Illegal parameter value"){}

        illegalParameterValue(char* theMessage)
        {message = theMessage;}

        void outputMessage() {cout << message << endl;}

    private:
        string message;
};

//The constructor definition
currency::currency(signType theSign, unsigned long theDollars, unsigned int theCents)
{
    setValue( theSign, theDollars, theCents);
}

//Three parameters initialization
void currency::setValue(signType theSign, unsigned long theDollars, unsigned int theCents)
{
    if(theCents > 99)
        throw illegalParameterValue("Cents should be < 100");

    amount = theDollars * 100 + theCents;
    if(theSign == minus) amount = -amount;
}

//Single parameter initialization
void currency::setValue(double theAmount)
{
    if (theAmount < 0) 
        amount = (long)  ((theAmount - 0.001) * 100);
    else
        amount = (long)  ((theAmount + 0.001) * 100);
}

//Reloading the "+" operator.The definition.
currency currency::operator+(const currency& x) const
{
    currency result;
    result.amount = amount + x.amount;
    return result;
}

//The definition of output function.
void currency::output(ostream& out) const
{
    long theAmount = amount;
    if (theAmount < 0) {out << '-'; theAmount = -theAmount;}
    long dollars = theAmount / 100;
    out << '$' << dollars << '.';
    int cents = theAmount -dollars * 100;
    if(cents < 10) out << '0';
    out << cents;
}

//Reloading the "<<" operator.The definition.
ostream& operator<<(ostream& out, const currency& x)
{ x.output(out); return out;}

TheClass.cpp

/********************************************************************
*Name:         TheClass.cpp
*Content:      main function
*Instructions: Testing can be used currency class normally
*Version:      V1.0
*Author:       Caddress
*Data:         20160226
*********************************************************************/
#include "TheClass.h"

/**********************************************************
*Function:     main
*Input:        void
*Output:       string of results
*Return:       0
*Data          Version      Author          Content
*----------------------------------------------------------
*20160226      V1.0         Caddress        create
***********************************************************/
int main()
{
 currency g, h(plus, 3, 50), i, j;


 g.setValue(minus, 2, 25);
 i.setValue(-6.45);


 j = h + g;

 cout << h << " + " << g << " = " << j << endl;


 j = i + g + h;
 cout << i << " + " << g << " + " << h << " =" << j << endl;


 cout << "Increment " << i << " by " << g << " and then add " << h << endl;

 j = (i += g) + h;
 cout << "Result is " << j << endl;
 cout << "Incremented object is " << i << endl;


 cout << "Attempting to initialize with cents = 152" << endl;
 try {i.setValue(plus, 3, 152);}
 catch (illegalParameterValue e)
 {
 cout << "Caught thrown exception" << endl;
 e.outputMessage();
 }
 getchar();
 return 0;
}

运行结果