類（Class）是一種資料型別，即符合該型別的資料都可以歸到你定義的類中。所以要定義一個類，就必須弄清楚其特點及其作用。計算機軟體各種功能不再是簡單的四則運算能實現的了，因此，需要建立區別與int、char、bool等資料型別的新資料型別——類，來實現更復雜的運算。

當然，不使用類也能完成複雜的運算，面向物件的概念出現之前就是靠工程師們強大的邏輯思維來實現的。但是當面向物件的程式設計思想出現後，類出現後，核心的技術人員將各個相同屬性的資料封裝成類，形成一個個物件，令上層的開發人員能直接通過對各物件進行排程來開發軟體。

這就好比積體電路，大量閘電路整合（封裝）為加法器，觸發器等邏輯電路。而各種邏輯電路又整合（封裝）為放大器，比較器等單元電路。而大量單元電路整合（封裝）為CPU，RAM，ROM。上層的設計師不瞭解內部構造，也不能修改封裝好的物件，但不影響其使用。反而加快了開發效率。這就是面向物件的程式設計思想，這就是類的作用。

本次實驗使用Visual Studio 2012，建立一個貨幣類，並且使用它。
例子來源：《資料結構、演算法與應用 C++語言描述》機械工業出版社

建立專案及相應的標頭檔案，源程式檔案

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;
}

執行結果