對於一個小程式，一般不需要編寫標頭檔案，但是對於一個複雜的大專案，模組化編寫程式，便於理解，且容易下手，將問題分解成一小塊一小塊，逐個擊破：

抽象一個點，一個圓，並判斷點與圓的關係。（在圓內還是圓外）

h檔案進行類的宣告；
cpp檔案為類的實現細節；主要注重的細節為：

在類外定義成員函式；
使用域作用符；

工程程式碼如下：

//MyCircle.h  
#pragma once  
#include"MyPoint.h"  
class MyCircle  
{  
public:  
    void setCir(double x, double y, double
 
 r);  

public:  
    charchar * judge(double x, double y);  
    charchar * judge(MyPoint &p);  

private:  
    double x1, y1; //圓心座標  
    double r1;  //圓半徑  
};  

//MyPoint.h  
#pragma once  
class MyPoint  
{  
private:  
    double x0, y0; //點座標  
public:  
    void setPoint(double x, double y);  
    double
 
 getX0();  
    double getY0();  
};  

//MyCircle.cpp  
#include "MyCircle.h"  

void MyCircle::setCir(double x, double y, double r)  
{  
    x1 = x; y1 = y; r1 = r;  
}  
char* MyCircle::judge(double x, double y)  
{  
    double s = (x1 - x)*(x1 - x) + (y1 - y)*(y1 - y) - r1*r1;  
    if (s > 0)  
    {  
        return
 
 "在圓外";  
    }  
    else  
    {  
        return "在圓內";  
    }  
}  
char* MyCircle::judge(MyPoint &p)  
{  
    double s = (x1 - p.getX0())*(x1 - p.getX0()) + (y1 - p.getY0())*(y1 - p.getY0()) - r1*r1;  
    if (s > 0)  
    {  
        return "在圓外";  
    }  
    else  
    {  
        return "在圓內";  
    }  
}  

//MyPoint.cpp  
#include "MyPoint.h"  

void MyPoint::setPoint(double x, double y)  
{  
    x0 = x;  
    y0 = y;  
}  
double MyPoint::getX0()  
{  
    return x0;  
}  
double MyPoint::getY0()  
{  
    return y0;  
}  

//MainTestClass.cpp  
#include "iostream"  
#include "MyCircle.h"  
#include "MyPoint.h"  

using namespace std;  

void main()  
{  
    MyCircle c1;  
    MyPoint  p1;  
    c1.setCir(2, 2, 3);  
    p1.setPoint(1, 1);  
    cout << c1.judge(p1.getX0(), p1.getY0()) << endl;  
    cout << c1.judge(p1) << endl;  

    system("pause");  
}