c++中 Hello world第一个程序

//引入io输入输出流
#include <iostream>

using namespace std;
using std::cin;
using std::printf;
//面向过程的方式，求圆的面积
void main1(void)
{
    printf("Hello World!");
    printf("\t");//Tab
    cout << "Hello" << endl;
    printf("你好啊。");
    printf("\n");//换行
    printf("我很好啊。");
    int c = 12;
    c += c -= c*c;
    cout << c << endl;//c的值为-264
    printf("\n");//换行
    
    double r = 0;
    double s = 0;

    cout << "求圆的面积：" << endl;
    cout << "请输入圆的半径：";
    cin >> r;
    cout << "r的值是：" << r << endl;
    s = 3.14*r*r;
    cout << "圆形的面积是s:" << s << endl;

    system("pause");

}

/**********
------------------------------------------------------------------------------------------------------------------------
******/
class MyCircle
{
public:
    double m_s;//圆的面积 属性 成员变量
    double m_r;//圆的半径

public:
    //成员函数
    void setR(double r)
    {
        m_r = r;
    }
    double getR()
    {
        return m_r;
    }
    double getS()
    {
        return 31.4*m_r*m_r;
    }

};

//用面向对象的方法
// 1  类的抽象 成员变量和成员函数

void main2()
{
    MyCircle c1, c2;//用类定义变量
    double r1,r2;

    //圆形1
    cout << "请输入c1圆形的半径：";
    cin >> r1;

    c1.setR(r1);
    cout << "c1圆的面积为：" << c1.getS() <<"\n";

    //圆形2
    cout << "请输入c2圆形的半径：";
    cin >> r2;

    c2.setR(r2);
    cout << "c2圆的面积为：" << c2.getS() << "\n";

    system("pause");
}
/**********
------------------------------------------------------------------------------------------------------------------------
******/

class Circle
{
public:
    double r;

    double p1 = 3.14;
    //这样写只会被初始化，不会被执行
    //double area = p1*r*r;

    double getS()
    {
        return p1*r*r;
    }

};

void main()
{
    Circle c1;
    cout << "please input your r:";
    cin >> c1.r;

    cout << "c1圆的面积为:" << c1.getS() << endl;

    system("pause");
}