贺老师的教学链接  本课讲解

命名空间namespace是个老相识

//正确程序1
#include <iostream>
using namespace std;
int main( )
{
    cout<<"hello world!"<<endl;
    return 0;
}

//出问题的程序
#include <iostream>


int main( )
{
    cout<<"hello world!"<<endl;
    return 0;
}

//再一种写法
#include <iostream>


int main( )
{
    std::cout<<"hello world!"<<std::endl;
    return 0;
}

吓人的错误！

//出错的程序
#include <iostream>
#include <math.h>
using namespace std;
class point
{
public:
    double x;
    double y;
};


double distance(point p1,point p2)
{
    double d=sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
    return d;
}


int main()
{
    point p1= {3,5},p2= {6,9};
    cout<<distance(p1,p2);
}