C++语言基础 例程 命名空间的使用

//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);
}

//改法1：std中的名称加限制
#include <iostream>
#include <math.h>
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};
    std::cout<<distance(p1,p2);  //出除了using namespace std，而在std中的cout前加上std::
}

//改法2：本地名称加限定
#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);  //保留using namespace std，而在distance前加上::，表示distance是“本地的”
    return 0;
}

//改法3：本地改名避冲突
#include <iostream>
#include <math.h>
using namespace std;
class point
{
public:
    double x;
    double y;
};

double dist(point p1,point p2)   //本地函数改名
{
    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<<dist(p1,p2);
    return 0;
}

//改法4：明确定义本地的命名空间
#include <iostream>
#include <math.h>
using std::cout;
namespace mypoint
{
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()
{
    mypoint::point p1= {3,5},p2= {6,9};
    cout<<distance(p1,p2);   //不使用std命名空间
    return 0;
}