using和名空间namespace

#include <iostream>
#include <string>
using namespace std;

namespace first
{
int x = 10;
int y = 2;
}
namespace second
{
double x = 3.2;
double y = 2.3;
}

int main{

using namespace first;
cout<<x<<‘\n‘;
cout<<y<<‘\n‘;
cout<<second::x<<‘\n‘;
cout<<second::y<<‘\n‘;
//另一种写法
using first::x;
using second::y;
cout<<x<<‘\n‘;
cout<<y<<‘\n‘;

}

输出：10

        2

        3.2

        2.3

        10

        2.3