#include <iostream>

using namespace std;

namespace first

{

int var = 5;

}

namespace second

{

double var = 3.1416;

}

int main () {

cout << first::var << endl;

cout << second::var << endl;

return 0;

}

关键字using可以帮助从namespace中引入名字到当前的声明区域

#include <iostream>

using namespace std;

namespace first

{

int x = 5;

int y = 10;

}

namespace second

{

double x = 3.1416;

double y = 2.7183;

}

int main () {

using first::x;

using second::y;

cout << x << endl;

cout << y << endl;

cout << first::y << endl;

cout << second::x << endl;

return 0;

}

using也可以导入整个的namespace

#include <iostream>

using namespace std;

namespace first

{

int x = 5;

int y = 10;

}

namespace second

{

double x = 3.1416;

double y = 2.7183;

}

int main () {

using namespace first;

cout << x << endl;

cout << y << endl;

cout << second::x << endl;

cout << second::y << endl;

return 0;

}