名字空间

Format:

namespace identifier
{
entities
}

The keyword using is used to introduce a name from a namespace into the current declarative region.
Example:

#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(void)
{
        using first::x;
        using second::y;
        cout << x << endl;
        cout << y << endl;
        cout << first::y << endl;
        cout << second::x << endl;

        return (0);
}

We can declare alternate names for existing namespaces：

namespace new_name = current_name;