// sample program to understand namespacee
using namespace std;

#include <iostream>
namespace myspace0{
        int n = 1001;
}
namespace myspace1{
        int m = 1002;
}

int main()
{
        cout << "testing namespace : before namespace declaration" << endl;
        myspace0::n = 2001;
        cout << "myspace0::n=" << myspace0::n << endl;
        cout << "myspace1::m=" << myspace1::m << endl;
        cout << "testing namespace : after namespace declaration" << endl;
        using namespace myspace1;
        using namespace myspace0;
        n = 3001;
        cout << "myspace0::n=" << n << endl;
        cout << "myspace1::m=" << m << endl;
        return 0;
}

==========================================================

$ ./a.exe
testing namespace : before namespace declaration
myspace0::n=2001
myspace1::m=1002
testing namespace : after namespace declaration
myspace0::n=3001
myspace1::m=1002