C++的命名空间

//chap1_2.cpp
//A simple app to demonstrate how to use namespace.
#include <iostream>
using namespace::std;

namespace One {
    int M = 200;
    int inf = 10;
}

namespace Two {
    int x;
    int inf = 100;
}

//using namespace::One

int main(int argc, char *argv[])
{
    using Two::x;
    cout << x << endl;
    x = -100;
    One::inf *= 1;
    cout << One::inf << endl;
    cout << One::M << endl;
    Two::inf *= 2;
    cout << Two::inf <<endl;
    cout << x << endl;
    return 0;
}