初步瞭解C++命名空間

聽說，一個好的工程都要有自己的命名空間，以體現C++的封裝性，最近學習了下，代碼不長，只爲弄懂原理。如有不當，請來者指教。

//file3.h

namespace A{

class One{
public:
int fun();
};
}
namespace B{
class Two{
public:
int fun();
};
}
class Three{
public:
int fun1();
void fun2();

};

//file3.cpp

#include <iostream>
using namespace std;
#include "file3.h"
int A::One::fun(){
return 1;
}
int B::Two::fun(){
return 2;
}
int Three::fun1(){
return 3;
}
void Three::fun2(){
cout<<"4"<<endl;
}

//main.cpp

//命名空間問題
#include <iostream>
#include "file3.h"
using namespace std;
using namespace A;
using namespace B;
void main(){
One one;
Two two;
Three three;
cout << "result = " <<one.fun() << endl;
cout << "result = " <<two.fun() << endl;
//cout << "result = "<<three.fun()<< endl;
three.fun1();  //無用，只做比照
cout << "result = " <<three.fun1() << endl;
cout << "result = " ;
three.fun2();
system("pause");
}