C++命名空间

#include <iostream>
using namespace std;
namespace Jack {
int age;
void sayHello() {
cout << "hello, I'm Jack" << endl;
}
}
namespace Johns {
int age = 20;
void sayHello() {
cout << "hello, I'm Johns" << endl;
}
}
int main() {
Jack::age = 10;
cout << Jack::age << endl;
Jack::sayHello();
cout << Johns::age << endl;
using Johns::sayHello;
sayHello();
return 0;
}