1 using namespace std;
 2 namespace A {
 3     int a = 100;
 4     namespace B            //嵌套一个命名空间B
 5     {
 6         int a = 20;
 7     }
 8 }
 9 
10 int a = 200;//定义一个全局变量
11 
12 
13 int main(int argc, char *argv[]) {
14     cout << "A::a =" << A::a << endl;        //A::a =100
15     cout << "A::B::a =" << A::B::a << endl;  //A::B::a =20
16     cout << "a =" << a << endl;              //a =200
17     cout << "::a =" << ::a << endl;          //::a =200
18 
19     using namespace A;
20     cout << "a =" << a << endl;     // Reference to 'a' is ambiguous // 命名空间冲突，编译期错误
21     cout << "::a =" << ::a << endl; //::a =200
22 
23     int a = 30;
24     cout << "a =" << a << endl;     //a =30
25     cout << "::a =" << ::a << endl; //::a =200
26 
27     //即：全局变量 a 表达为 ::a，用于当有同名的局部变量时来区别两者。
28 
29     using namespace A;
30     cout << "a =" << a << endl;     // a =30  // 当有本地同名变量后，优先使用本地，冲突解除
31     cout << "::a =" << ::a << endl; //::a =200
32 
33 
34     return 0;
35 }