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 }
