命名空间里的运算符

 1 #include <iostream>
 2 using namespace std;
 3 
 4 class X {
 5 public:
 6     void operator+(int) {
 7         cout << "from class X" << endl;
 8     }
 9 };
10 
11 int main() {
12     X x;
13     x + 1;    // from class X
14 
15     return 0;
16 }

 1 #include <iostream>
 2 using namespace std;
 3 
 4 class X {
 5 public:
 6 };
 7 
 8 class Y {
 9 public:
10 };
11 
12 
13 int main() {
14     X x;
15     Y y;
16 
17     void operator+(X& x, Y& y);
18 
19     x + y;    // from function
20 
21     return 0;
22 }
23 
24 void operator+(X& x, Y& y) {
25     cout << "from function" << endl;
26 }

 1 #include <iostream>
 2 using namespace std;
 3 
 4 namespace NX {
 5     class X {
 6     public:
 7         /*void operator+(int) {
 8             cout << "from class X" << endl;
 9         }*/
10     };
11 
12     void operator+(X& x, int) {
13         cout << "form namespace NX" << endl;
14     }
15 }
16 
17 int main() {
18     NX::X x;
19     x + 1;    // from namespace NX
20 
21     return 0;
22 }

 1 #include <iostream>
 2 using namespace std;
 3 
 4 class X {
 5 public:
 6 };
 7 
 8 namespace NY {
 9     class Y {
10     public:
11     };
12 
13     void operator+(X& x, Y& y) {
14         cout << "from namespace NY" << endl;
15     }
16 }
17 
18 int main() {
19     X x;
20     NY::Y y;
21 
22     x + y;    // from namespace NY
23 
24     return 0;
25 }