Re-learning C ++ (2) -- pointer to class member (from: http://www.wangchao.net.cn/bbsdetail_73812.html)

class A　　{　　 int _val;　　 int val();　　};　　int (A::*p_val) = &A::_val;　　int ( A::*p_func )() = &A::val;

Pointer to class members, which is rarely used in the impression. When I re-learn C ++, I found that I ignored a very important thing, I used to think that the member functions of the class cannot be used as Callback functions. Therefore, many C Programs cannot be transplanted to C ++ until now, this is because you do not know the pointer to a class member.
1. pointer to non-static members
In fact, it is easy to point to non-static class members. The only difference between them and common pointers is that they are restricted by classes. As follows:
　　
As you can see, yes. The difference with a normal pointer is that the pointer to a class member must also carry the class. In the above example, :: put this qualifier together, then? The usage is the same as that of normal pointers.
2. pointer to a static member
Pointer to a static member. The declaration method is exactly the same as that of a normal pointer, but the class qualifier must be added when values are assigned. Why? I think it can be understood that the existence of non-static members depends on the class. When the class disappears, non-static members die. Therefore, the declaration must be tied to the class qualifier, static members are not attached to the class. Therefore, the class qualifier is not required. As follows:
　　

class A　　{　　 static int _val;　　 static int val();　　};　　int *p_val = &A::_val;　　int (*p_func) = &A::val;

3. Benefits:
One advantage is that by pointing to a member's function pointer, you can easily call each member function. Another advantage is that for a static member function, you can become a callback function in C.
The following is an example to further understand:
　　

#include <iostream>　　#include <string>　　　　using namespace std;　　　　typedef void (*funchandler)();　　　　void register_func(funchandler f)　　{　　 cout << "register_func" << endl;　　 (*f)();　　}　　　　class A　　{　　public:　　 A() : _val( 0 ) { cout << "create A..." << endl; }　　 void test() { cout << "test..." << endl; }　　 void test1() { cout << "test1..." << endl; }　　 void test2() { cout << "test2..." << endl; }　　 int val() { return _val; }　　 static void test3() { cout << "test3..." << endl; }　　 int _val;　　private: 　　};　　　　　　　　int main()　　{　　 A a;　　 int ( A::*p_val ) = 0;　　 p_val = &A::_val;　　 cout << "a.*p_val: " << a.*p_val << endl;　　 　　 void (A::*p_func)();　　 p_func = &A::test;　　 　　 a.test();　　 (a.*p_func)();　　 　　 p_func = &A::test1;　　 ( a.*p_func )();　　 p_func = &A::test2;　　 ( a.*p_func )();　　 　　 void (* pp_func)();　　 pp_func = &A::test3;　　 (*pp_func)();　　 　　 register_func( pp_func );　　 return 0;　　 　　}