about function pointer in c++

zlly20

I have a program in c++

in myclass.h

1. 
   
2. class MyClass
   
3. {
   
4. public:
   
5.      MyClass();
   
6.      struct test_struct
   
7.      {
   
8.         //a is an array
   
9.         double *a;
   
10.         //sigma is a function pointer point to a member function of MyClass
    
11.         double (*sigma)(int, int);
    
12.      };
    
13.      
    
14.      void use_struct_example();
    
15. 
    
16. private:
    
17.      double somefunction(int, int);
    
18. }
    
19. 
    
20. in myclass.cpp
    
21. 
    
22. [CODE]
    
23. //constructor
    
24. MyClass::MyClass()
    
25. {
    
26. }
    
27. 
    
28. MyClass::use_struct_example()
    
29. {
    
30.      test_struct  ts;
    
31.      int myarray[5] = {1,2,3,4,5};
    
32.      ts.a = myarray;
    
33.      ts.sigma = &MyClass::somefunction;
    
34. 
    
35.      //some other operation here.....
    
36. }
    
37. MyClass::somefunction(int a, int b)
    
38. {
    
39. }
    

复制代码

when compile this program, there are two error reported by compiler:
first one is ISO C++ forbids taking the address of an unqualified non-static member function to form a pointer to member function.
second one is can not convert from type MyClass:: to type double.....

is there any one can explain how funciton pointer can be used in c++, particularly when you are in the situaion that you have to use a library that written in C, and in your c++ code, you have to pass a member function's address to a struct member, which obviously also written in C, encoded in that library.....

any attempts to solove my problem is much appreciated....

rickxbx

这个……
调用c++类里面的非静态成员函数，跟普通的函数不一样，有可能会多传一个参数（如gcc），有可能在某个寄存器里面放置一个值（如vc）（这个多加的一个参数，应该已经猜出来了，就是this）。所以呢，也不能像其他的函数指针那样操作类成员函数。stl 或者 boost 库（记不得是哪个了，说不定是两个）好象有对成员函数操作的封装，有兴趣可以参考参考

Buddhist[CHinA]

楼上正解。
不过把你的somefunction声明为static member function就可以了。

这个问题和为什么普通成员函数不能作为回调函数一个解释

marsbible

楼主,我查了一下资料,改了一下你的代码,可以跑了.主要就是要知道在c++中指向成员函数的指针在定义和调用时都要有作用域。

#include <iostream>

using namespace std;
class MyClass
{
public:
     MyClass();
     struct test_struct
     {
        //a is an array
         int *a;
        //sigma is a function pointer point to a member function of MyClass
         double (MyClass::*sigma)(int, int);
     };
     
     void use_struct_example();

private:
     double somefunction(int, int);
};

MyClass::MyClass()
{
}     

void MyClass::use_struct_example()
{
     test_struct  ts;
     int myarray[5] = {1,2,3,4,5};
     ts.a = myarray;
     ts.sigma = &MyClass::somefunction;
     (this->*(ts.sigma))(0,1);
     
}
double MyClass::somefunction(int a, int b)
{
   
    cout<<a<<b<<endl;
}
int main()
{
    int aa;
    MyClass a;
    a.use_struct_example();
    return 0;
}