Question

This question is similar to what I’m trying to do Calling C++ member function pointer from a struct .
However my structure contains a member function pointer that is defined in a different class then the one the structure is defined and used in. Here is some example code of how my classes, structures and function pointers are laid out.

// Alpha.h:
class Alpha{
public:
    void function1(char name[], int number);
    void function2(char name[], int number);
    void function3(char name[], int number);

    typedef void (Alpha::*My_func_ptr)(char name[], int number);

    static My_func_ptr functionTable[];
};

// Alpha.cpp:
#include "Alpha.h"

Alpha::My_func_ptr Alpha::functionTable[] = {
    &Alpha::function1, 
    &Alpha::function2, 
    &Alpha::function3
};

void Alpha::function1(char name[], int number)
{
    //some stuff
}

void Alpha::function2(char name[], int number)
{
    //some stuff
}

void Alpha::function3(char name[], int number)
{
    //some stuff
}

// Beta.h:
#include "Alpha.h"

typdef struct{
    char bName[10];
    Alpha::My_func_ptr fptr;
}ptr_structure;

class Beta{
public:
      void betafunction();

      Alpha alphaobject;
      ptr_structure str_array[3];
};

// Beta.cpp:
#include "Beta.h"

void betafunction()
{
    str_array[0].fptr = alphaobject.functionTable[0];
    str_array[1].fptr = alphaobject.functionTable[1];
    str_array[2].fptr = alphaobject.functionTable[2];

    (str_array[0].fptr)("name", 1); //gives error expression must have 
                                    //(pointer-to-)  function type

    (this->*str_array[0].fptr)("name", 1);
    //error pointer-to-member selection class types are incompatible "Beta" and "Alpha"

    //sample function pointer call using function table from other class,
    //this syntax compiles and runs without error.
    (alphaobject.*Alpha::functionTable[0]("name", 1); 
}

As you can see I can call the function pointer from an array, but can’t seem to figure out how to call a function pointer from inside an array of structures.

Answer 1

When calling a through member function pointer, you need to have an instance of the object associated with that pointer:

 (alphaobject.*(str_array[0].fptr))("name", 1)
  ^^^^^^^^^^^