How Can I Correctly Pass Member Function Pointers in C ?

Passing Member Function Pointers in C

In object-oriented programming, it is often necessary to pass a member function as an argument to another function. However, doing this correctly can be a bit tricky, especially when using the this pointer.

Let's consider a specific example from the code snippet provided:

class testMenu : public MenuScreen {
  // ...
  void test2() {
    draw = true;
  }
};

MenuButton<testMenu> x;

testMenu() : MenuScreen("testMenu") {
  x.SetButton(100, 100, ..., &test2);
}

Here, the test2 member function is assigned to the ButtonFunc member of the MenuButton using the SetButton function:

template <class object>
void MenuButton::SetButton(..., void (object::*ButtonFunc)()) {
  this->ButtonFunc = &ButtonFunc;
}

The problem arises when calling the test2 function from the MenuButton class. To do this, we need both a pointer to the object (i.e., testMenu) and a pointer to the function (i.e., &test2). In the modified version of SetButton:

template <class object>
void MenuButton::SetButton(..., object *ButtonObj, void (object::*ButtonFunc)()) {
  this->ButtonObj = ButtonObj;
  this->ButtonFunc = ButtonFunc;
}

We pass a reference to the object, which can be accessed using the ButtonObj pointer. The member function pointer is then invoked using ((ButtonObj)->*(ButtonFunc))().

Finally, the corrected testMenu constructor:

testMenu() : MenuScreen("testMenu") {
  x.SetButton(100, 100, ..., this, &test2);
}

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