How Can I Pass a Class Member Function Pointer to an External Function in C ?

Passing Member Function Pointers with Class Objects

When attempting to pass a member function within a class to an external function that requires a member function pointer, it is essential to understand the necessary steps. This article addresses a specific scenario where a class member function is being passed to a function in a separate class.

Within the provided code, the testMenu class includes a member function called test2() and a MenuButton that calls SetButton(). The challenge lies in correctly passing the test2() function pointer using the this pointer.

The key to solving this issue is to provide the external function with both a pointer to the object and a pointer to the specific function within the object. In the modified version of MenuButton::SetButton():

template <class object>
void MenuButton::SetButton(int xPos, int yPos, LPCWSTR normalFilePath,
        LPCWSTR hoverFilePath, LPCWSTR pressedFilePath,
        int Width, int Height, object *ButtonObj, void (object::*ButtonFunc)())
{
  BUTTON::SetButton(xPos, yPos, normalFilePath, hoverFilePath, pressedFilePath, Width, Height);

  this->ButtonObj = ButtonObj;
  this->ButtonFunc = ButtonFunc;
}

The object and function pointers are stored within the MenuButton object. The function can then be invoked using both pointers:

((ButtonObj)->*(ButtonFunc))();

Finally, within the testMenu class constructor, when setting the MenuButton, it is crucial to pass a pointer to the testMenu object:

testMenu::testMenu()
:MenuScreen("testMenu")
{
  x.SetButton(100,100,TEXT("buttonNormal.png"), TEXT("buttonHover.png"),
        TEXT("buttonPressed.png"), 100, 40, this, test2);
  draw = false;
}

By incorporating these modifications, the member function pointer can be successfully passed, allowing the external function to access and execute the desired member function within the testMenu class.

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