This is the simpliest menu declaration I would use:

Fl_Menu_Item menu_item[] = {
 {"name", 0, nullptr, (void*)(int_ptr)},
 {0}
};

The o->copy code is looking for the zero-filled item as a terminator.

Thanks a lot, I have a bit of an issue though...

Here is my code (roughly, my memory isn't perfect):

Fl_Menu_Item menu_item[] = { {"name", 0, nullptr, int_ptr } }; // here int_ptr is a int* which exists for the entire duration of the program
Fl_Menu_Button* o = new Fl_Menu_Button(400, 30, 40, 15);

o->copy(menu_item); // < here, it crashes and complains about menu_item.size()

Do you know what this might be caused by, and how it could be fixed?