So simply I can create an object which can hold the method address and then give it to callback as the second argument. Sure.
If FLTK could accept class member addresses there would be no need for this workarounds. Is that possible? callback() should have had an overload to get a thiscall void*() right?
[edit]
I miss read your explanation about pointers to objects. I thought you said only member functions cant be dereferenced.
[edit2]
Actually thiscall void*(void*) or something...
