Reading Nymia's diary entries makes me suspect that the method used as a callback is not static. The "this" pointer is usually given as first argument of a method, unless the method is static. If you need to call methods of the class in the callback pass a pointer to an instantiation of the class. eg.
void SomeClass::callback(void* arg) { SomeClass* that = (SomeClass*)arg; that->SomeMethod(); }