// Windows crap, no pimpl
class MyThing
{
  HMODULE this_thing;
  HICON that_thing;
public:
  DoCrap();
};

// Unix crap no pimpl
class MyThing
{
  FILE this_thing;
  void* that_thing;
public:
  DoCrap();
};

// PIMPL:
class MyThing
{
  struct MyThingImplementation;
  MyThingImplementation* m_Pimpl;

public:
  DoCrap();
};

// now you have different CPP files for win/nix, which is ok. m_Pimpl size is same across platforms sizeof(pointer type), pretty much always 4 bytes.