Microsoft's GDI+ defines many empty classes to be treated as handles internally. For example, (source GdiPlusGpStubs.h)
//Approach 1
class GpGraphics {};
class GpBrush {};
class GpTexture : public GpBrush {};
class GpSolidFill : public GpBrush {};
class GpLineGradient : public GpBrush {};
class GpPathGradient : public GpBrush {};
class GpHatch : public GpBrush {};
class GpPen {};
class GpCustomLineCap {};
There are other two ways to define handles. They're,
//Approach 2
class BOOK; //no need to define it!
typedef BOOK *PBOOK;
typedef PBOOK HBOOK; //handle to be used internally
//Approach 3
typedef void* PVOID;
typedef PVOID HBOOK; //handle to be used internally
I just want to know the advantages and disadvantages of each of these approaches.
One advantage with Microsoft's approach is that, they can define type-safe hierarchy of handles using empty classes, which (I think) is not possible with the other two approaches, though I wonder what advantages this hierarchy would bring to the implementation? Anyway, what else?
EDIT:
One advantage with the second approach (i.e using incomplete classes) is that we can prevent clients from dereferencing the handles (that means, this approach appears to support encapsulation strongly, I suppose). The code would not even compile if one attempts to dereference handles. What else?
The same advantage one has with third approach as well, that you cannot dereference the handles.
Approach #1 is some mid-way between C style and C++ interface. Instead of member functions you have to pass the handle as argument. The advantage of exposed polymorphism is that you can reduce the amount of functions in interface and the types are checked compile time. Usually most experts prefer pimpl idiom (sometimes called compilation firewall) to such interface. You can not use approach #1 to interface with C so better go full C++.
Approach #2 is C style encapsulation and information hiding. The pointer may be (and often is) a pointer to real thing, so it is not over-engineered. User of library may not dereference that pointer. Disadvantage is that it does not expose any polymorphism. Advantage is that you may use it when interfacing with modules written in C.
Approach #3 is over-abstracted C-style encapsulation. The pointer may be really not a pointer at all since user of library should not cast, deallocate or dereference it. Advantage is that it may so carry exception or error values, disadvantage is that most of it has to be checked run time.
I agree with DeadMG that language-neutral object-oriented interfaces are very easy and elegant to use from C++, but these also involve more run-time checks than compile time checks and are overkill when i don't need to interface with other languages. So i personally prefer Approach #2 if it needs to interface with C or Pimpl idiom when it is C++ only.