Hi all,

Currently working a school project which happens to be a multiplayer 2D platform / fighting game, I'm now organizing all the classes in different subgroups, mainly to allow a certain level of abstraction when handling game objects.

Basically right now this is what worked out for object events / handling / viewing:
[indent=1]An Object class allows abstraction for game object updating.
[indent=1]An Input class represents a state of user input (keyboard, mouse, joystick).
[indent=1]A View class provides an interface for display, and will probably be extended by an Animation class.

[indent=1]Associated Observer classes update these Subjects' state (Observer pattern)

The idea is to do the 3rd party-specific operations in the Observer classes (events -> Listener, rendering -> Canvas).

For example, for the Input module:
[source lang="cpp"]
struct Input
{
class Mouse
{
public:
enum Button
{
Any = 0,
Left = 1,
Right = 2,
// etc...
};
bool IsDown(Button = Any) const;

protected:
friend class Listener;
void setDown(Button);
void setPos(int, int);

private:
int btns_down_;
int x_, y_;
} mouse;

class Keyboard
{
public:
enum Key
{
A, B, C, // etc...
};
bool IsDown(Key) const;

protected:
friend class Listener;
void setDown(Key);

private:
std::set<Key> keys_down_;
} keyboard;

// similar for joystick

};
[/source]

But so far I can't figure out how to do this specifically in the observers, without introducing dependencies in the Subject classes. This problem is mainly present concerning an object's view: concrete classes need knowledge about camera, alpha blending, coloring, etc...

Besides the obvious impossible solutions like "code up SFML / SDL / Allegro / whatever", what options do I have?
Is it actually possible (in practical cases) to separate these APIs from the game core, and if so, is this overkill?

The typical approach is to encapsulate the 3rd party library within a class or possibly a group of classes, which then become the public interface used by the rest of your program.
Why is Input a struct? It looks to me like you're trying to use it as a namespace. Just use namespaces

Encapsulating the library is exactly my point, this was what I was going for at first, but in that case I would roughly need to write wrappers and callbacks for a bunch of classes.
Is there a way to avoid this, or should I just shut up and code?

Oh and Input used to be a namespace actually , I can't remember why it was decided to change it to a struct though.

I can't really think of a way to avoid writing wrappers. The trick is to design the wrappers well. You're unlikely to use the entirety of a given library, so there's little use wrapping what you won't need for your project. Also, you may be able to simplify some calls, or generalize the interface if you intend to allow a choice of libraries. This would require writing a wrapper for each library, but so long as they share an interface (using an abstract class in c++) then you could swap one for the other with no change needed to the rest of your code.