I'm starting to learn C++ and coming from a C# background I'm having a lot of problems.
What I want to do is trying to replicate exactly the same thing that I'm doing in the following C# snippet, with C++. This is just a simple implementation of the MVP pattern, which I use quite a lot.
I tried in many different ways to make this work with proper and modern C++ but it's constantly giving me compilation errors which I do not understand properly. Most of them are due to the fact that I'm not able to find a proper way to pass an Interface as a constructor parameter and also to store an Interface as a field of a class. Could someone translate this C# code into a proper C++ code, or at least give me some advice please? Thanks in advance.
NOTE: I'm trying to make headers files with only classes declaration and cpp files with actual implementation.
// This is my C# implementation that I want to convert in C++
public class Program
{
public static void Main()
{
IGameView view = new GameView();
Game game = new Game(view);
view.SetPresenter(game);
}
}
public interface IGameView
{
void SetPresenter(Game game);
}
public class GameView : IGameView
{
public void SetPresenter(Game game)
{
_game = game;
}
Game _game;
}
public class Game
{
public Game(IGameView view)
{
_view = view;
}
IGameView _view;
}
This is the C++ code that I'm trying to make compile. I've put everything here without .h and .cpp for clarity and brevity, but as I said I'm actually separating classes from implementation.
class Game
{
public:
(IGameView& view) : _view(view)
{ }
Game operator=(const Game &);
private:
IGameView& _view;
};
class IGameView
{
public:
virtual ~IGameView() {}
virtual void SetPresenter(const Game) = 0;
};
class GameView : public IGameView
{
public:
GameView();
void SetPresenter(const Game game) override
{
_game = game;
}
private:
Game& _game;
};
int main()
{
IGameView view;
Game game(view);
view.SetPresenter(game);
}
elgonzo is right. You shouldn't start learning a language by doing translations, esp. going between something like C# and C++. The only similarity between them is naming conventions for keywords.
In this case the important difference (aside from how to declare interfaces in C++) is that all types in C++ are held by value, whereas C# classes are held by reference. You can't create an instance of an interface in either language (i.e. you can't do new IGameView()
in C#).
Thus, your Game
type can't hold an interface type by value. It needs to be a pointer or reference instead. This is all very different from C#, and I suggest you do as the other commenters have said and learn C++ fundamentals first and come back to this later.
Edit:
Here's a working form of the C++ code you posted. It has comments explaining why/when to do what it does.
// C++ requires declaring types before you use them.
// if we want to use Game before defining it we must at least declare that it exists.
class Game;
// this is an interface because it contains abstract (pure virtual) functions.
// you cannot create an instance of an abstract type - but you can make a reference or pointer to one.
struct IGameView
{
// we might want polymorphic deletion - so to be safe we'll make a virtual dtor.
virtual ~IGameView() {};
// Game is probably expensive to copy - pass it by reference.
// = 0 makes this an abstract method, which makes this an abstract type.
virtual void SetPresenter(Game &game) = 0;
};
// --------------------------------------------
class Game
{
public:
// take a reference to the (interface) object to use (we can't pass an abstract type by value)
Game(IGameView &view) : _view(view) { }
private:
// hold a reference to the IGameView (interface) object.
// if you ever wanted this to refer to something else this would need to be pointer instead.
// references are kind of like pointers that cannot be repointed to something else.
IGameView &_view;
};
class GameView : public IGameView
{
public:
GameView();
virtual void SetPresenter(Game &game) override
{
_game = &game;
}
private:
// hold a pointer to the Game object.
// this has to be a pointer because SetPresenter() needs to be able to repoint it (refences can't do that).
// for safety, initialize this to null.
Game *_game = nullptr;
};
// ---------------------------------------------
int main()
{
GameView view; // create the game view to use
Game game(view); // create the game object and use that view
view.SetPresenter(game); // set the view to use the game object
return 0;
}