如何在 C++ 的构造函数和方法中正确使用接口？

Question

我开始学习 C++ 并且有 C# 背景，但我遇到了很多问题。

我想做的是尝试使用 C++ 复制我在以下 C# 片段中所做的完全相同的事情。这只是我经常使用的 MVP 模式的简单实现。

我尝试了许多不同的方法来使它与适当的现代 C++ 一起工作，但它不断给我编译错误，我无法正确理解这些错误。其中大部分是因为我无法找到将接口作为构造函数参数传递以及将接口存储为 class 的字段的正确方法。有人可以将此 C# 代码翻译成正确的 C++ 代码，或者至少给我一些建议吗？提前致谢。

注意：我正在尝试制作仅包含 classes 声明的头文件和包含实际实现的 cpp 文件。

// 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;
}

这是我要编译的 C++ 代码。为了清楚和简洁起见，我把没有 .h 和 .cpp 的所有东西放在这里，但正如我所说，我实际上是将 classes 与实现分开。

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);
}

Answer 1

elgonzo 是对的。你不应该开始通过翻译来学习语言，尤其是。介于 C# 和 C++ 之间。它们之间唯一的相似之处是关键字的命名约定。

在这种情况下，重要的区别（除了如何在 C++ 中声明接口之外）是 C++ 中的所有类型都是按值保存的，而 C# 类是按引用保存的。您不能使用任何一种语言创建接口的实例（即您不能在 C# 中执行 new IGameView()）。

因此，您的 Game 类型不能按值保存接口类型。它需要是一个指针或引用。这与 C# 完全不同，我建议您按照其他评论者所说的那样，先学习 C++ 基础知识，然后再回来学习。

编辑：

这是您发布的 C++ 代码的工作形式。它有注释解释 why/when 做什么。

// 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;
}

如何在 C++ 的构造函数和方法中正确使用接口？

How do I use interfaces in constructors and methods in C++ properly?

c#

c++

mvp

interface