状态模式C++

Question

我正在尝试引入一个简单的状态模式，在下面介绍了一些优秀的教程之后：http://gameprogrammingpatterns.com/state.html

在本教程中，我已经完成了一半，我试图通过将它们包含在基类中来复制每个状态的静态实例。然而，当涉及到切换状态时，g++抛出了这个错误。

   state_test.cpp: In member function ‘virtual void Introduction::handleinput(Game&, int)’:
state_test.cpp:55:16: error: cannot convert ‘Playing*’ to ‘GameState*’ in assignment
    game.state_ = &GameState::play;
                ^

现在，我理解了这个错误涉及指针的转换，但是我真的很难知道如何修复它。当我遵循这些家伙的代码时，我有点希望它能起作用，但是因为他正在改变它，并试图加强最佳实践，所以我没有他完整的源代码可以遵循。但是，我觉得在本教程的其余部分开始之前，在这个阶段理解代码是很重要的。

下面是我创建的代码，试图复制他的状态系统：

#include <iostream>

class Game;
class Introduction;
class Playing;

class GameState
{
public:

    static Introduction intro;
    static Playing play;

    virtual ~GameState() {std::cout << "an undefined GameState has been destroyed" << std::endl;}
    virtual void handleinput(Game& game, int arbitary) {}
    virtual void update(Game& game) {}

};

class Game
{
public:

    Game()
    {}
    ~Game()
    {}

    virtual void handleinput(int arbitary)
        {
            state_->handleinput(*this, arbitary);
        }

    virtual void update()
        {
            state_->update(*this);
        }

//private: 
    GameState* state_;
};

class Introduction : public GameState
{
public:

    Introduction()  
    {
        std::cout << "constructed Introduction state" << std::endl;
    }

    virtual void handleinput(Game& game, int arbitary) 
        {
            if (arbitary == 1)
            game.state_ = &GameState::play;
        }

    virtual void update(Game& game) {}
};

class Playing : public GameState
{
public:
    Playing()   {std::cout << "constructed Playing state" << std::endl;}

    virtual void handleinput(Game& game, int arbitary) 
        {
            if (arbitary == 0)
            game.state_ = &GameState::intro;
        }

    virtual void update(Game& game) {}
};

int main(int argc, char const *argv[])
{
    Game thisgame;

    return 0;
}

知道为什么我的实现没有编译吗？

编辑：

因此，为了回应先前的辅导，我对此非常感激，我修改了代码。首先，我把所有这些放在单独的文件中，但这比这么少量的测试代码更麻烦。我只需重写一个声明类的头文件，然后在.cpp文件中定义它们。

下面是.h文件：

class Introduction;
class Playing;
class Game;
class GameState;

class GameState
{
    public:

    static Introduction intro;
    static Playing play;

    virtual ~GameState();
    virtual void handleinput(Game& game, int arbitary);
    virtual void update(Game& game);

};


class Introduction : public GameState
{
public:

    Introduction();

    virtual void handleinput(Game& game, int arbitary); 

    virtual void update(Game& game);

};

class Playing : public GameState
{
public:
    Playing();

    virtual void handleinput(Game& game, int arbitary);

    virtual void update(Game& game);    
};


class Game
{
public:

    Game();

    ~Game();

    virtual void handleinput(int arbitary);

    virtual void update();

    GameState* state_;

};

下面是.cpp文件：

#include <iostream>
#include "state.h"


GameState::~GameState() 
    {std::cout << "Exiting Game State Instance" << std::endl;}
void GameState::handleinput(Game& game, int arbitary) 
    {}
void GameState::update(Game& game) 
    {}



Game::Game()
    {}
Game::~Game()
    {}
void Game::handleinput(int arbitary)
        {
            state_->handleinput(*this, arbitary);
        }

void Game::update()
        {
            state_->update(*this);
        }


Introduction::Introduction()    
    {
        std::cout << "constructed Introduction state" << std::endl;
    }

void Introduction::handleinput(Game& game, int arbitary) 
        {
            if (arbitary == 1)
            game.state_ = &GameState::play;
        }

void Introduction::update(Game& game) {}


Playing::Playing()  
        {
            std::cout << "constructed Playing state" << std::endl;
        }

void Playing::handleinput(Game& game, int arbitary) 
        {
            if (arbitary == 0)
            game.state_ = &GameState::intro;
        }

void Playing::update(Game& game) {}



int main(int argc, char const *argv[])
{
    Game mygame;
    return 0;
}

但我还是不能让它起作用。前面的错误已经消失了，但是我很难访问"introduction“的静态实例，并在基类中运行。引发的错误是：

/tmp/ccH87ioX.o: In function `Introduction::handleinput(Game&, int)':
state_test.cpp:(.text+0x1a9): undefined reference to `GameState::play'
/tmp/ccH87ioX.o: In function `Playing::handleinput(Game&, int)':
state_test.cpp:(.text+0x23f): undefined reference to `GameState::intro'
collect2: error: ld returned 1 exit status

我以为我被调查过了！太沮丧了！

我要补充的是，RustyX提供的答案确实是编译的，但是我必须将“that”和"introduction“的实例移到类定义之外，然后我不能再将它们设置为静态的，我认为这很重要，因为我只需要每个实例的一个实例，我希望尽早初始化它们。

Answer 1

问题是编译器从上到下读取文件。在包含

game.state_ = &GameState::play;

他仍然不知道Playing是从GameState继承来的。它只知道Playing是以后将声明的类。

您应该将类声明与方法实现分开。先拥有所有的类声明，然后才有方法实现。在更大的项目中，您会将它们分割到单独的*.h和*.cpp文件中，这种排序将自然发生。

缩短的例子：

class Playing : public GameState
{
public:
    Playing();

    virtual void handleinput(Game& game, int arbitary);

    virtual void update(Game& game);
};

// Declarations of other classes...


Playing::Playing() {
    std::cout << "constructed Playing state" << std::endl;
}

void Playing::handleinput(Game& game, int arbitrary) {
    if (arbitary == 0)
        game.state_ = &GameState::intro;
    }
}

void Playing::update(Game& game) {
}

您可以在类声明中保留一些方法。通常情况下，如果方法很小，会从内联中受益，并且不存在这种循环依赖问题，就会这样做。

Answer 2

将函数的实现移到所有类的定义之后。

编译器必须完整地查看继承的类Playing和Introduction，才能知道它们是从GameState继承的。

#include <iostream>

class Game;
class Introduction;
class Playing;

class GameState
{
public:

    static Introduction intro;
    static Playing play;

    virtual ~GameState() {std::cout << "an undefined GameState has been destroyed" << std::endl;}
    virtual void handleinput(Game& game, int arbitary) {}
    virtual void update(Game& game) {}

};

class Game
{
public:

    Game()
    {}
    ~Game()
    {}

    virtual void handleinput(int arbitary)
        {
            state_->handleinput(*this, arbitary);
        }

    virtual void update()
        {
            state_->update(*this);
        }

//private: 
    GameState* state_;
};

class Introduction : public GameState
{
public:

    Introduction()  
    {
        std::cout << "constructed Introduction state" << std::endl;
    }

    virtual void handleinput(Game& game, int arbitary);

    virtual void update(Game& game) {}
};

class Playing : public GameState
{
public:
    Playing()   {std::cout << "constructed Playing state" << std::endl;}

    virtual void handleinput(Game& game, int arbitary);

    virtual void update(Game& game) {}
};

void Introduction::handleinput(Game& game, int arbitary) 
{
    if (arbitary == 1)
        game.state_ = &GameState::play;
}

void Playing::handleinput(Game& game, int arbitary) 
{
    if (arbitary == 0)
        game.state_ = &GameState::intro;
}

Introduction GameState::intro;
Playing GameState::play;


int main(int argc, char const *argv[])
{
    Game thisgame;

    return 0;
}