状态模式 C++

Question

在学习了这里的一些优秀教程之后，我正在尝试创建一个简单的状态模式：http://gameprogrammingpatterns.com/state.html

我正在完成当前教程的一半，我正在尝试复制每个状态的静态实例，方法是将它们包含在基础 class 中。但是，在切换状态时，g++ 会抛出此错误。

   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，我明白了错误涉及指针的转换，但我真的很难看到如何修复它。当我遵循这个人的代码时，我有点希望它能工作，但是因为他正在不断地改变它并试图加强最佳实践，所以我没有他的完整源代码可以遵循。但是，我觉得在我完成本教程的其余部分之前，在这个阶段理解代码对我来说很重要。

以下是我创建的代码，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;
}

为什么我的实现没有编译有什么想法吗？

编辑：

所以为了响应之前的辅导，对此我非常感谢，我修改了代码。我首先将它们全部放在单独的文件中，但是对于这么少量的测试代码来说，这带来的麻烦超过了它的价值。我只是重写了一个声明 classes 的头文件，然后在 .cpp 文件中定义了它们。

这是 .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_;

};

这是 .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;
}

而且我仍然无法让它工作。之前的错误已经消失，但我正在努力访问 "introduction" 的静态实例并在基础 class 内部播放。抛出的错误是：

/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

我以为我怀疑了！好沮丧！

我应该补充一点，RustyX 提供的答案确实可以编译，但是我必须将 "playing" 和 "introduction" 的实例移到 class 定义之外，然后我可以不再将它们设置为静态，我认为这很重要，因为我只需要每个实例，我希望它们尽早初始化。

Answer 1

问题是编译器是从上到下读取文件的。在包含

的行

game.state_ = &GameState::play;

他还不知道Playing继承自GameState。它只知道Playing是一个class，后面会声明

您应该将 class 声明与方法实现分开。先声明所有 class ，再声明方法。在更大的项目中，您会将它们全部拆分为单独的 *.h 和 *.cpp 文件，并且这种排序会自然发生。

简化示例：

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 class 声明中保留一些方法。通常如果方法很小，会从内联中受益并且没有这种循环依赖问题。

Answer 2

在所有类.

的定义之后将函数的实现移到
编译器必须完整地看到继承的类 Playing 和 Introduction 才能知道它们继承自 GameState.

#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; }

状态模式 C++

state pattern C++

c++

virtual

class

state-pattern