Question

我在尝试了一些简单的状态模式之后，遵循了一些优秀的教程：http://gameprogrammingpatterns.com/state.html

我正在完成当前教程的一半，并且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。但是，当谈到切换状态时，g ++会抛出这个错误。

   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，我理解错误涉及指针的转换，但我真的很难看到如何解决它。当我跟随这些人的代码时，我有点期待它的工作，但是因为他正在改变它，并试图强化最佳实践，我没有完整的源代码可以遵循。但是，在我完成本教程的其余部分之前，我认为在此阶段理解代码非常重要。

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;
}

为什么我的实施没有编译？

编辑：

因此，为了回应早期的课外辅导，我非常感激，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，这比它的价值更麻烦。我只是重写了一个声明类的头文件，然后在.cpp文件中定义它们。

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;
}

我仍然无法让它发挥作用。之前的错误已经消失，但我正在努力访问＆＃34;介绍＆＃34;的静态实例。并在基类内部玩。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

我以为我怀疑了！太沮丧了！

我应该补充一点，RustyX提供的答案确实可以编译，但是我必须移动＆＃34;播放＆＃34;和＆＃34;介绍＆＃34;在类定义之外，我可以不再将它们设置为静态，我相信这很重要，因为我只需要一个实例，我希望它们尽早初始化。

Answer 1

问题是编译器从上到下读取文件。在包含

的行

game.state_ = &GameState::play;

他仍然不知道Playing继承自GameState。它只知道Playing是一个稍后将被声明的类。

您应该从方法实现中拆分类声明。首先是所有类声明，然后是方法实现。在更大的项目中，您可以将它们全部拆分为单独的* .h和* .cpp文件，这种排序会自然发生。

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，可以从内联中受益，并且不会出现这种循环依赖问题。

Answer 2

在所有类的定义之后，将函数的实现移到之外。

编译器必须完全看到继承的类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; }

状态模式C ++

2 个答案: