state_pattern.cpp

#include <iostream>

class Heroine;

enum class Input
{
    PressDown,
    ReleaseDown
};

class HeroineState
{
public:
    virtual ~HeroineState() {}
    virtual void enter() {}
    virtual HeroineState *handleInput(Heroine &heroine, Input input) = 0;
    virtual void update(Heroine &heroine) {}
};

class StandingState : public HeroineState
{
public:
    void enter() override
    {
        std::cout << "now standing." << std::endl;
    }

    HeroineState *handleInput(Heroine &heroine, Input input) override;
};

class DuckingState : public HeroineState
{
public:
    DuckingState()
        : m_chargeTime(0)
    {
    }

    void enter() override
    {
        std::cout << "now ducking." << std::endl;
    }

    HeroineState *handleInput(Heroine &heroine, Input input) override;

    void update(Heroine &heroine) override
    {
        m_chargeTime++;
    }

private:
    int m_chargeTime;
};

class Heroine
{
public:
    Heroine()
    {
        m_state = new StandingState();
        m_state->enter();
    }

    virtual void handleInput(Input input)
    {
        HeroineState *state = m_state->handleInput(*this, input);
        if (state)
        {
            delete m_state;
            m_state = state;
            m_state->enter();
        }
    }

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

private:
    HeroineState *m_state;
};

HeroineState *StandingState::handleInput(Heroine &heroine, Input input)
{
    if (input == Input::PressDown)
    {
        return new DuckingState();
    }
    return nullptr;
}

HeroineState *DuckingState::handleInput(Heroine &heroine, Input input)
{
    if (input == Input::ReleaseDown)
    {
        return new StandingState();
    }
    return nullptr;
}

int main()
{
    Heroine heroine;                         // "now standing."
    heroine.handleInput(Input::PressDown);   // "now docking."
    heroine.handleInput(Input::ReleaseDown); // "now standing."
    heroine.handleInput(Input::ReleaseDown); // no output
    heroine.handleInput(Input::PressDown);   // "now docking."
}

Game Programming Patterns, Chapter 7

• 複数のstateを持たせる場合もある
• さらに継承を使って階層的state machineを表現することもできる
• プッシュダウンオートマトンを使うと、一つ前のstateに戻ることもできる
• ゲームAIのような複雑な用途ではビヘイビアツリーとかが流行っている