State

#include <iostream>

using namespace std;

#define DESTROY_POINTER(ptr) if (ptr) { delete ptr; ptr = NULL; }

class Context;

class DbState
{
public:
    DbState(Context* pContext) { m_pContext = pContext; }
    virtual void Open()=;
    virtual void Close()=;
    virtual void Read()=;
    virtual void Write()=;

protected:
    Context* m_pContext;
};

class OpenState : public DbState
{
public:
    OpenState(Context* pContext) : DbState(pContext) {}

    void Open() { cout<<"Already open!"<<endl; }
    void Close();
    void Read() { cout<<"Finish Reading"<<endl; }
    void Write() { cout<<"Finish Writing"<<endl; }
};

class CloseState : public DbState
{
public:
    CloseState(Context* pContext) : DbState(pContext) {}

    void Open();
    void Close() { cout<<"Already Closing"<<endl; }
    void Read() { cout<<"Already Closing, Can't read"<<endl; }
    void Write() { cout<<"Already Closing, Can't write"<<endl; }
};

class ReadingState : public DbState
{
public:
    ReadingState(Context* pContext) : DbState(pContext) {}

    void Open() { cout<<"Already Open"<<endl; }
    void Close();
    void Read() { cout<<"Reading, Try again"<<endl; }
    void Write() { cout<<"Reading, Can't write"<<endl; }
};

class WritingState : public DbState
{
public:
    WritingState(Context* pContext) : DbState(pContext) {}

    void Open() { cout<<"Already Open"<<endl; }
    void Close();
    void Read() { cout<<"Writing, Can't read"<<endl; }
    void Write() { cout<<"Writing, Try again"<<endl; }
};

class BusyState : public DbState
{
public:
    BusyState(Context* pContext) : DbState(pContext) {}

    void Open() { cout<<"Already Open"<<endl; }
    void Close() { cout<<"Busy, Can't Close"<<endl; }
    void Read() { cout<<"Busy, Can't read"<<endl; }
    void Write() { cout<<"Busy, Can't write"<<endl; }
};

class Context
{
public:
    Context() : m_pState(NULL) {}
    ~Context() { DESTROY_POINTER(m_pState); }

    void SetState(DbState* pState) { DESTROY_POINTER(m_pState); m_pState = pState; }

    void Open() { m_pState->Open(); }
    void Close() { m_pState->Close(); }
    void Read() { m_pState->Read(); }
    void Write() { m_pState->Write(); }

private:
    DbState* m_pState;
};

void OpenState::Close() { cout<<"Finish closing"<<endl; m_pContext->SetState(new CloseState(m_pContext)); }
void CloseState::Open() { cout<<"Finish Opening"<<endl; m_pContext->SetState(new OpenState(m_pContext)); }
void ReadingState::Close() { cout<<"Finish Closing"<<endl; m_pContext->SetState(new CloseState(m_pContext)); }
void WritingState::Close() { cout<<"Finish Closing"<<endl; m_pContext->SetState(new CloseState(m_pContext)); }

int main(int argc, char *argv[])
{
    Context context;
    context.SetState(new OpenState(&context));
    context.Open();
    context.Close();

    context.Open();
    context.Read();
    context.Write();

    context.Close();
    context.Write();

    context.Open();
    context.Write();

    return ;
}