Question

这是我的代码：

class Number;

class Memento
{
  public:
    Memento(int val)
    {
        _state = val;
    }
  private:
    friend class Number; // not essential, but p287 suggests this
    int _state;
};

class Number
{
  public:
    Number(int value)
    {
        _value = value;
    }
    void dubble()
    {
        _value = 2 * _value;
    }
    void half()
    {
        _value = _value / 2;
    }
    int getValue()
    {
        return _value;
    }
    Memento *createMemento()
    {
        return new Memento(_value);
    }
    void reinstateMemento(Memento *mem)
    {
        _value = mem->_state;
    }
  private:
    int _value;
};

class Command
{
  public:
    typedef void(Number:: *Action)();
    Command(Number *receiver, Action action)
    {
        _receiver = receiver;
        _action = action;
    }
    virtual void execute()
    {
        _mementoList[_numCommands] = _receiver->createMemento();
        _commandList[_numCommands] = this;
        if (_numCommands > _highWater)
          _highWater = _numCommands;
        _numCommands++;
        (_receiver-> *_action)();
    }
    static void undo()
    {
        if (_numCommands == 0)
        {
            cout << "*** Attempt to run off the end!! ***" << endl;
            return ;
        }
        _commandList[_numCommands - 1]->_receiver->reinstateMemento
          (_mementoList[_numCommands - 1]);
        _numCommands--;
    }
    void static redo()
    {
        if (_numCommands > _highWater)
        {
            cout << "*** Attempt to run off the end!! ***" << endl;
            return ;
        }
        (_commandList[_numCommands]->_receiver->*(_commandList[_numCommands]
          ->_action))();
        _numCommands++;
    }
  protected:
    Number *_receiver;
    Action _action;
    static Command *_commandList[20];
    static Memento *_mementoList[20];
    static int _numCommands;
    static int _highWater;
};

Command *Command::_commandList[];
Memento *Command::_mementoList[];
int Command::_numCommands = 0;
int Command::_highWater = 0;

int main()
{
  int i;
  cout << "Integer: ";
  cin >> i;
  Number *object = new Number(i);

  Command *commands[3];
  commands[1] = new Command(object, &Number::dubble);
  commands[2] = new Command(object, &Number::half);

  cout << "Exit[0], Double[1], Half[2], Undo[3], Redo[4]: ";
  cin >> i;

  while (i)
  {
    if (i == 3)
      Command::undo();
    else if (i == 4)
      Command::redo();
    else
      commands[i]->execute();
    cout << "   " << object->getValue() << endl;
    cout << "Exit[0], Double[1], Half[2], Undo[3], Redo[4]: ";
    cin >> i;
  }
}

声明typedef void(Number:: *Action)()在上述计划中有何意义？它是函数指针，那么函数定义在哪里？

Answer 1

声明的意义是什么：“typedef void(Number::*Action)()”在上面的程序中?????它是函数指针，那么函数defination ???

alias为pointer to a member-function。该成员函数属于一个名为Number的类，而成员函数不作为参数，只返回void

它可以使用名称Action作为类型说明符来声明一个类型如前所述的变量。

请参阅ISO C++'s FAQ about Pointers to Members

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1 个答案: