Memento дизайн шаблона

Вот мой код:

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;
}
}

Каково значение заявления typedef void(Number:: *Action)() в вышеуказанной программе? Это указатель на функцию, тогда где находится определение функции?