Question

我有一些操作...查看，编辑，结帐等。业务逻辑规定如果文档已经签出，则所有视图都会变为编辑。

有一种简洁的OO方式可以做这样的事情：

class Action
{
public:
  Document document;
  virtual void execute();
};
class View, Edit, Checkout : public Action;

View::execute()
{
  if (document.isCheckedOut)
  {
    delete this;
    this = new Edit;
    this->execute();
  }
  /* execute view */
}

更新：你们怎么看待这个：

class Action
{
public:
  static int type;
  Document document;
  virtual void execute();
  static Action* construct(int type, Document document) = 0;
private:
  Action();
};
class View, Edit: public Action;

Action* View::construct(Document document)
{
  if (document.isCheckedOut)
    return new Edit(document);
  return new View(document);
}

Action* Edit::construct(Document document)
{
  return new Edit(document);
}

void onViewButton(Document document)
{
  Action *action = View::construct(document);
  action->execute();
  delete action;
}

Answer 1

没有办法像这样重新分配'this'指针。您可以使用智能指针实现通过添加间接层来执行类似的操作。

class action_ptr  
{
  Action* m_Data;
public:
  action_ptr(Action* value)
  {
     m_Data = value;
  }

  Action* operator->()
  {
      if(m_Data->isView() && m_Data->isCheckedOut())
      {
        delete m_Data;
        m_Data = new Edit();
      }
      return m_Data;
  }
};


 // usage
action_ptr pAction(new View());
pAction->DoViewStuff();
pAction->Checkout();
pAction->OtherStuff();  // If isCheckedOut returned true, this now magically operates on an Edit instead!

这绝不应该是一个完整的实现，仅仅是一个例子 - 存在内存泄漏和许多缺少的功能。

Answer 2

我可能会用策略模式来解决这个问题，其中策略具有转换成员函数。沿着这些方向的东西（省略了无趣的代码部分）：

struct action_strategy {
  virtual ~action_strategy();

  virtual action_strategy *convert_strategy(document const &doc);
  virtual void execute() = 0;
};

class action { // Hur, hur :D
public:
  // construct with heap-allocated strategy
  action(action_strategy *strategy);

  void execute() {
    strategy_.reset(strategy_->convert_strategy(doc_);
    strategy_->execute();
  }

private:
  document doc_;
  std::auto_ptr<action_strategy> strategy_;
};

// Default: do nothing
action_strategy *action_strategy::convert_strategy(document const &doc) {
  return this;
}

class view : public action_strategy {
  // overload as necessary
  virtual action_strategy *convert_strategy(document const &doc);
};

action_strategy *view::convert_strategy(document const &doc) {
  if(doc.is_checked_out()) {
    return new edit();
  }

  return this;
}

一个实例使用新实例进行热交换

2 个答案: