意图

该模式可以在不暴露对象实现细节的情况下保存和恢复对象之前的状态。

问题

现在需要实现一个编辑器，编辑器需要实现保存和恢复状态功能，怎么做到既实现这些功能，又能不暴露类的内部细节呢？

解决方案

备忘录模式将创建状态快照（Snapshot）的工作委派给实际状态的拥有者原发器（Originator）对象。这样其他对象就不再需要从 “外部” 复制编辑器状态了，编辑器类拥有其状态的完全访问权，因此可以自行生成快照。

模式建议将对象状态的副本存储在一个名为备忘录（Memento） 的特殊对象中。除了创建备忘录的对象外，任何对象都不能访问备忘录的内容。其他对象必须使用受限接口与备忘录进行交互，它们可以获取快照的元数据（创建时间和操作名称等），但不能获取快照中原始对象的状态。

这种限制策略允许你将备忘录保存在通常被称为负责人（Caretakers） 的对象中。由于负责人仅通过受限接口与备忘录互动，故其无法修改存储在备忘录内部的状态。同时，原发器拥有对备忘录所有成员的访问权限，从而能随时恢复其以前的状态。

在文字编辑器的示例中，我们可以创建一个独立的历史（History）类作为负责人。编辑器每次执行操作前，存储在负责人中的备忘录栈都会生长。你甚至可以在应用的 UI 中渲染该栈，为用户显示之前的操作历史。

当用户触发撤销操作时，历史类将从栈中取回最近的备忘录，并将其传递给编辑器以请求进行回滚。由于编辑器拥有对备忘录的完全访问权限，因此它可以使用从备忘录中获取的数值来替换自身的状态。

示例

/**
 * The Memento interface provides a way to retrieve the memento's metadata, such
 * as creation date or name. However, it doesn't expose the Originator's state.
 */
class Memento {
 public:
  virtual std::string GetName() const = 0;
  virtual std::string date() const = 0;
  virtual std::string state() const = 0;
};

/**
 * The Concrete Memento contains the infrastructure for storing the Originator's
 * state.
 */
class ConcreteMemento : public Memento {
 private:
  std::string state_;
  std::string date_;

 public:
  ConcreteMemento(std::string state) : state_(state) {
    this->state_ = state;
    std::time_t now = std::time(0);
    this->date_ = std::ctime(&now);
  }
  /**
   * The Originator uses this method when restoring its state.
   */
  std::string state() const override {
    return this->state_;
  }
  /**
   * The rest of the methods are used by the Caretaker to display metadata.
   */
  std::string GetName() const override {
    return this->date_ + " / (" + this->state_.substr(0, 9) + "...)";
  }
  std::string date() const override {
    return this->date_;
  }
};

/**
 * The Originator holds some important state that may change over time. It also
 * defines a method for saving the state inside a memento and another method for
 * restoring the state from it.
 */
class Originator {
  /**
   * @var string For the sake of simplicity, the originator's state is stored
   * inside a single variable.
   */
 private:
  std::string state_;

  std::string GenerateRandomString(int length = 10) {
    const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";
    int stringLength = sizeof(alphanum) - 1;

    std::string random_string;
    for (int i = 0; i < length; i++) {
      random_string += alphanum[std::rand() % stringLength];
    }
    return random_string;
  }

 public:
  Originator(std::string state) : state_(state) {
    std::cout << "Originator: My initial state is: " << this->state_ << "\n";
  }
  /**
   * The Originator's business logic may affect its internal state. Therefore,
   * the client should backup the state before launching methods of the business
   * logic via the save() method.
   */
  void DoSomething() {
    std::cout << "Originator: I'm doing something important.\n";
    this->state_ = this->GenerateRandomString(30);
    std::cout << "Originator: and my state has changed to: " << this->state_ << "\n";
  }

  /**
   * Saves the current state inside a memento.
   */
  Memento *Save() {
    return new ConcreteMemento(this->state_);
  }
  /**
   * Restores the Originator's state from a memento object.
   */
  void Restore(Memento *memento) {
    this->state_ = memento->state();
    std::cout << "Originator: My state has changed to: " << this->state_ << "\n";
  }
};

/**
 * The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
 * doesn't have access to the originator's state, stored inside the memento. It
 * works with all mementos via the base Memento interface.
 */
class Caretaker {
  /**
   * @var Memento[]
   */
 private:
  std::vector<Memento *> mementos_;

  /**
   * @var Originator
   */
  Originator *originator_;

 public:
  Caretaker(Originator *originator) : originator_(originator) {
    this->originator_ = originator;
  }

  void Backup() {
    std::cout << "\nCaretaker: Saving Originator's state...\n";
    this->mementos_.push_back(this->originator_->Save());
  }
  void Undo() {
    if (!this->mementos_.size()) {
      return;
    }
    Memento *memento = this->mementos_.back();
    this->mementos_.pop_back();
    std::cout << "Caretaker: Restoring state to: " << memento->GetName() << "\n";
    try {
      this->originator_->Restore(memento);
    } catch (...) {
      this->Undo();
    }
  }
  void ShowHistory() const {
    std::cout << "Caretaker: Here's the list of mementos:\n";
    for (Memento *memento : this->mementos_) {
      std::cout << memento->GetName() << "\n";
    }
  }
};
/**
 * Client code.
 */

void ClientCode() {
  Originator *originator = new Originator("Super-duper-super-puper-super.");
  Caretaker *caretaker = new Caretaker(originator);
  caretaker->Backup();
  originator->DoSomething();
  caretaker->Backup();
  originator->DoSomething();
  caretaker->Backup();
  originator->DoSomething();
  std::cout << "\n";
  caretaker->ShowHistory();
  std::cout << "\nClient: Now, let's rollback!\n\n";
  caretaker->Undo();
  std::cout << "\nClient: Once more!\n\n";
  caretaker->Undo();

  delete originator;
  delete caretaker;
}

int main() {
  std::srand(static_cast<unsigned int>(std::time(NULL)));
  ClientCode();
  return 0;
}

运行结果：

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: uOInE8wmckHYPwZS7PtUTwuwZfCIbz

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: te6RGmykRpbqaWo5MEwjji1fpM1t5D

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: hX5xWDVljcQ9ydD7StUfbBt5Z7pcSN

Caretaker: Here's the list of mementos:
Sat Oct 19 18:09:37 2019
 / (Super-dup...)
Sat Oct 19 18:09:37 2019
 / (uOInE8wmc...)
Sat Oct 19 18:09:37 2019
 / (te6RGmykR...)

Client: Now, let's rollback!

Caretaker: Restoring state to: Sat Oct 19 18:09:37 2019
 / (te6RGmykR...)
Originator: My state has changed to: te6RGmykRpbqaWo5MEwjji1fpM1t5D

Client: Once more!

Caretaker: Restoring state to: Sat Oct 19 18:09:37 2019
 / (uOInE8wmc...)
Originator: My state has changed to: uOInE8wmckHYPwZS7PtUTwuwZfCIbz