C++ 备忘录模式讲解和代码示例

/** * 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 ~Memento() {} 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"; delete memento; } }; /** * 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) { } ~Caretaker() { for (auto m : mementos_) delete m; } 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; } 

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