Hourra ! La version française est enfin sortie ! Laissez-nous un message svp, si vous voulez nous faire part de vos commentaires ou signaler une erreur.
Mémento

Mémento en C++

Le Mémento est un patron de conception comportemental qui permet de prendre des instantanés de l’état d’un objet et de les restaurer plus tard.

Le mémento ne compromet pas l’intégrité de la structure interne de l’objet avec lequel il interagit, ni celle des données contenues dans l’instantané.

Utilisation du patron de conception en C++

Complexité :

Popularité :

Exemples d’utilisation : Le principe du mémento peut être imité en utilisant la sérialisation, ce qui est assez commun en C++. Ce n’est pas forcément la manière la plus efficace de prendre des instantanés de l’état d’un objet, mais il permet de stocker des sauvegardes de l’état tout en protégeant la structure du créateur des autres objets.

Exemple conceptuel

Dans cet exemple, nous allons voir la structure du Mémento. Nous allons répondre aux questions suivantes :

  • Que contiennent les classes ?
  • Quels rôles jouent-elles ?
  • Comment les éléments du patron sont-ils reliés ?

main.cc: Exemple conceptuel

/**
 * 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;
}

Output.txt: Résultat de l’exécution

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

Mémento dans les autres langues

Patrons de conception : Mémento en Java Patrons de conception : Mémento en C# Patrons de conception : Mémento en PHP Patrons de conception : Mémento en Python Patrons de conception : Mémento en Ruby Patrons de conception : Mémento en Swift Patrons de conception : Mémento en TypeScript Patrons de conception : Mémento en Go