Memento を C++ で
Memento は、 振る舞いに関するデザインパターンの一つで、 オブジェクトの状態のスナップショットを作成し、 それを将来復元します。
Memento は、 その対象オブジェクトの内部構造やスナップショットの内部に保存されるデータの機密を守ります。
複雑度：
人気度：
使用例： Memento の原則は、 シリアライゼーションを使って達成することが可能で、 C++ ではよく見かけます。 これは、 オブジェクトの状態のスナップショットを作る上で唯一の方法でも、 最も効率のいい方法でもありませんが、 オリジネーターの構造を他のオブジェクトから隠蔽しつつ状態のバックアップを取ることはできます。
概念的な例
この例は、 Memento デザインパターンの構造を説明するためのものです。 以下の質問に答えることを目的としています：
- どういうクラスからできているか？
- それぞれのクラスの役割は？
- パターンの要素同士はどう関係しているのか？
main.cc: 概念的な例
/**
* 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";
}
};
/**
* 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;
}
Output.txt: 実行結果
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
