C++ 备忘录模式讲解和代码示例
备忘录是一种行为设计模式， 允许生成对象状态的快照并在以后将其还原。
备忘录不会影响它所处理的对象的内部结构， 也不会影响快照中保存的数据。
复杂度：
流行度：
使用示例： 备忘录的基本功能可用序列化来实现， 这在 C++ 语言中很常见。 尽管备忘录不是生成对象状态快照的唯一或最有效的方法， 但它能在保护原始对象的结构不暴露给其他对象的情况下保存对象状态的备份。
概念示例
本例说明了备忘录设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.cc: 概念示例
/**
* 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";
}
};
/**
* 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: 执行结果
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
