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C++로 작성된 싱글턴
싱글턴은 같은 종류의 객체가 하나만 존재하도록 하고 다른 코드의 해당 객체에 대한 단일 접근 지점을 제공하는 생성 디자인 패턴입니다.
싱글턴은 전역 변수들과 거의 같은 장단점을 가지고 있습니다: 매우 편리하나 코드의 모듈성을 깨뜨립니다.
싱글턴에 의존하는 클래스를 다른 콘텍스트에서 사용하려면 싱글턴도 다른 콘텍스트로 전달해야 합니다. 대부분의 경우 이 제한 사항은 유닛 테스트를 생성하는 동안 발생합니다.
복잡도:
인기도:
사용 사례들: 많은 개발자는 싱클턴을 안티패턴으로 간주합니다. 그래서 C# 코드에서의 사용이 감소하고 있습니다.
식별: 싱글턴은 같은 캐싱 된 객체를 반환하는 정적 생성 메서드로 식별될 수 있습니다.
기본 싱글턴
조잡한 싱글턴을 구현하는 것은 매우 쉽습니다. 생성자를 숨기고 정적 생성 메서드를 구현하기만 하면 됩니다.
같은 클래스는 다중 스레드 환경에서 잘못 작동합니다. 여러 스레드가 생성 메서드를 동시에 호출할 수 있으며 싱글턴 클래스의 여러 인스턴스를 가져올 수 있기 때문입니다.
main.cc: 개념적인 예시
/**
* The Singleton class defines the `GetInstance` method that serves as an
* alternative to constructor and lets clients access the same instance of this
* class over and over.
*/
class Singleton
{
/**
* The Singleton's constructor should always be private to prevent direct
* construction calls with the `new` operator.
*/
protected:
Singleton(const std::string value): value_(value)
{
}
static Singleton* singleton_;
std::string value_;
public:
/**
* Singletons should not be cloneable.
*/
Singleton(Singleton &other) = delete;
/**
* Singletons should not be assignable.
*/
void operator=(const Singleton &) = delete;
/**
* This is the static method that controls the access to the singleton
* instance. On the first run, it creates a singleton object and places it
* into the static field. On subsequent runs, it returns the client existing
* object stored in the static field.
*/
static Singleton *GetInstance(const std::string& value);
/**
* Finally, any singleton should define some business logic, which can be
* executed on its instance.
*/
void SomeBusinessLogic()
{
// ...
}
std::string value() const{
return value_;
}
};
Singleton* Singleton::singleton_= nullptr;;
/**
* Static methods should be defined outside the class.
*/
Singleton *Singleton::GetInstance(const std::string& value)
{
/**
* This is a safer way to create an instance. instance = new Singleton is
* dangeruous in case two instance threads wants to access at the same time
*/
if(singleton_==nullptr){
singleton_ = new Singleton(value);
}
return singleton_;
}
void ThreadFoo(){
// Following code emulates slow initialization.
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
Singleton* singleton = Singleton::GetInstance("FOO");
std::cout << singleton->value() << "\n";
}
void ThreadBar(){
// Following code emulates slow initialization.
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
Singleton* singleton = Singleton::GetInstance("BAR");
std::cout << singleton->value() << "\n";
}
int main()
{
std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
"If you see different values, then 2 singletons were created (booo!!)\n\n" <<
"RESULT:\n";
std::thread t1(ThreadFoo);
std::thread t2(ThreadBar);
t1.join();
t2.join();
return 0;
}
Output.txt: 실행 결과
If you see the same value, then singleton was reused (yay!
If you see different values, then 2 singletons were created (booo!!)
RESULT:
BAR
FOO
스레드로부터 안전한 싱글턴
이 문제를 해결하려면 싱글턴 객체를 처음 생성하는 동안 스레드들을 동기화해야 합니다.
main.cc: 개념적인 예시
/**
* The Singleton class defines the `GetInstance` method that serves as an
* alternative to constructor and lets clients access the same instance of this
* class over and over.
*/
class Singleton
{
/**
* The Singleton's constructor/destructor should always be private to
* prevent direct construction/desctruction calls with the `new`/`delete`
* operator.
*/
private:
static Singleton * pinstance_;
static std::mutex mutex_;
protected:
Singleton(const std::string value): value_(value)
{
}
~Singleton() {}
std::string value_;
public:
/**
* Singletons should not be cloneable.
*/
Singleton(Singleton &other) = delete;
/**
* Singletons should not be assignable.
*/
void operator=(const Singleton &) = delete;
/**
* This is the static method that controls the access to the singleton
* instance. On the first run, it creates a singleton object and places it
* into the static field. On subsequent runs, it returns the client existing
* object stored in the static field.
*/
static Singleton *GetInstance(const std::string& value);
/**
* Finally, any singleton should define some business logic, which can be
* executed on its instance.
*/
void SomeBusinessLogic()
{
// ...
}
std::string value() const{
return value_;
}
};
/**
* Static methods should be defined outside the class.
*/
Singleton* Singleton::pinstance_{nullptr};
std::mutex Singleton::mutex_;
/**
* The first time we call GetInstance we will lock the storage location
* and then we make sure again that the variable is null and then we
* set the value. RU:
*/
Singleton *Singleton::GetInstance(const std::string& value)
{
std::lock_guard<std::mutex> lock(mutex_);
if (pinstance_ == nullptr)
{
pinstance_ = new Singleton(value);
}
return pinstance_;
}
void ThreadFoo(){
// Following code emulates slow initialization.
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
Singleton* singleton = Singleton::GetInstance("FOO");
std::cout << singleton->value() << "\n";
}
void ThreadBar(){
// Following code emulates slow initialization.
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
Singleton* singleton = Singleton::GetInstance("BAR");
std::cout << singleton->value() << "\n";
}
int main()
{
std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
"If you see different values, then 2 singletons were created (booo!!)\n\n" <<
"RESULT:\n";
std::thread t1(ThreadFoo);
std::thread t2(ThreadBar);
t1.join();
t2.join();
return 0;
}
Output.txt: 실행 결과
If you see the same value, then singleton was reused (yay!
If you see different values, then 2 singletons were created (booo!!)
RESULT:
FOO
FOO
