Singleton em C++
O Singleton é um padrão de projeto criacional, que garante que apenas um objeto desse tipo exista e forneça um único ponto de acesso a ele para qualquer outro código.
O Singleton tem quase os mesmos prós e contras que as variáveis globais. Embora sejam super úteis, eles quebram a modularidade do seu código.
Você pode usar classes que dependem de singletons em algumas outras situações. Você terá que levar a classe singleton também. Na maioria das vezes, essa limitação surge durante a criação de testes de unidade.
Complexidade:
Popularidade:
Exemplos de uso: Muitos desenvolvedores consideram o padrão Singleton um antipadrão. É por isso que seu uso está diminuindo no código C++.
Identificação: O Singleton pode ser reconhecido por um método de criação estático, que retorna o mesmo objeto em cache.
Singleton ingênuo
É muito fácil implementar um Singleton desleixado. Você só precisa ocultar o construtor e implementar um método de criação estático.
A mesma classe se comporta incorretamente em um ambiente multithread. Vários threads podem chamar o método de criação simultaneamente e obter várias instâncias da classe Singleton.
main.cc: Exemplo conceitual
/**
* 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: Resultados da execução
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
Singleton seguro para threads
Para corrigir o problema, você deve sincronizar os threads durante a primeira criação do objeto Singleton.
main.cc: Exemplo conceitual
/**
* 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: Resultados da execução
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
