C++ weak_ptr

weak_ptr

weak_ptr

• <memory> 라이브러리

• shared_ptr의 순환 의존성 문제를 해결할 수 있다.

  #include <iostream>
  #include <memory>
  
  class Person
  {
    std::string            name_;
    std::weak_ptr<Person>    partner_;
  
  public:
    Person(const std::string& name) : name_(name)
    {
      std::cout << name_ << " created\n";
    }
  
    ~Person()
    {
      std::cout << name_ << " destroyed\n";
    }
  
    friend bool partnerUp(std::shared_ptr<Person>& p1, std::shared_ptr<Person>& p2)
    {
      if (!p1 || !p2)
        return false;
  
      p1->partner_ = p2;
      p2->partner_ = p1;
  
      std::cout << p1->name_ << " is partnered with " << p2->name_ << '\n';
  
      return true;
    }
  
    const std::string& getName() const
    {
      return name_;
    }
  };
  
  int            main()
  {
    auto lucy = std::make_shared<Person>("Lucy");
    auto ricky = std::make_shared<Person>("Ricky");
  
    partnerUp(lucy, ricky);
  }
  
  /* stdout stderr
  Lucy created
  Ricky created
  Lucy is partnered with Ricky
  Ricky destroyed
  Lucy destroyed
  */

• mutex처럼 lock이 걸린 상태에서만 개수를 세기 때문에, 인스턴스를 사용 중이 아니면 소멸자를 호출할 수 있는 원리이다.

  • lock 함수를 사용하면 shared_ptr를 반환한다.

    #include <iostream>
    #include <memory>
    
    class Person
    {
      std::string                name_;
      std::weak_ptr<Person>    partner_;
    
    public:
      Person(const std::string& name) : name_(name)
      {
        std::cout << name_ << " created\n";
      }
    
      ~Person()
      {
        std::cout << name_ << " destroyed\n";
      }
    
      friend bool partnerUp(std::shared_ptr<Person>& p1, std::shared_ptr<Person>& p2)
      {
        if (!p1 || !p2)
          return false;
    
        p1->partner_ = p2;
        p2->partner_ = p1;
    
        std::cout << p1->name_ << " is partnered with " << p2->name_ << '\n';
    
        return true;
      }
    
      const std::string& getName() const
      {
        return name_;
      }
    
      const std::shared_ptr<Person> getPartner() const
      {
        return partner_.lock();
      }
    };
    
    int            main()
    {
      auto lucy = std::make_shared<Person>("Lucy");
      auto ricky = std::make_shared<Person>("Ricky");
    
      partnerUp(lucy, ricky);
    
      std::cout << lucy->getName() << std::endl;
      std::cout << lucy->getPartner()->getName() << std::endl;
    }
    
    /* stdout stderr
    Lucy created
    Ricky created
    Lucy is partnered with Ricky
    Lucy
    Ricky
    Ricky destroyed
    Lucy destroyed
    */