C++ 동적 형변환 (Dynamic Casting)

동적 형변환 (Dynamic Casting)

• dynamic_cast는 캐스팅이 실패하면 nullptr를 반환한다.

---

예제

• 간단한 예제

  #include <iostream>
  
  class Base
  {
  public:
    int    i_ = 0;
  
    virtual void print()
    {
      std::cout << "I'm Base\n";
    }
  };
  
  class Derived1 : public Base
  {
  public:
    int    j_ = 1024;
  
    virtual void print() override
    {
      std::cout << "I'm Derived\n";
    }
  };
  
  int            main()
  {
    using namespace std;
  
    Derived1    d1;
    Base*        base = &d1;
  
    d1.j_ *= 2;
  
    auto*        base_to_d1 = dynamic_cast<Derived1*>(base);
    cout << base_to_d1->j_ << endl;
    base_to_d1->j_ /= 8;
  
    cout << d1.j_ << endl;
  }
  
  /* stdout
  2048
  256
  */

  ---

• 캐스팅 실패 예제

  #include <iostream>
  
  class Base
  {
  public:
    int    i_ = 0;
  
    virtual void print()
    {
      std::cout << "I'm Base\n";
    }
  };
  
  class Derived1 : public Base
  {
  public:
    int    j_ = 1024;
  
    virtual void print() override
    {
      std::cout << "I'm Derived\n";
    }
  };
  
  class Derived2 : public Base
  {
  public:
    std::string    name_ = "Dr. Two";
  
    virtual void print() override
    {
      std::cout << "I'm Derived\n";
    }
  };
  
  int            main()
  {
    using namespace std;
  
    Derived1    d1;
    Base*        base = &d1;
  
    d1.j_ *= 2;
  
    auto*        base_to_d1 = dynamic_cast<Derived2*>(base);
    if (base_to_d1 != nullptr)
      base_to_d1->print();
    else
      cout << "Failed to cast\n";
  }
  
  /* stdout
  Failed to cast
  */

  ---

정적 형변환 (Static Casting)

• 최대한 변환되도록 캐스팅한다.

• 위의 경우 가급적 Dynamic Casting을 사용하는게 안전하다.

  #include <iostream>
  
  class Base
  {
  public:
    int    i_ = 0;
  
    virtual void print()
    {
      std::cout << "I'm Base\n";
    }
  };
  
  class Derived1 : public Base
  {
  public:
    int    j_ = 1024;
  
    virtual void print() override
    {
      std::cout << "I'm Derived\n";
    }
  };
  
  class Derived2 : public Base
  {
  public:
    std::string    name_ = "Dr. Two";
  
    virtual void print() override
    {
      std::cout << "I'm Derived\n";
    }
  };
  
  int            main()
  {
    using namespace std;
  
    Derived1    d1;
    Base*        base = &d1;
  
    d1.j_ *= 2;
  
    auto*        base_to_d1 = static_cast<Derived2*>(base);
    if (base_to_d1 != nullptr)
      base_to_d1->print();
    else
      cout << "Failed to cast\n";
  }
  
  /* stdout
  I'm Derived
  */