C++ 대입 연산자 오버로딩 (Assignment Operator Overloading)

대입 연산자 오버로딩 (Assignment Operator Overloading)

• 자기 자신을 assignment할 때(hello = hello 등) 발생할 수 있는 문제를 미리 방지할 수 있다.

• 깊은 복사를 직접 구현할 수 있다.

  #include <iostream>
  #include <cassert>
  
  class MyString
  {
      char* data_ = nullptr;
      int        len_ = 0;
  
  public:
      MyString(const char* source = "")
      {
          assert(source);
  
          len_ = std::strlen(source) + 1;
          data_ = new char[len_];
  
          for (int i = 0; i < len_; ++i)
              data_[i] = source[i];
          data_[len_ - 1] = '\0';
      }
  
      ~MyString()
      {
          delete[] data_;
      }
  
      MyString(const MyString& source)
      {
          std::cout << "Copy constructor\n";
  
          len_ = source.len_;
  
          if (source.data_ != nullptr)
          {
              data_ = new char[len_];
  
              for (int i = 0; i < len_; ++i)
                  data_[i] = source.data_[i];
          }
          else
              data_ = nullptr;
      }
  
      char*& getString() { return data_; }
  
      MyString& operator = (const MyString& source)
      {
          std::cout << "Assignment operator\n";
  
          if (this == &source)  // prevent self-assignment
              return *this;
  
          delete[] data_;
  
          len_ = source.len_;
  
          if (source.data_ != nullptr)
          {
              data_ = new char[len_];
  
              for (int i = 0; i < len_; ++i)
                  data_[i] = source.data_[i];
          }
          else
              data_ = nullptr;
      }
  };
  
  int        main()
  {
      using namespace std;
  
      MyString    hello("Hello");
  
      cout << (int*)hello.getString() << endl;
      cout << hello.getString() << endl;
  
      {
          MyString    copy;
  
          copy = hello;
  
          cout << (int*)copy.getString() << endl;
          cout << copy.getString() << endl;
      }
  
      cout << hello.getString() << endl;
  }
  
  /* stdout
  00C0EAD8
  Hello
  Assignment operator
  00C0E5D0
  Hello
  Hello
  */

• 같은 자료형을 인자로 넣는 초기화는 Copy Constructor를 호출한다.

  • 대입 연산자를 통해 값을 넣으려면 초기화가 아닌 대입을 해야한다.

    #include <iostream>
    #include <cassert>
    
    class MyString
    {
        char* data_ = nullptr;
        int        len_ = 0;
    
    public:
        MyString(const char* source = "")
        {
            assert(source);
    
            len_ = std::strlen(source) + 1;
            data_ = new char[len_];
    
            for (int i = 0; i < len_; ++i)
                data_[i] = source[i];
            data_[len_ - 1] = '\0';
        }
    
        ~MyString()
        {
            delete[] data_;
        }
    
        MyString(const MyString& source)
        {
            std::cout << "Copy constructor\n";
    
            len_ = source.len_;
    
            if (source.data_ != nullptr)
            {
                data_ = new char[len_];
    
                for (int i = 0; i < len_; ++i)
                    data_[i] = source.data_[i];
            }
            else
                data_ = nullptr;
        }
    
        char*& getString() { return data_; }
    
        MyString& operator = (const MyString& source)
        {
            std::cout << "Assignment operator\n";
    
            if (this == &source)  // prevent self-assignment
                return *this;
    
            delete[] data_;
    
            len_ = source.len_;
    
            if (source.data_ != nullptr)
            {
                data_ = new char[len_];
    
                for (int i = 0; i < len_; ++i)
                    data_[i] = source.data_[i];
            }
            else
                data_ = nullptr;
        }
    };
    
    int        main()
    {
        using namespace std;
    
        MyString    hello("Hello");
    
        MyString    str1 = hello;
    
        MyString    str2(hello);
    
        MyString    str3{ hello };
    
        MyString    str4;
    
        str4 = hello;
    }
    
    /* stdout
    Copy constructor
    Copy constructor
    Copy constructor
    Assignment operator
    */