(c++) Copy Constructor

Copy Constructor & Swallow/Deep Copy

def

The copy constructor is called whenever an object is initialized (by direct-initialization or copy-initialization) from another object of the same type.

when object initialized?

1. initialization

   Class T
   T a=b;
   T a(b);

2. function argument passing

   void func(T t);
   f(a)

3. function argument return

   T func()
   return a

Implicitly-declared copy constructor(Swallow Copy)

If no user-defined copy constructors are provided for a class type (struct, class, or union), the compiler will always declare a copy constructor as a non-explicit inline public member of its class. This implicitly-declared copy constructor has the form

Example 1) Error code

class String
{
private:
	char* m_Buffer;
	unsigned int m_Size;
public:
	String(const char* string)
	{
		m_Size = strlen(string);
		m_Buffer = new char[m_Size+1];
		memcpy(m_Buffer, string, m_Size);
		m_Buffer[m_Size] = NULL;
	}
	~String()
	{
		cout << "소멸자" << endl;
		delete[] m_Buffer;
	}
	char* GetBuffer() const
	{
		return m_Buffer;
	}
};

std::ostream& operator<<(std::ostream& stream, const String string) {
	stream << string.GetBuffer();
	return stream;
}

int main()
{
	String string = "Cherno";
	cout << string << endl;
	
	return 0;
}

solution 1)

std::ostream& operator<<(std::ostream& stream, const String string)

const String string (매우 안좋은 코드)

1. 입력값으로 객체를 복사
2. 복사된 객체가 없어지면서 소멸함수에 의해 m_Buffer가 삭제된다.

const String& string
객체를 참조함으로서 불필요한 객체의 생성을 막는다.

Example 2)

String string = "Cherno";
String second = string;

cout << string << endl;
cout << string << endl;

solution 2)

Reference

1. 이것이 C++ 이다, 최호성
2. cppreference - copy constructors
3. TheChernoProject