5-4 overloading assignment operator in C++

5.4.1 What is “Operator Overloading”

Same operator can define various function. It’s called “Operator Overloading”. First we need assign operator to resolve that assign between object.

5.4.2 Implementing the Assignment Operator Overloading as a Member Function

let’s type “operator=()” instread =.

#include <iostream>
using namespace std;

class String
{
public:
    String(char ch, int nSize);
    ~String();
    void operator=(const String& s);
    void SetData();
private:
    int nLength;
    char *pBuffer;
};

String::String(char ch, int nSize)
{
    nLength = nSize;
    pBuffer = new char[nLength + 1];
    memset(pBuffer, ch, nLength);
    pBuffer[nLength] = '\0';
}

String::~String()
{
    delete[] pBuffer;
}

void String::operator=(const String& s)
{
    delete pBuffer;
    nLength = s.nLength;
    pBuffer = new char[nLength + 1];
    strcpy(pBuffer, s.pBuffer);
}

void String::SetData()
{
    cout << "pBuffer: " << this->pBuffer << endl;
    cout << "nLength: " << this->nLength << endl;
}

int main() {
    String str1('A', 3), str2('B', 5);
    cout << "before assign str2";
    str2.SetData();
    
    str2 = str1;
    cout << "after assign str2";
    str2.SetData();

    return 0;
}

• str2 = str1; is same mean str2.operator=(str1);

// overloading object assignment operator 
void String::operator=(const String& s)
{
    delete pBuffer;
    nLength = s.nLength;
    pBuffer = new char[nLength + 1];
    strcpy(pBuffer, s.pBuffer);
}

1. delete pointer
2. make new heap memory array and copy

5.4.3 Handling self-assignment of an object

• how work self-assignment between object. like, str1 = str1
• itsn’t work, because can’t copy value. so we have to edit code.

void String::operator=(const String& s)
{
    if(&s == this)
        return;
    delete pBuffer;
    nLength = s.nLength;
    pBuffer = new char[nLength + 1];
    strcpy(pBuffer, s.pBuffer);
}

5.4.4 Handling Multiple Assignments of an Object

How work 3 over object assignmnets like str1 = str2 = str3. it’s have to assigned str1 = (str2 = str3);

#include <iostream>
using namespace std;

class String
{
public:
    String(char ch, int nSize);
    ~String();
    String& operator=(const String& s);
    void SetData();
private:
    int nLength;
    char *pBuffer;
};

String::String(char ch, int nSize)
{
    nLength = nSize;
    pBuffer = new char[nLength + 1];
    memset(pBuffer, ch, nLength);
    pBuffer[nLength] = '\0';
}
String::~String()
{
    delete pBuffer;
}
String& String::operator=(const String& s)
{
    if(&s == this)
        return *this;
    delete pBuffer;
    nLength = s.nLength;
    pBuffer = new char[nLength + 1];
    strcpy(pBuffer, s.pBuffer);
    return *this;
}
void String::SetData()
{
    cout << endl;
    cout << "pBuffer: " << this->pBuffer << endl;
    cout << "nLength: " << this->nLength << endl;

}

int main() {
    String str1('A', 3), str2('B', 3), str3('C', 4);
    cout << "before assign str1";
    str1.SetData();
    str1 = str2 = str3;
    cout << "after assign str1";
    str1.SetData();

    return 0;
}

• add return *this; on assign operator

Q2)

/*
String 클래스를 기반으로 한다.
MyString 클래스라는 이름으로 새로 생성한다.
기능은 String과 동일하다.

두 개의 객체 str1과 str2를 선언한다.
이 때 str1 생성 시 생성자의 전달인자는 (‘A’, 5),
str2 생성 시 생성자의 전달인자는 (‘Z’, 10) 이라고 하자.

이 때 str1에 str2를 대입한다. (str1 = str2) 그리고, 대입이 된 str1의 멤버변수를
화면에 출력하자. 대입 시 문제가 발생하였다면 대입 연산자 오버로딩을 통해 문제를
해결하도록 한다.
*/
#include <iostream>
using namespace std;

class MyString {
 private:
  char* ch;
  int size;

 public:
  MyString(char c, int s) {
    size = s;
    ch = new char[size + 1];
    memset(ch, c, size);
    ch[size] = '\0';

    cout << ch << endl;
  }
  ~MyString() { delete[] ch; }
  MyString& operator=(const MyString& s) {
    if (&s == this) return *this;
    delete ch;
    size = s.size;
    ch = new char[size + 1];
    strcpy(ch, s.ch);
    return *this;
  }
};

int main() {
  MyString str1('A', 5);
  MyString str2('B', 10);

  str1 = str2;

  return 0;
}

📚 reference
- book - C++ 프로그래밍과 STL (이창현)
- Photo by Tachina Lee on Unsplash