Copy Assignment Operator and Move Assignment Operator in C++

The copy assignment operator and move assignment operator are special member functions in C++ that handle the assignment of one object to another.

• Copy assignment operator: T& operator=(const T b) or (T& b):
  • don't pass non-constant rhs ref, like (T& b). It's allowed but may cause issue.
  • If undefined: shallow-copy
• Move assignment operator: T& operator=(T&& b)
  • Called when an object appears on the left-hand side of an assignment expression, where the right-hand side is an rvalue of the same or implicitly convertible type.

Code Example

This example demonstrates how to use the copy assignment operator and move assignment operator in C++ to manage object assignments efficiently. It also shows examples of copy constructor and move constructor, which helps to highlight their differences.

Explanation

Copy Constructor

MyClass(const MyClass& a) { // copy constructor
  val = a.val;
  cout << "copy constructor, val = " << this->val << endl;
}
Copy

• Creates a new object as a copy of an existing object.
• It copies the value of val from the source object a to the new object.

Copy Assignment Operator

MyClass& operator=(const MyClass& a) { // copy assignment operator
  val = -a.val;
  cout << "copy assignment operator, val = " << this->val << endl;
  return *this;
}
Copy

• Assigns the value of one existing object to another.
• It negates the value of val from the source object a and assigns it to the current object.
• In this example, the behavior of the copy assignment operator is deliberately to be different from the copy constructor, so their outcome can be differented.

Move Constructor

MyClass(MyClass&& a) {
  val = a.val;
  cout << "move constructor, val = " << this->val << endl;
}
Copy

• Transfers ownership of resources from a temporary or moved object a to the new object.
• The val of the source object is assigned to the new object.

Move Assignment Operator

MyClass& operator=(MyClass&& a) {
  val = -a.val;
  cout << "move assignment operator, val = " << this->val << endl;
  return *this;
}
Copy

• Transfers ownership of resources from a temporary or moved object a to an existing object.
• It negates the value of val from the source object a and assigns it to the current object.
• The behavior here is also deliberately created to be different from the move constructor.

main function

int main() {
  MyClass m1; // default constructor
  MyClass m2 = m1; // copy constructor
  MyClass m3; // default constructor
  m3 = m1; // copy assignment operator
  MyClass m4 = move(m1); // move constructor
  MyClass m5; // default constructor
  m5 = move(m4); // move assignment operator
  return 0;
}
Copy

• MyClass m1;: Creates an object m1 using the default constructor.
• MyClass m2 = m1;: Creates an object m2 using the copy constructor.
• MyClass m3;: Creates an object m3 using the default constructor.
• m3 = m1;: Uses the copy assignment operator to assign the value of m1 to m3.
• MyClass m4 = move(m1);: Creates an object m4 using the move constructor.
• MyClass m5;: Creates an object m5 using the default constructor.
• m5 = move(m4);: Uses the move assignment operator to assign the value of m4 to m5.

int main() {
    MyClass m1; // default constructor
    MyClass m2 = m1; // copy constructor
    MyClass m3; // default constructor
    m3 = m1; // copy assignment operator
    MyClass m4 = move(m1); // move constructor
    MyClass m5; // default constructor
    m5 = move(m4); // move assignment operator
    return 0;
  }
  Copy

Conclusion

Understanding the copy assignment operator and move assignment operator is essential for efficient object management and resource handling in C++. These operators help ensure that objects are assigned correctly, preventing resource leaks and unnecessary copies.