move assignment to object with const value

Question

I have a struct like this:

struct OBJ {
  int x;
  const int y;
  OBJ& operator=(OBJ &&oth)
  {
    y = oth.y; // this is disallowed
    return *this;
  }
}

And an example code

void func() {
  static OBJ obj;
  OBJ other; // random values
  if(conditon)
    obj = std::move(other); //move
}

I understand this as obj is Non const OBJ with const member y . I can't change just y but I should be able to change whole object (call destructor and constructor). Is this possible or the only proper solution is to remove my const before y , and remember to don't change by accident?

I need to store my static obj between func call but if condition is true i want to move other object in place of this static object.

Answer 1

You're doing constructors wrong. Constructors should initialize , not assign:

OBJ(OBJ &&oth) : y(oth.y) {}
//             ^^^^^^^^^^

Also, constructors cannot return *this , since they have no return type.

An assignment operator for your class doesn't make sense since the class has unassignable members (namely constants). (You could of course write a custom assignment that doesn't modify the const member, but then you'd have a truly weird class that has extremely surprising behaviour.)

Answer 2

What about writing the move-assignment operator this way:

OBJ& operator=(OBJ&& other) {
    this->~OBJ();
    new(this) OBJ(other.x, other.y);
    return *this;
}

You would also need a constructor:

OBJ(const int x, const int y)
    : x(x), y(y)
{
}

Answer 3

I would suggest moving to std::unique_ptr :

void func() {
  static std::unique_ptr<OBJ> obj = std::make_unique<OBJ>();
  std::unique_ptr<OBJ> other = std::make_unique<OBJ>(); // random values
  if(condition)
    obj = std::move(other); //move
}

This should be your choice in many cases where there is a need to move something that cannot be moved, to hold an unknown polymorphic type or any other case where you cannot deal with the actual type.