Move constructor vs. Move assignment

Question

As an extension to This question, i am trying to get my move assignment correct.

I have the following code:

// copy assignment operator
LinkedList<T>& operator= (LinkedList<T> other) noexcept
{
    swap(*this, other);
    return *this;
}

// move assignment operator
LinkedList<T>& operator= (LinkedList<T>&& other) noexcept
{
    swap(*this, other);
    return *this;
}

But when i try to use it, my code fails to compile.

First some code:

LinkedList<int> generateLinkedList()
{
    LinkedList<int> List;   
    List.add(123);
    return List;
}


int main()
{
    LinkedList<int> L;   
    L = generateLinkedList();
      ^ get an error here...

I get the following error:

main.cpp(24): error C2593: 'operator =' is ambiguous

linkedlist.h(79): note: could be 'LinkedList &LinkedList::operator =(LinkedList &&) noexcept'(Points to the move assignment operator)

linkedlist.h(63): note: or 'LinkedList &LinkedList::operator =(LinkedList) noexcept' (Points to the copy assignment operator)

main.cpp(24): note: while trying to match the argument list '(LinkedList, LinkedList)'

Are my move assignment operator wrong, or do i use it the wrong way?

Good explanation in the C++11 section of the first answer to What is the copy-and-swap idiom?. Might even be good enough. — user4581301
– user4581301, Commented Nov 26, 2019 at 15:59

Max Langhof · Accepted Answer · 2019-11-27 08:27:33Z

5

The copy assignment operator would take a const LinkedList<T>& other, not a LinkedList<T> other.

This

LinkedList<T>& operator= (LinkedList<T> other) noexcept
{
    swap(*this, other);
    return *this;
}

is how one would implement both copy and move assignment at once using copy-and-swap. By re-using the copy and move constructors (other is either copy-constructed or move-constructed), and you just swap this with other. other dies at the end of the function, taking with it the old state of this. This implementation is totally fine, but then you don't need a second overload for temporaries (which is indeed ambiguous).

If you want to provide separate copy assignment operators for copy vs move assignment, the signatures would be

// copy assignment operator
LinkedList<T>& operator=(const LinkedList<T>& other) noexcept
{
  //...
}

// move assignment operator
LinkedList<T>& operator=(LinkedList<T>&& other) noexcept
{
  //...
}

But since you already have swap and the copy+move constructors, using copy-and-swap is preferable.

PS: As these appear to be inline definitions (i.e. within the class body), you can skip the <T> template argument - within the LinkedList template class definition, writing LinkedList automatically refers to the "current instantiation" (i.e. LinkedList<T>).

edited Nov 27, 2019 at 8:27

answered Nov 26, 2019 at 15:59

Max Langhof

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7 Comments

tannic Over a year ago

Now i have updated the copy assignment operator to receive a const LinkedList<T>&. Now that wont compile. I was following the examples in (stackoverflow.com/questions/3279543)

Max Langhof Over a year ago

@tannic Sorry, I originally meant to remove the bodies in the second code snippet as they wouldn't make sense. Indeed, you cannot swap with a const LinkedList& (because it's const) - you'd have to construct a copy manually first (i.e. LinkedList copy(other); swap(*this, copy); return *this;), but that's better done by doing copy-and-swap the normal way.

Tanveer Badar Over a year ago

Didn't know about this trick. I use operator=(other) in implementing the move constructor though.

Max Langhof Over a year ago

@TanveerBadar If you already have swap for your type, you can just implement the move constructor as default-constructor + swap(*this, other) (but maybe there's a more efficient implementation - default-constructing and then swapping some plain int value is wasted effort, you might as well copy it in the first place).

tannic Over a year ago

@MaxLanghof: if i looks in the link above (copy/swap paradime), i see this codesnip, which was actually what i used: dumb_array& operator=(dumb_array other) // (1) { swap(*this, other); // (2) return *this; }

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