[lemmyg]

Move assignment has nothing to do with 'clearing fields' or whatever it's that the moved-to value steals what it can and leaves the moved from value in a valid but unspecified state. Notably, you should still be able to assign-to and destroy a moved-from value.

It's an invariant of the 'DirectorySearchResult' class that query is non-null, as the destructor deletes it without checking it for nullity. The most straight-forward and efficient way to achieve both the stealing and the valid state is to swap the pointers.

[jemfinch]

> It's an invariant of the 'DirectorySearchResult' class that query is non-null, as the destructor deletes it without checking it for nullity.

Deleting a null pointer is a fully specified no-op in C++.

[lemmyg]

That's true. And I see the other example answers they have written do assume the pointer can be null, although the original class makes no indication of this.

To make the question clearer they should have added a function on the original class that checks (or doesn't check) the pointer before dereferencing it, thereby clarifying whether query being null is valid or not.