1792638 - Cannot update MOZ_KNOWN_LIVE members in move assign operator override

Status: NEW

(Developer Infrastructure :: Source Code Analysis, enhancement, P3)

Description

[Masayuki Nakano [:masayuki] (he/him)(JST, +0900)]

class MOZ_STACK_CLASS Foo {
 public:
  Foo(Foo&& aOther) : mContent(std::move(aOther.mContent)) {}  // no problem
  Foo& operator=(Foo&& aOther) {
    mContent = std::move(aOther.mContent);  // error: MOZ_KNOWN_LIVE members can only be modified by constructors and destructors
    return *this;
  }
 private:
  MOZ_KNOWN_LIVE nsCOMPtr<nsIContent> mContent;
};

I'm not sure whether assigning the value in move assign operator causes some troubles or not. However, Foo& operator=(Foo&& aOther) = default works. Therefore, they are not consistent result at least.

Comment 1

[Masayuki Nakano [:masayuki] (he/him)(JST, +0900)]

FYI: I want to define it for validating the members with MOZ_ASSERT.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:andi, could you have a look please?

For more information, please visit auto_nag documentation.

Comment 3

[Andi [:andi]]

This was added by https://phabricator.services.mozilla.com/D146023 maybe let's ask :saschanaz what was the reasoning behind it.

Comment 4

[Kagami Rosylight [:saschanaz] (they/them)]

(I believe Masayuki and I had interaction while adding that.)

The intention was to give some real restriction to MOZ_KNOWN_LIVE rather than just randomly declaring it's "live". It's tricky here since we can't make sure the assign operator never overwrites the existing non-null pointer. I'm not sure what to do here tbh, maybe we can require MOZ_ASSERT(!mContent) in the operator?

Comment 5

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:andi, could you have a look please?

For more information, please visit auto_nag documentation.