857645 - Make it possible to copy-initialize an nsCOMPtr from a superclass already_AddRefed

Status: RESOLVED FIXED

(Core :: XPCOM, defect)

Description

[Boris Zbarsky [:bzbarsky]]

Basically, want code like this:

  nsRefPtr<nsTextNode> foo;
  nsCOMPtr<nsIContent> baz = foo.forget();

to compile.  Right now it doesn't.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Attachment: Make it possible to copy-initialize an nsCOMPtr<T> from an already_AddRefed<U> as long as U inherits from T.

Comment 2

[(no longer active)]

Comment on attachment 732899 [details] [diff] [review]
Make it possible to copy-initialize an nsCOMPtr<T> from an already_AddRefed<U> as long as U inherits from T.

Review of attachment 732899 [details] [diff] [review]:
-----------------------------------------------------------------

::: xpcom/glue/nsCOMPtr.h
@@ +555,5 @@
> +        {
> +          // But make sure that U actually inherits from T, not the
> +          // other way around.
> +          T* unused = aSmartPtr.mRawPtr;
> +          (void) unused;

Instead of this, please use a static assert with mozilla::IsBaseOf.

Comment 3

[Boris Zbarsky [:bzbarsky]]

Can I use that in nsCOMPtr?  If so, happy to do it.

Comment 4

[Boris Zbarsky [:bzbarsky]]

Attachment: Let's try this

Comment 5

[(no longer active)]

Comment on attachment 733157 [details] [diff] [review]
Let's try this

Review of attachment 733157 [details] [diff] [review]:
-----------------------------------------------------------------

Great, thanks!

Comment 6

[Boris Zbarsky [:bzbarsky]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/8396d7543197

Comment 7

[Phil Ringnalda (:philor)]

Backed out in https://hg.mozilla.org/integration/mozilla-inbound/rev/8fc33d047ed6 - nsHTMLEditRules.o was unhappy a la https://tbpl.mozilla.org/php/getParsedLog.php?id=21460075&tree=Mozilla-Inbound

Comment 8

[Boris Zbarsky [:bzbarsky]]

Attachment: Make it possible to copy-initialize an nsCOMPtr<T> from an already_AddRefed<U> as long as U inherits from T.

Fix that by making our operator= also accept any relevant already_AddRefed.

Comment 9

[[e-mail]]

Comment on attachment 733750 [details] [diff] [review]
Make it possible to copy-initialize an nsCOMPtr<T> from an already_AddRefed<U> as long as U inherits from T.

Why do you need to static_cast to T* once you know U is a subclass of T?

Comment 10

[Boris Zbarsky [:bzbarsky]]

Because I'm assigning into an nsISupports*, and U may have ambiguous inheritance from nsISupports, but T is known not to.

Comment 11

[Boris Zbarsky [:bzbarsky]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/d93dab0b19fc

Comment 12

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/d93dab0b19fc