1544860 - Make it easier to cast from RefPtr<T>/already_AddRefed<T> and RefPtr<U>/already_AddRefed<U>

Status: RESOLVED WORKSFORME

(Core :: MFBT, defect)

Description

[Beth Rennie [:beth] (she/her)]

I commonly run into the pattern where a method returns a pointer to a base class but I need to act on the specific subclass (and I know it actually returns that subclass), e.g.:

// inside CompositorOGL:

RefPtr<CompositingRenderTargetOGL> renderTarget;
{
  RefPtr<CompositingRenderTarget> genericRenderTarget = CreateRenderTarget(/* ... */);
  renderTarget = static_cast<CompositingRenderTargetOGL*>(genericRenderTarget.get());
}

It would be great if I could do something like:

RefPtr<CompositingRenderTargetOGL> renderTarget = CreateRenderTarget(/* ... */)
    .static_cast<CompositingRenderTargetOGL>();

This would also have the side effect of getting rid of an AddRef() and Release() pair.

Comment 1

[Nathan Froyd [:froydnj]]

already_AddRefed has a downcast method for exactly this case:

https://searchfox.org/mozilla-central/source/mfbt/AlreadyAddRefed.h#150-169