On Oct 29, 2009, at 12:56 PM, Darin Adler wrote:
On Oct 27, 2009, at 10:55 AM, Jens Alfke wrote:
Looking at how refcounting is implemented in WebCore, I was
surprised to find that there are a lot of functions/methods that
take PassRefPtr<>s as parameters instead of regular pointers to
those objects. I can't see any benefit to this, and it adds the
overhead of a ref() and deref() at every call-site.
Have you read the RefPtr document? It is at <http://webkit.org/coding/RefPtr.html
>.
Only functions that take ownership of the passed-in objects should
take PassRefPtr. This makes it so we can optimally handle the case
where that object was just created and lets us hand off the
reference instead of having to do a round of reference count thrash.
If the function is going to be storing the value in a RefPtr, then
the PassRefPtr does not introduce any reference count thrash even if
the passed in object was not just created.
It’s wrong for the HTMLNameCollection constructor to take a
PassRefPtr for its document argument because it doesn’t take
ownership of the document. So that one is definitely a mistake.
However, its base class HTMLCollection can take ownership. The
HTMLNameCollection constructor misses out on the optimization though:
HTMLNameCollection::HTMLNameCollection(PassRefPtr<Document> document,
CollectionType type, const String& name)
: HTMLCollection(document.get(), type, document-
>nameCollectionInfo(type, name))
, m_name(name)
{
}
Note that it passes document.get(), but also uses document after that
so it couldn't just pass document. Thus, it probably shouldn't take a
PassRefPtr (unless there is a clever way to fix it that I'm not seeing).
- Maciej
It is true that PassRefPtr arguments can be tricky as you mentioned
and the RefPtr document also mentions.
-- Darin
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