[webkit-dev] about hashmap staticDOMObjects

Maciej Stachowiak mjs at apple.com
Wed Apr 22 23:26:00 PDT 2009

Re-adding list (I assume it was omitted by mistake).

On Apr 22, 2009, at 11:12 PM, ZHOU Xiao-bo wrote:

> thanks, but I have another question:
> Is every DOM wrapper that is inserted into the hashtable been  
> protected, I mean whether they are added in
> the protectset?
> If yes, when are they inserted, if not, I think they may become wild  
> pointer after Collector::collect is called.

Objects in this hashtable are not protected from garbage collection.  
Instead, all DOM object wrappers that are eligible to be in this hash  
table will automatically remove themselves when collected. This  
ensures that the hashtable does not contain bad pointers after a GC.

  - Maciej

> 2009/4/23 Maciej Stachowiak <mjs at apple.com>
> On Apr 22, 2009, at 12:27 AM, ZHOU Xiao-bo wrote:
> hi all:
>     I still don't understand the purpose of the HashMap:
> static DOMObjectMap& domObjects()
> {
>    // Don't use malloc here. Calling malloc from a mark function can  
> deadlock.
>    static DOMObjectMap staticDOMObjects;
>    return staticDOMObjects;
> }
> what kind of DOMObjects should be stored in it? And why?
> I searched the source codes and I found that these classes below  
> will do that:
> Document, Event, HTMLCollection, XMLHttpRequest, CanvasGradient,  
> CanvasPattern, CanvasRenderingContext2D, DOMCoreException,  
> DOMImplementation, DOMParser, EventException, History, NamedNodeMap,  
> NodeFilter, NodeIterator, NodeList, Range, RangeException,  
> TreeWalker, XMLHttpRequestException, XMLSerializer, Clipboard
> but what's the reason? Is it because these classes are essentially  
> simple and just acting as a tool?
> The hash map is to ensure that DOM wrappers are unique but created  
> on demand, without consuming a pointer in the underlying C++ object.  
> For DOM Nodes, we use a hash table in the Node's owner document, but  
> in the case of non-Node objects and the Document itself (which has  
> no owner document), we need this global hash table.
> Regards,
> Maciej

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