325636 - XPConnect wrapping makes object identity testing hard

Status: RESOLVED INACTIVE

(Core :: XPConnect, defect)

Description

[Dan Mosedale (:dmosedale, :dmose)]

In COM itself, clients are allowed to compare any two interface pointers to determine whether they point to the same object (Essential COM, chapter 4, "Uniqueness and Identity" section).  In C++, XPCOM allows this as well.  Furthermore, various interfaces in Mozilla depend on this by having interface pointers as in params which are used as keys into lists of objects.

Because XPCOM interface pointers can be mapped to JS objects in variety of different ways, doing object identity tests when implementing such interfaces is complicated.  It would be really nice if this could be made easier.  One possibility suggested by bz would be to make |x == x.wrappedJSObject| return true.

Comment 1

[Dan Mosedale (:dmosedale, :dmose)]

"when implementing such interfaces" should read "when implementing such interfaces in JS".

Comment 2

[Dan Mosedale (:dmosedale, :dmose)]

It's been pointed out to me that the pointer equivalence rule I mentioned is specific to IUnknown in COM, and not general to all interface pointers.

Comment 3

[timeless]

Just in case people need a solution sooner...

var SIP=Components.Constructor("@mozilla.org/supports-interface-pointer;1", Components.interfaces.nsISupportsInterfacePointer);

function compare_js_object_to_xpconnect_object(jsObject, xpconnectObject)
{
  var sip = new SIP;
  sip.data = jsObject;
  return sip.data == xpconnectObject;
}

Comment 4

[BMO Automation]

Per policy at https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Inactive_Bugs. If this bug is not an enhancement request or a bug not present in a supported release of Firefox, then it may be reopened.