xpcom/base/HoldDropJSObjects.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_HoldDropJSObjects_h
   8 #define mozilla_HoldDropJSObjects_h
   9
  10 #include <type_traits>
  11 #include "nsCycleCollectionNoteChild.h"
  12
  13 class nsISupports;
  14 class nsScriptObjectTracer;
  15
  16 namespace JS {
  17 class Zone;
  18 }
  19
  20 // Only HoldJSObjects and DropJSObjects should be called directly.
  21
  22 namespace mozilla {
  23 namespace cyclecollector {
  24
  25 void HoldJSObjectsImpl(void* aHolder, nsScriptObjectTracer* aTracer,
  26                        JS::Zone* aZone = nullptr);
  27 void HoldJSObjectsImpl(nsISupports* aHolder);
  28 void DropJSObjectsImpl(void* aHolder);
  29 void DropJSObjectsImpl(nsISupports* aHolder);
  30
  31 }  // namespace cyclecollector
  32
  33 template <class T, bool isISupports = std::is_base_of<nsISupports, T>::value>
  34 struct HoldDropJSObjectsHelper {
  35   static void Hold(T* aHolder) {
  36     cyclecollector::HoldJSObjectsImpl(aHolder,
  37                                       NS_CYCLE_COLLECTION_PARTICIPANT(T));
  38   }
  39   static void Drop(T* aHolder) { cyclecollector::DropJSObjectsImpl(aHolder); }
  40 };
  41
  42 template <class T>
  43 struct HoldDropJSObjectsHelper<T, true> {
  44   static void Hold(T* aHolder) {
  45     cyclecollector::HoldJSObjectsImpl(ToSupports(aHolder));
  46   }
  47   static void Drop(T* aHolder) {
  48     cyclecollector::DropJSObjectsImpl(ToSupports(aHolder));
  49   }
  50 };
  51
  52 /**
  53   Classes that hold strong references to JS GC things such as `JSObjects` and
  54   `JS::Values` (e.g. `JS::Heap<JSObject*> mFoo;`) must use these, generally by
  55   calling `HoldJSObjects(this)` and `DropJSObjects(this)` in the ctor and dtor
  56   respectively.
  57
  58   For classes that are wrapper cached and hold no other strong references to JS
  59   GC things, there's no need to call these; it will be taken care of
  60   automatically by nsWrapperCache.
  61 **/
  62 template <class T>
  63 void HoldJSObjects(T* aHolder) {
  64   HoldDropJSObjectsHelper<T>::Hold(aHolder);
  65 }
  66
  67 template <class T>
  68 void DropJSObjects(T* aHolder) {
  69   HoldDropJSObjectsHelper<T>::Drop(aHolder);
  70 }
  71
  72 }  // namespace mozilla
  73
  74 #endif  // mozilla_HoldDropJSObjects_h