gecko-dev/xpcom/glue/nsCycleCollectionNoteChild.h
Nathan Froyd e07aa30b4b Bug 1097861 - make CycleCollectionNoteChild more easily forward-declarable; r=mccr8
Forward declaring functions with default arguments is difficult.  If you try to say:

template<typename T>
inline void
CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
                         T* aChild, const char* aName, uint32_t aFlags);

and then later have:

template<typename T>
inline void
CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
                         T* aChild, const char* aName, uint32_t aFlags = 0);
{
  ...
}

the compiler complains that default arguments cannot be added to a
function template that has already been declared.  If you attempt to
mollify the compiler by declaring instead:

template<typename T>
inline void
CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
                         T* aChild, const char* aName, uint32_t aFlags = 0);

the compiler then complains about redefining the default argument (!)
when an actual definition is found.

To circumvent this, manually implement "default" arguments by providing
a three-argument form of CycleCollectionNoteChild, which simply forwards
to the four-argument version.
2014-11-12 15:22:32 -05:00

102 lines
3.3 KiB
C++

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
// This header will be included by headers that define refpointer and array classes
// in order to specialize CC helpers such as ImplCycleCollectionTraverse for them.
#ifndef nsCycleCollectionNoteChild_h__
#define nsCycleCollectionNoteChild_h__
#include "nsCycleCollectionTraversalCallback.h"
#include "mozilla/Likely.h"
#include "mozilla/TypeTraits.h"
enum
{
CycleCollectionEdgeNameArrayFlag = 1
};
// Just a helper for appending "[i]". Didn't want to pull in string headers here.
void
CycleCollectionNoteEdgeNameImpl(nsCycleCollectionTraversalCallback& aCallback,
const char* aName,
uint32_t aFlags = 0);
// Should be inlined so that in the no-debug-info case this is just a simple if().
MOZ_ALWAYS_INLINE void
CycleCollectionNoteEdgeName(nsCycleCollectionTraversalCallback& aCallback,
const char* aName,
uint32_t aFlags = 0)
{
if (MOZ_UNLIKELY(aCallback.WantDebugInfo())) {
CycleCollectionNoteEdgeNameImpl(aCallback, aName, aFlags);
}
}
#define NS_CYCLE_COLLECTION_INNERCLASS \
cycleCollection
#define NS_CYCLE_COLLECTION_INNERNAME \
_cycleCollectorGlobal
#define NS_CYCLE_COLLECTION_PARTICIPANT(_class) \
_class::NS_CYCLE_COLLECTION_INNERCLASS::GetParticipant()
template<typename T>
nsISupports*
ToSupports(T* aPtr, typename T::NS_CYCLE_COLLECTION_INNERCLASS* aDummy = 0)
{
return T::NS_CYCLE_COLLECTION_INNERCLASS::Upcast(aPtr);
}
// The default implementation of this class template is empty, because it
// should never be used: see the partial specializations below.
template<typename T,
bool IsXPCOM = mozilla::IsBaseOf<nsISupports, T>::value>
struct CycleCollectionNoteChildImpl
{
};
template<typename T>
struct CycleCollectionNoteChildImpl<T, true>
{
static void Run(nsCycleCollectionTraversalCallback& aCallback, T* aChild)
{
aCallback.NoteXPCOMChild(ToSupports(aChild));
}
};
template<typename T>
struct CycleCollectionNoteChildImpl<T, false>
{
static void Run(nsCycleCollectionTraversalCallback& aCallback, T* aChild)
{
aCallback.NoteNativeChild(aChild, NS_CYCLE_COLLECTION_PARTICIPANT(T));
}
};
// We declare CycleCollectionNoteChild in 3-argument and 4-argument variants,
// rather than using default arguments, so that forward declarations work
// regardless of header inclusion order.
template<typename T>
inline void
CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
T* aChild, const char* aName, uint32_t aFlags)
{
CycleCollectionNoteEdgeName(aCallback, aName, aFlags);
CycleCollectionNoteChildImpl<T>::Run(aCallback, aChild);
}
template<typename T>
inline void
CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
T* aChild, const char* aName)
{
CycleCollectionNoteChild(aCallback, aChild, aName, 0);
}
#endif // nsCycleCollectionNoteChild_h__