gecko-dev/mfbt/RefPtr.h

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/* -*- 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
2012-05-21 11:12:37 +00:00
* 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/. */
/* Helpers for defining and using refcounted objects. */
#ifndef mozilla_RefPtr_h
#define mozilla_RefPtr_h
#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/Attributes.h"
#include "mozilla/NullPtr.h"
#include "mozilla/RefCountType.h"
#include "mozilla/TypeTraits.h"
#if defined(MOZILLA_INTERNAL_API)
#include "nsXPCOM.h"
#endif
#if defined(MOZILLA_INTERNAL_API) && \
(defined(DEBUG) || defined(FORCE_BUILD_REFCNT_LOGGING))
#define MOZ_REFCOUNTED_LEAK_CHECKING
#endif
namespace mozilla {
template<typename T> class RefCounted;
template<typename T> class RefPtr;
template<typename T> class TemporaryRef;
template<typename T> class OutParamRef;
template<typename T> OutParamRef<T> byRef(RefPtr<T>&);
/**
* RefCounted<T> is a sort of a "mixin" for a class T. RefCounted
* manages, well, refcounting for T, and because RefCounted is
* parameterized on T, RefCounted<T> can call T's destructor directly.
* This means T doesn't need to have a virtual dtor and so doesn't
* need a vtable.
*
* RefCounted<T> is created with refcount == 0. Newly-allocated
* RefCounted<T> must immediately be assigned to a RefPtr to make the
* refcount > 0. It's an error to allocate and free a bare
* RefCounted<T>, i.e. outside of the RefPtr machinery. Attempts to
* do so will abort DEBUG builds.
*
* Live RefCounted<T> have refcount > 0. The lifetime (refcounts) of
* live RefCounted<T> are controlled by RefPtr<T> and
* RefPtr<super/subclass of T>. Upon a transition from refcounted==1
* to 0, the RefCounted<T> "dies" and is destroyed. The "destroyed"
* state is represented in DEBUG builds by refcount==0xffffdead. This
* state distinguishes use-before-ref (refcount==0) from
* use-after-destroy (refcount==0xffffdead).
*
* Note that when deriving from RefCounted or AtomicRefCounted, you
* should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
* section of your class, where ClassName is the name of your class.
*/
namespace detail {
#ifdef DEBUG
const MozRefCountType DEAD = 0xffffdead;
#endif
// When building code that gets compiled into Gecko, try to use the
// trace-refcount leak logging facilities.
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
class RefCountLogger
{
public:
static void logAddRef(const void* aPointer, MozRefCountType aRefCount,
const char* aTypeName, uint32_t aInstanceSize)
{
MOZ_ASSERT(aRefCount != DEAD);
NS_LogAddRef(const_cast<void*>(aPointer), aRefCount, aTypeName,
aInstanceSize);
}
static void logRelease(const void* aPointer, MozRefCountType aRefCount,
const char* aTypeName)
{
MOZ_ASSERT(aRefCount != DEAD);
NS_LogRelease(const_cast<void*>(aPointer), aRefCount, aTypeName);
}
};
#endif
// This is used WeakPtr.h as well as this file.
enum RefCountAtomicity
{
AtomicRefCount,
NonAtomicRefCount
};
template<typename T, RefCountAtomicity Atomicity>
class RefCounted
{
friend class RefPtr<T>;
protected:
RefCounted() : mRefCnt(0) {}
~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }
public:
// Compatibility with nsRefPtr.
void AddRef() const
{
// Note: this method must be thread safe for AtomicRefCounted.
MOZ_ASSERT(int32_t(mRefCnt) >= 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
++mRefCnt;
#else
const char* type = static_cast<const T*>(this)->typeName();
uint32_t size = static_cast<const T*>(this)->typeSize();
const void* ptr = static_cast<const T*>(this);
MozRefCountType cnt = ++mRefCnt;
detail::RefCountLogger::logAddRef(ptr, cnt, type, size);
#endif
}
void Release() const
{
// Note: this method must be thread safe for AtomicRefCounted.
MOZ_ASSERT(int32_t(mRefCnt) > 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
MozRefCountType cnt = --mRefCnt;
#else
const char* type = static_cast<const T*>(this)->typeName();
const void* ptr = static_cast<const T*>(this);
MozRefCountType cnt = --mRefCnt;
// Note: it's not safe to touch |this| after decrementing the refcount,
// except for below.
detail::RefCountLogger::logRelease(ptr, cnt, type);
#endif
if (0 == cnt) {
// Because we have atomically decremented the refcount above, only
// one thread can get a 0 count here, so as long as we can assume that
// everything else in the system is accessing this object through
// RefPtrs, it's safe to access |this| here.
#ifdef DEBUG
mRefCnt = detail::DEAD;
#endif
delete static_cast<const T*>(this);
}
}
// Compatibility with wtf::RefPtr.
void ref() { AddRef(); }
void deref() { Release(); }
MozRefCountType refCount() const { return mRefCnt; }
bool hasOneRef() const
{
MOZ_ASSERT(mRefCnt > 0);
return mRefCnt == 1;
}
private:
mutable typename Conditional<Atomicity == AtomicRefCount,
Atomic<MozRefCountType>,
MozRefCountType>::Type mRefCnt;
};
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T) \
virtual const char* typeName() const { return #T; } \
virtual size_t typeSize() const { return sizeof(*this); }
#else
#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T)
#endif
// Note that this macro is expanded unconditionally because it declares only
// two small inline functions which will hopefully get eliminated by the linker
// in non-leak-checking builds.
#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T) \
const char* typeName() const { return #T; } \
size_t typeSize() const { return sizeof(*this); }
} // namespace detail
template<typename T>
class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount>
{
public:
~RefCounted()
{
static_assert(IsBaseOf<RefCounted, T>::value,
"T must derive from RefCounted<T>");
}
};
namespace external {
/**
* AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
* reference counter.
*
* NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
* instead.
*/
template<typename T>
class AtomicRefCounted :
public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount>
{
public:
~AtomicRefCounted()
{
static_assert(IsBaseOf<AtomicRefCounted, T>::value,
"T must derive from AtomicRefCounted<T>");
}
};
} // namespace external
/**
* RefPtr points to a refcounted thing that has AddRef and Release
* methods to increase/decrease the refcount, respectively. After a
* RefPtr<T> is assigned a T*, the T* can be used through the RefPtr
* as if it were a T*.
*
* A RefPtr can forget its underlying T*, which results in the T*
* being wrapped in a temporary object until the T* is either
* re-adopted from or released by the temporary.
*/
template<typename T>
class RefPtr
{
// To allow them to use unref()
friend class TemporaryRef<T>;
friend class OutParamRef<T>;
struct DontRef {};
public:
RefPtr() : mPtr(0) {}
RefPtr(const RefPtr& aOther) : mPtr(ref(aOther.mPtr)) {}
MOZ_IMPLICIT RefPtr(const TemporaryRef<T>& aOther) : mPtr(aOther.drop()) {}
MOZ_IMPLICIT RefPtr(T* aVal) : mPtr(ref(aVal)) {}
template<typename U>
RefPtr(const RefPtr<U>& aOther) : mPtr(ref(aOther.get())) {}
~RefPtr() { unref(mPtr); }
RefPtr& operator=(const RefPtr& aOther)
{
assign(ref(aOther.mPtr));
return *this;
}
RefPtr& operator=(const TemporaryRef<T>& aOther)
{
assign(aOther.drop());
return *this;
}
RefPtr& operator=(T* aVal)
{
assign(ref(aVal));
return *this;
}
template<typename U>
RefPtr& operator=(const RefPtr<U>& aOther)
{
assign(ref(aOther.get()));
return *this;
}
TemporaryRef<T> forget()
{
T* tmp = mPtr;
mPtr = nullptr;
return TemporaryRef<T>(tmp, DontRef());
}
T* get() const { return mPtr; }
operator T*() const { return mPtr; }
T* operator->() const { return mPtr; }
T& operator*() const { return *mPtr; }
template<typename U>
operator TemporaryRef<U>() { return TemporaryRef<U>(mPtr); }
private:
void assign(T* aVal)
{
unref(mPtr);
mPtr = aVal;
}
T* mPtr;
static MOZ_ALWAYS_INLINE T* ref(T* aVal)
{
if (aVal) {
aVal->AddRef();
}
return aVal;
}
static MOZ_ALWAYS_INLINE void unref(T* aVal)
{
if (aVal) {
aVal->Release();
}
}
};
/**
* TemporaryRef<T> represents an object that holds a temporary
* reference to a T. TemporaryRef objects can't be manually ref'd or
* unref'd (being temporaries, not lvalues), so can only relinquish
* references to other objects, or unref on destruction.
*/
template<typename T>
class TemporaryRef
{
// To allow it to construct TemporaryRef from a bare T*
friend class RefPtr<T>;
typedef typename RefPtr<T>::DontRef DontRef;
public:
MOZ_IMPLICIT TemporaryRef(T* aVal) : mPtr(RefPtr<T>::ref(aVal)) {}
TemporaryRef(const TemporaryRef& aOther) : mPtr(aOther.drop()) {}
template<typename U>
TemporaryRef(const TemporaryRef<U>& aOther) : mPtr(aOther.drop()) {}
~TemporaryRef() { RefPtr<T>::unref(mPtr); }
T* drop() const
{
T* tmp = mPtr;
mPtr = nullptr;
return tmp;
}
private:
TemporaryRef(T* aVal, const DontRef&) : mPtr(aVal) {}
mutable T* mPtr;
TemporaryRef() MOZ_DELETE;
void operator=(const TemporaryRef&) MOZ_DELETE;
};
/**
* OutParamRef is a wrapper that tracks a refcounted pointer passed as
* an outparam argument to a function. OutParamRef implements COM T**
* outparam semantics: this requires the callee to AddRef() the T*
* returned through the T** outparam on behalf of the caller. This
* means the caller (through OutParamRef) must Release() the old
* object contained in the tracked RefPtr. It's OK if the callee
* returns the same T* passed to it through the T** outparam, as long
* as the callee obeys the COM discipline.
*
* Prefer returning TemporaryRef<T> from functions over creating T**
* outparams and passing OutParamRef<T> to T**. Prefer RefPtr<T>*
* outparams over T** outparams.
*/
template<typename T>
class OutParamRef
{
friend OutParamRef byRef<T>(RefPtr<T>&);
public:
~OutParamRef()
{
RefPtr<T>::unref(mRefPtr.mPtr);
mRefPtr.mPtr = mTmp;
}
operator T**() { return &mTmp; }
private:
explicit OutParamRef(RefPtr<T>& p) : mRefPtr(p), mTmp(p.get()) {}
RefPtr<T>& mRefPtr;
T* mTmp;
OutParamRef() MOZ_DELETE;
OutParamRef& operator=(const OutParamRef&) MOZ_DELETE;
};
/**
* byRef cooperates with OutParamRef to implement COM outparam semantics.
*/
template<typename T>
OutParamRef<T>
byRef(RefPtr<T>& aPtr)
{
return OutParamRef<T>(aPtr);
}
} // namespace mozilla
#endif /* mozilla_RefPtr_h */