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/*
* Copyright 2012 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkWeakRefCnt_DEFINED
#define SkWeakRefCnt_DEFINED
#include "SkRefCnt.h"
#include <atomic>
/** \class SkWeakRefCnt
SkWeakRefCnt is the base class for objects that may be shared by multiple
objects. When an existing strong owner wants to share a reference, it calls
ref(). When a strong owner wants to release its reference, it calls
unref(). When the shared object's strong reference count goes to zero as
the result of an unref() call, its (virtual) weak_dispose method is called.
It is an error for the destructor to be called explicitly (or via the
object going out of scope on the stack or calling delete) if
getRefCnt() > 1.
In addition to strong ownership, an owner may instead obtain a weak
reference by calling weak_ref(). A call to weak_ref() must be balanced by a
call to weak_unref(). To obtain a strong reference from a weak reference,
call try_ref(). If try_ref() returns true, the owner's pointer is now also
a strong reference on which unref() must be called. Note that this does not
affect the original weak reference, weak_unref() must still be called. When
the weak reference count goes to zero, the object is deleted. While the
weak reference count is positive and the strong reference count is zero the
object still exists, but will be in the disposed state. It is up to the
object to define what this means.
Note that a strong reference implicitly implies a weak reference. As a
result, it is allowable for the owner of a strong ref to call try_ref().
This will have the same effect as calling ref(), but may be more expensive.
Example:
SkWeakRefCnt myRef = strongRef.weak_ref();
... // strongRef.unref() may or may not be called
if (myRef.try_ref()) {
... // use myRef
myRef.unref();
} else {
// myRef is in the disposed state
}
myRef.weak_unref();
*/
class SK_API SkWeakRefCnt : public SkRefCnt {
public:
/** Default construct, initializing the reference counts to 1.
The strong references collectively hold one weak reference. When the
strong reference count goes to zero, the collectively held weak
reference is released.
*/
SkWeakRefCnt() : SkRefCnt(), fWeakCnt(1) {}
/** Destruct, asserting that the weak reference count is 1.
*/
~SkWeakRefCnt() override {
#ifdef SK_DEBUG
SkASSERT(getWeakCnt() == 1);
fWeakCnt.store(0, std::memory_order_relaxed);
#endif
}
#ifdef SK_DEBUG
/** Return the weak reference count. */
int32_t getWeakCnt() const {
return fWeakCnt.load(std::memory_order_relaxed);
}
void validate() const {
this->INHERITED::validate();
SkASSERT(getWeakCnt() > 0);
}
#endif
private:
/** If fRefCnt is 0, returns 0.
* Otherwise increments fRefCnt, acquires, and returns the old value.
*/
int32_t atomic_conditional_acquire_strong_ref() const {
int32_t prev = fRefCnt.load(std::memory_order_relaxed);
do {
if (0 == prev) {
break;
}
} while(!fRefCnt.compare_exchange_weak(prev, prev+1, std::memory_order_acquire,
std::memory_order_relaxed));
return prev;
}
public:
/** Creates a strong reference from a weak reference, if possible. The
caller must already be an owner. If try_ref() returns true the owner
is in posession of an additional strong reference. Both the original
reference and new reference must be properly unreferenced. If try_ref()
returns false, no strong reference could be created and the owner's
reference is in the same state as before the call.
*/
bool SK_WARN_UNUSED_RESULT try_ref() const {
if (atomic_conditional_acquire_strong_ref() != 0) {
// Acquire barrier (L/SL), if not provided above.
// Prevents subsequent code from happening before the increment.
return true;
}
return false;
}
/** Increment the weak reference count. Must be balanced by a call to
weak_unref().
*/
void weak_ref() const {
SkASSERT(getRefCnt() > 0);
SkASSERT(getWeakCnt() > 0);
// No barrier required.
(void)fWeakCnt.fetch_add(+1, std::memory_order_relaxed);
}
/** Decrement the weak reference count. If the weak reference count is 1
before the decrement, then call delete on the object. Note that if this
is the case, then the object needs to have been allocated via new, and
not on the stack.
*/
void weak_unref() const {
SkASSERT(getWeakCnt() > 0);
// A release here acts in place of all releases we "should" have been doing in ref().
if (1 == fWeakCnt.fetch_add(-1, std::memory_order_acq_rel)) {
// Like try_ref(), the acquire is only needed on success, to make sure
// code in internal_dispose() doesn't happen before the decrement.
#ifdef SK_DEBUG
// so our destructor won't complain
fWeakCnt.store(1, std::memory_order_relaxed);
#endif
this->INHERITED::internal_dispose();
}
}
/** Returns true if there are no strong references to the object. When this
is the case all future calls to try_ref() will return false.
*/
bool weak_expired() const {
return fRefCnt.load(std::memory_order_relaxed) == 0;
}
protected:
/** Called when the strong reference count goes to zero. This allows the
object to free any resources it may be holding. Weak references may
still exist and their level of allowed access to the object is defined
by the object's class.
*/
virtual void weak_dispose() const {
}
private:
/** Called when the strong reference count goes to zero. Calls weak_dispose
on the object and releases the implicit weak reference held
collectively by the strong references.
*/
void internal_dispose() const override {
weak_dispose();
weak_unref();
}
/* Invariant: fWeakCnt = #weak + (fRefCnt > 0 ? 1 : 0) */
mutable std::atomic<int32_t> fWeakCnt;
typedef SkRefCnt INHERITED;
};
#endif
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