Android高级——智能指针

智能指针

智能指针是一种能够自动维护对象引用计数的技术

引用了一个实际使用的对象，而不是一个指针
智能指针构造时，增加它所引用的对象的引用计数
智能指针析构时，减少它所引用的对象的引用计数

但智能指针无法解决循环引用问题，引入强引用计数和弱引用计数

对象的生命周期只受强引用计数控制
将有关联的对象划分为“父-子”和“子-父”关系
在“父-子”关系中，“父”对象通过强引用计数来引用“子”对象
而在“子-父”关系中，“子”对象通过弱引用计数来引用“父”对象
如A通过强引用计数来引用B，而B通过弱引用计数来引用A，A的生命周期不受B的影响，当A不再使用时可以安全地释放
当B想要使用A时，先要成功地将A的弱引用计数升级为强引用计数，若升级失败，那么就说明A已经被释放，不能再使用

Android系统提供了三种类型的C++智能指针

轻量级指针——引用计数
强指针——强引用计数
弱指针——弱引用技术

轻量级指针

LightRefBase

一个类的对象要支持使用轻量级指针，需继承LightRefBase，其在frameworks/rs/cpp/util/RefBase.h（Android13）

template <class T>
class LightRefBase
{
public:inline LightRefBase() : mCount(0) { }inline void incStrong(__attribute__((unused)) const void* id) const {__sync_fetch_and_add(&mCount, 1);}inline void decStrong(__attribute__((unused)) const void* id) const {if (__sync_fetch_and_sub(&mCount, 1) == 1) {delete static_cast<const T*>(this);}}//! DEBUGGING ONLY: Get current strong ref count.inline int32_t getStrongCount() const {return mCount;}typedef LightRefBase<T> basetype;protected:inline ~LightRefBase() { }private:friend class ReferenceMover;inline static void moveReferences(void*, void const*, size_t,const ReferenceConverterBase&) { }private:mutable volatile int32_t mCount;
};

模板类，T表示对象的实际类型，需继承LightRefBase
mCount 描述对象的引用计数值
incStrong和decStrong 用来增加/减少它所引用对象的引用计数，减少到0释放内存

sp

sp用于轻量级指针，同时也用于强指针，其在frameworks/rs/cpp/util/StrongPointer.h（Android13）

template <typename T>
class sp
{
public:inline sp() : m_ptr(0) { }sp(T* other);  // NOLINT, implicitsp(const sp<T>& other);template<typename U> sp(U* other);  // NOLINT, implicittemplate<typename U> sp(const sp<U>& other);  // NOLINT, implicit~sp();// Assignmentsp& operator = (T* other);sp& operator = (const sp<T>& other);template<typename U> sp& operator = (const sp<U>& other);template<typename U> sp& operator = (U* other);//! Special optimization for use by ProcessState (and nobody else).void force_set(T* other);// Resetvoid clear();// Accessorsinline  T&      operator* () const  { return *m_ptr; }inline  T*      operator-> () const { return m_ptr;  }inline  T*      get() const         { return m_ptr; }// OperatorsCOMPARE(==)COMPARE(!=)COMPARE(>)COMPARE(<)COMPARE(<=)COMPARE(>=)private:template<typename Y> friend class sp;template<typename Y> friend class wp;void set_pointer(T* ptr);T* m_ptr;
};

模板类，T表示对象的实际类型，需继承LightRefBase
m_ptr 在构造函数中初始化，指向实际引用的对象

sp类的构造函数如下，初始化m_ptr（指向LightRefBase子类），调用LightRefBase类incStrong增加它的引用计数

template<typename T>
sp<T>::sp(T* other)
: m_ptr(other){if (other) other->incStrong(this);}

sp类的析构函数如下，调用 LightRefBase类decStrong 减少它的引用计数

template<typename T>
sp<T>::~sp()
{if (m_ptr) m_ptr->decStrong(this);
}

实例分析

external/lightpointer/lightpointer.cpp

强指针和弱指针

RefBase

一个类的对象要支持使用轻量级指针，需继承RefBase，其在frameworks/rs/cpp/util/RefBase.h（Android13）

class RefBase
{
public:void            incStrong(const void* id) const;void            decStrong(const void* id) const;void            forceIncStrong(const void* id) const;//! DEBUGGING ONLY: Get current strong ref count.int32_t         getStrongCount() const;class weakref_type{public:RefBase*            refBase() const;void                incWeak(const void* id);void                decWeak(const void* id);// acquires a strong reference if there is already one.bool                attemptIncStrong(const void* id);// acquires a weak reference if there is already one.// This is not always safe. see ProcessState.cpp and BpBinder.cpp// for proper use.bool                attemptIncWeak(const void* id);//! DEBUGGING ONLY: Get current weak ref count.int32_t             getWeakCount() const;//! DEBUGGING ONLY: Print references held on object.void                printRefs() const;//! DEBUGGING ONLY: Enable tracking for this object.// enable -- enable/disable tracking// retain -- when tracking is enable, if true, then we save a stack trace//           for each reference and dereference; when retain == false, we//           match up references and dereferences and keep only the//           outstanding ones.void                trackMe(bool enable, bool retain);};weakref_type*   createWeak(const void* id) const;weakref_type*   getWeakRefs() const;//! DEBUGGING ONLY: Print references held on object.inline  void            printRefs() const { getWeakRefs()->printRefs(); }//! DEBUGGING ONLY: Enable tracking of object.inline  void            trackMe(bool enable, bool retain){getWeakRefs()->trackMe(enable, retain);}typedef RefBase basetype;protected:RefBase();virtual                 ~RefBase();//! Flags for extendObjectLifetime()enum {OBJECT_LIFETIME_STRONG  = 0x0000,OBJECT_LIFETIME_WEAK    = 0x0001,OBJECT_LIFETIME_MASK    = 0x0001};void            extendObjectLifetime(int32_t mode);//! Flags for onIncStrongAttempted()enum {FIRST_INC_STRONG = 0x0001};virtual void            onFirstRef();virtual void            onLastStrongRef(const void* id);virtual bool            onIncStrongAttempted(uint32_t flags, const void* id);virtual void            onLastWeakRef(const void* id);private:friend class ReferenceMover;static void moveReferences(void* d, void const* s, size_t n,const ReferenceConverterBase& caster);private:friend class weakref_type;class weakref_impl;RefBase(const RefBase& o);RefBase&        operator=(const RefBase& o);weakref_impl* const mRefs;
};

不同于LightRefBase，使用weakref_impl对象（即mRefs）维护对象的引用计数，而非整数

weakref_impl

weakref_impl同时为对象提供了强引用计数和弱引用计数，其在./system/core/libutils/RefBase.cpp（Android13）

class RefBase::weakref_impl : public RefBase::weakref_type
{
public:std::atomic<int32_t>    mStrong;std::atomic<int32_t>    mWeak;RefBase* const          mBase;std::atomic<int32_t>    mFlags;#if !DEBUG_REFSexplicit weakref_impl(RefBase* base): mStrong(INITIAL_STRONG_VALUE), mWeak(0), mBase(base), mFlags(OBJECT_LIFETIME_STRONG){}void addStrongRef(const void* /*id*/) { }void removeStrongRef(const void* /*id*/) { }void renameStrongRefId(const void* /*old_id*/, const void* /*new_id*/) { }void addWeakRef(const void* /*id*/) { }void removeWeakRef(const void* /*id*/) { }void renameWeakRefId(const void* /*old_id*/, const void* /*new_id*/) { }void printRefs() const { }void trackMe(bool, bool) { }#else......
#endif
};

weakref_impl 继承了 weakref_type，并实现其中方法
mStrong / mWeak 描述强/弱引用计数，mBase指向它所引用对象的地址

mFlags 描述对象生命周期控制方法

0：只受强引用计数影响
OBJECT_LIFETIME_WEAK：同时受强/弱引用计数影响
OBJECT_LIFETIME_FOREVER：完全不受强/弱引用计数影响

强指针

sp

强指针实现类为sp，同上

sp构造函数

T为继承RefBase的子类

template<typename T>
sp<T>::sp(T* other)
: m_ptr(other){if (other) other->incStrong(this);}

实际调用RefBase的incStrong，RefBase在构造函数初始化mRefs

RefBase::RefBase(): mRefs(new weakref_impl(this))
{
}void RefBase::incStrong(const void* id) const
{weakref_impl* const refs = mRefs;refs->incWeak(id);refs->addStrongRef(id);const int32_t c = refs->mStrong.fetch_add(1, std::memory_order_relaxed);ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFSALOGD("incStrong of %p from %p: cnt=%d\n", this, id, c);
#endifif (c != INITIAL_STRONG_VALUE)  {return;}int32_t old __unused = refs->mStrong.fetch_sub(INITIAL_STRONG_VALUE, std::memory_order_relaxed);// A decStrong() must still happen after us.ALOG_ASSERT(old > INITIAL_STRONG_VALUE, "0x%x too small", old);refs->mBase->onFirstRef();
}void RefBase::weakref_type::incWeak(const void* id)
{weakref_impl* const impl = static_cast<weakref_impl*>(this);impl->addWeakRef(id);const int32_t c __unused = impl->mWeak.fetch_add(1,std::memory_order_relaxed);ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}

调用 incWeak 将this转为weakref_impl并调用mWeak.fetch_add增加弱引用计数
调用 mStrong.fetch_add 增加强引用计数
若是第一次被强指针引用，调用fetch_sub将强引用计数值设置为1（默认为INITIAL_STRONG_VALUE），然后调用 onFirstRef 处理相关业务，默认为空实现，需要子类自行实现
弱引用计数>=强引用计数

sp析构函数

同理，T为继承RefBase的子类

template<typename T>
sp<T>::~sp()
{if (m_ptr) m_ptr->decStrong(this);
}

实际调用RefBase的decStrong

void RefBase::decStrong(const void* id) const
{weakref_impl* const refs = mRefs;refs->removeStrongRef(id);const int32_t c = refs->mStrong.fetch_sub(1, std::memory_order_release);
#if PRINT_REFSALOGD("decStrong of %p from %p: cnt=%d\n", this, id, c);
#endifLOG_ALWAYS_FATAL_IF(BAD_STRONG(c), "decStrong() called on %p too many times",refs);if (c == 1) {std::atomic_thread_fence(std::memory_order_acquire);refs->mBase->onLastStrongRef(id);int32_t flags = refs->mFlags.load(std::memory_order_relaxed);if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {delete this;// The destructor does not delete refs in this case.}}refs->decWeak(id);
}RefBase::~RefBase()
{int32_t flags = mRefs->mFlags.load(std::memory_order_relaxed);if ((flags & OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_WEAK) {if (mRefs->mWeak.load(std::memory_order_relaxed) == 0) {delete mRefs;}} else if (mRefs->mStrong.load(std::memory_order_relaxed) == INITIAL_STRONG_VALUE) {ALOGW("RefBase: Explicit destruction, weak count = %d (in %p). Use sp<> to manage this ""object.",mRefs->mWeak.load(), this);#if CALLSTACK_ENABLEDCallStack::logStack(LOG_TAG);
#endif}// For debugging purposes, clear mRefs.  Ineffective against outstanding wp's.const_cast<weakref_impl*&>(mRefs) = nullptr;
}void RefBase::weakref_type::decWeak(const void* id)
{weakref_impl* const impl = static_cast<weakref_impl*>(this);impl->removeWeakRef(id);const int32_t c = impl->mWeak.fetch_sub(1, std::memory_order_release);LOG_ALWAYS_FATAL_IF(BAD_WEAK(c), "decWeak called on %p too many times",this);if (c != 1) return;atomic_thread_fence(std::memory_order_acquire);int32_t flags = impl->mFlags.load(std::memory_order_relaxed);if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {if (impl->mStrong.load(std::memory_order_relaxed)== INITIAL_STRONG_VALUE) {// Decrementing a weak count to zero when object never had a strong// reference.  We assume it acquired a weak reference early, e.g.// in the constructor, and will eventually be properly destroyed,// usually via incrementing and decrementing the strong count.// Thus we no longer do anything here.  We log this case, since it// seems to be extremely rare, and should not normally occur. We// used to deallocate mBase here, so this may now indicate a leak.ALOGW("RefBase: Object at %p lost last weak reference ""before it had a strong reference", impl->mBase);} else {// ALOGV("Freeing refs %p of old RefBase %p\n", this, impl->mBase);delete impl;}} else {impl->mBase->onLastWeakRef(id);delete impl->mBase;}
}

调用 mStrong.fetch_sub 减少强引用计数
当为0时调用 onLastStrongRef 处理相关业务，默认为空实现，需要子类自行实现
判断 mFlags 标志位，检查生命周期是否受弱引用计数控制，若为否则释放内存并调用RefBase的析构函数
在RefBase的析构函数中若发现弱引用计数为0则释放mRefs

当一个对象的强引用计数值为0时

当弱引用计数值大于0，只能释放RefBase，不能释放其内部的weakref_impl（因为还有其他的弱指针通过该weakref_impl对象来引用实际的对象）
当弱引用计数值为0，才可以释放weakref_impl

调用 decWeak 将this转为weakref_impl并调用 mWeak.fetch_sub 减少弱引用计数，若为0

若对象的生命周期只受强引用计数控制，且对象从来没有被强指针引用过，则打印输出记录（impl->mBase好像没释放？）
若对象的生命周期只受强引用计数控制，且被强指针引用过，当若弱引用计数为0时，decStrong释放该对象，decWeak释放其内部的weakref_impl
若对象的生命周期受弱引用计数控制或完全不受强/弱引用计数控制，调用onLastWeakRef处理业务并释放对象

弱指针

wp

wp用于弱指针，其在frameworks/rs/cpp/util/RefBase.h（Android13）

template <typename T>
class wp
{
public:typedef typename RefBase::weakref_type weakref_type;inline wp() : m_ptr(0) { }explicit wp(T* other);wp(const wp<T>& other);explicit wp(const sp<T>& other);template<typename U> explicit wp(U* other);template<typename U> explicit wp(const sp<U>& other);template<typename U> explicit wp(const wp<U>& other);~wp();// Assignmentwp& operator = (T* other);wp& operator = (const wp<T>& other);wp& operator = (const sp<T>& other);template<typename U> wp& operator = (U* other);template<typename U> wp& operator = (const wp<U>& other);template<typename U> wp& operator = (const sp<U>& other);void set_object_and_refs(T* other, weakref_type* refs);// promotion to spsp<T> promote() const;// Resetvoid clear();// Accessorsinline  weakref_type* get_refs() const { return m_refs; }inline  T* unsafe_get() const { return m_ptr; }// OperatorsCOMPARE_WEAK(==)COMPARE_WEAK(!=)COMPARE_WEAK(>)COMPARE_WEAK(<)COMPARE_WEAK(<=)COMPARE_WEAK(>=)inline bool operator == (const wp<T>& o) const {return (m_ptr == o.m_ptr) && (m_refs == o.m_refs);}template<typename U>inline bool operator == (const wp<U>& o) const {return m_ptr == o.m_ptr;}inline bool operator > (const wp<T>& o) const {return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);}template<typename U>inline bool operator > (const wp<U>& o) const {return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);}inline bool operator < (const wp<T>& o) const {return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);}template<typename U>inline bool operator < (const wp<U>& o) const {return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);}inline bool operator != (const wp<T>& o) const { return m_refs != o.m_refs; }template<typename U> inline bool operator != (const wp<U>& o) const { return !operator == (o); }inline bool operator <= (const wp<T>& o) const { return !operator > (o); }template<typename U> inline bool operator <= (const wp<U>& o) const { return !operator > (o); }inline bool operator >= (const wp<T>& o) const { return !operator < (o); }template<typename U> inline bool operator >= (const wp<U>& o) const { return !operator < (o); }private:template<typename Y> friend class sp;template<typename Y> friend class wp;T*              m_ptr;weakref_type*   m_refs;
};

模板类，T表示对象的实际类型，需继承RefBase
m_ptr 在构造函数中初始化，指向实际引用的对象
使用weakref_type类型的变量m_refs维护对象的弱指针引用
弱指针不可以直接操作它所引用的对象，因为它所引用的对象可能是不受弱引用计数控制的
如果需要操作一个弱指针所引用的对象，需要通过promote升级为强指针

wp构造函数

T为继承RefBase的子类

template<typename T>
wp<T>::wp(T* other): m_ptr(other)
{if (other) m_refs = other->createWeak(this);
}

实际调用RefBase的createWeak、incWeak在上面有讲

RefBase::weakref_type* RefBase::createWeak(const void* id) const
{mRefs->incWeak(id);return mRefs;
}

wp析构函数

实际调用RefBase的decWeak,在上面有讲

template<typename T>
wp<T>::~wp()
{if (m_ptr) m_refs->decWeak(this);
}

wp promote函数

wp没有重载*和->操作符，故不能直接操作它引用的对象，需要通过promote将弱引用升级为强引用

template<typename T>
sp<T> wp<T>::promote() const
{sp<T> result;if (m_ptr && m_refs->attemptIncStrong(&result)) {result.set_pointer(m_ptr);}return result;
}template<typename T>
void sp<T>::set_pointer(T* ptr) {m_ptr = ptr;
}

m_refs是该对象内部的弱引用计数器，通过调用attemptIncStrong试图增加该对象的强引用计数，若成功则转变为sp

bool RefBase::weakref_type::attemptIncStrong(const void* id)
{incWeak(id);weakref_impl* const impl = static_cast<weakref_impl*>(this);int32_t curCount = impl->mStrong.load(std::memory_order_relaxed);ALOG_ASSERT(curCount >= 0,"attemptIncStrong called on %p after underflow", this);while (curCount > 0 && curCount != INITIAL_STRONG_VALUE) {// we're in the easy/common case of promoting a weak-reference// from an existing strong reference.if (impl->mStrong.compare_exchange_weak(curCount, curCount+1,std::memory_order_relaxed)) {break;}// the strong count has changed on us, we need to re-assert our// situation. curCount was updated by compare_exchange_weak.}if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {// we're now in the harder case of either:// - there never was a strong reference on us// - or, all strong references have been releasedint32_t flags = impl->mFlags.load(std::memory_order_relaxed);if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {// this object has a "normal" life-time, i.e.: it gets destroyed// when the last strong reference goes awayif (curCount <= 0) {// the last strong-reference got released, the object cannot// be revived.decWeak(id);return false;}// here, curCount == INITIAL_STRONG_VALUE, which means// there never was a strong-reference, so we can try to// promote this object; we need to do that atomically.while (curCount > 0) {if (impl->mStrong.compare_exchange_weak(curCount, curCount+1,std::memory_order_relaxed)) {break;}// the strong count has changed on us, we need to re-assert our// situation (e.g.: another thread has inc/decStrong'ed us)// curCount has been updated.}if (curCount <= 0) {// promote() failed, some other thread destroyed us in the// meantime (i.e.: strong count reached zero).decWeak(id);return false;}} else {// this object has an "extended" life-time, i.e.: it can be// revived from a weak-reference only.// Ask the object's implementation if it agrees to be revivedif (!impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id)) {// it didn't so give-up.decWeak(id);return false;}// grab a strong-reference, which is always safe due to the// extended life-time.curCount = impl->mStrong.fetch_add(1, std::memory_order_relaxed);// If the strong reference count has already been incremented by// someone else, the implementor of onIncStrongAttempted() is holding// an unneeded reference.  So call onLastStrongRef() here to remove it.// (No, this is not pretty.)  Note that we MUST NOT do this if we// are in fact acquiring the first reference.if (curCount != 0 && curCount != INITIAL_STRONG_VALUE) {impl->mBase->onLastStrongRef(id);}}}impl->addStrongRef(id);654,17        79%break;}// the strong count has changed on us, we need to re-assert our// situation. curCount was updated by compare_exchange_weak.}if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {// we're now in the harder case of either:// - there never was a strong reference on us// - or, all strong references have been releasedint32_t flags = impl->mFlags.load(std::memory_order_relaxed);if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {// this object has a "normal" life-time, i.e.: it gets destroyed// when the last strong reference goes awayif (curCount <= 0) {// the last strong-reference got released, the object cannot// be revived.decWeak(id);return false;}// here, curCount == INITIAL_STRONG_VALUE, which means// there never was a strong-reference, so we can try to// promote this object; we need to do that atomically.while (curCount > 0) {if (impl->mStrong.compare_exchange_weak(curCount, curCount+1,std::memory_order_relaxed)) {break;}// the strong count has changed on us, we need to re-assert our// situation (e.g.: another thread has inc/decStrong'ed us)// curCount has been updated.}if (curCount <= 0) {// promote() failed, some other thread destroyed us in the// meantime (i.e.: strong count reached zero).decWeak(id);return false;}} else {// this object has an "extended" life-time, i.e.: it can be// revived from a weak-reference only.// Ask the object's implementation if it agrees to be revivedif (!impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id)) {// it didn't so give-up.decWeak(id);return false;}// grab a strong-reference, which is always safe due to the// extended life-time.curCount = impl->mStrong.fetch_add(1, std::memory_order_relaxed);// If the strong reference count has already been incremented by// someone else, the implementor of onIncStrongAttempted() is holding// an unneeded reference.  So call onLastStrongRef() here to remove it.// (No, this is not pretty.)  Note that we MUST NOT do this if we// are in fact acquiring the first reference.if (curCount != 0 && curCount != INITIAL_STRONG_VALUE) {impl->mBase->onLastStrongRef(id);}}}impl->addStrongRef(id);#if PRINT_REFSALOGD("attemptIncStrong of %p from %p: cnt=%d\n", this, id, curCount);
#endif// curCount is the value of mStrong before we incremented it.// Now we need to fix-up the count if it was INITIAL_STRONG_VALUE.// This must be done safely, i.e.: handle the case where several threads// were here in attemptIncStrong().// curCount > INITIAL_STRONG_VALUE is OK, and can happen if we're doing// this in the middle of another incStrong.  The subtraction is handled// by the thread that started with INITIAL_STRONG_VALUE.if (curCount == INITIAL_STRONG_VALUE) {impl->mStrong.fetch_sub(INITIAL_STRONG_VALUE,std::memory_order_relaxed);}return true;
}