Android智能指针

智能指针的目标

在使用指针的时候容易出现的问题不外乎下面几个。首先，指针在使用之前都必须初始化，这个还算容易解决，在创建指针变量的时候同步初始化就好了；第二个问题就是经常忘记delete，就我的经验来看，这个还是很容易忘记的，在一个大型程序中要是有那么几个地方忘记执行delete，长久来看系统内存肯定会被消耗完；第三个问题就是就算记得delete，但是也不是说delete就delete的，要是还有别的对象在引用这个对象，然后被delete了，那么在其他地方访问这个对象的时候程序肯定会奔溃的。

有了智能指针后，上面的问题就好办了，首先创建智能指针的时候也同时创建需要管理的对象，然后将需要管理的对象委托给智能指针来管理就好了。在智能指针内部管理着一个引用计数值，当有新的智能指针引用这个对象时，引用计数值加一，当本来引用该对象的智能指针指向其他对象时，那么引用计数值就减一，当引用计数值减为0的时候就智能指针会帮忙将对象delete，也就解决了忘记delete以及在错误时刻进行delete的困境了。这样上面出现的问题就都能很好的解决了。

轻量级指针

template <class T>
class LightRefBase
{
public:inline LightRefBase() : mCount(0) { }inline void incStrong(__attribute__((unused)) const void* id) const {android_atomic_inc(&mCount);}inline void decStrong(__attribute__((unused)) const void* id) const {if (android_atomic_dec(&mCount) == 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 renameRefs(size_t n, const ReferenceRenamer& renamer) { }inline static void renameRefId(T* ref,const void* old_id, const void* new_id) { }private:mutable volatile int32_t mCount;
};

首先看一下轻量级指针，轻量级指针属于模板类，要想使用轻量级指针的功能只要在创建自身类的时候继承这个类就OK了，然后在这个类内部组织管理着一个引用计数值。这个值的功能和前面说的一样，起到控制对象生命周期的作用。incStrong和decStrong起到减少和增加引用计数值的功能。

因为LightRefBase内没有用来保存待委托对象的指针，所以LightRefBase不算是智能指针，大概算是指针的升级版本。以这个类作为基类的类还需要搭配真正的智能指针才能发挥作用。之后会介绍真正的智能指针sp和wp。

强指针

sp类

sp类相对简单，大家看一下代码的实现基本也能了解，下面会配合Refbase进行简单的分析。

template<typename T>
class sp {
public:inline sp() : m_ptr(0) { }sp(T* other);sp(const sp<T>& other);template<typename U> sp(U* other);template<typename U> sp(const sp<U>& other);~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;
};

强指针使用的引用计数类是RefBase，它比LightRefBase复杂得多，所以后者才会被称为轻量级指针。下面看一下RefBase的代码：

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 weakref_type;class weakref_impl;RefBase(const RefBase& o);RefBase&        operator=(const RefBase& o);private:friend class ReferenceMover;static void renameRefs(size_t n, const ReferenceRenamer& renamer);static void renameRefId(weakref_type* ref,const void* old_id, const void* new_id);static void renameRefId(RefBase* ref,const void* old_id, const void* new_id);weakref_impl* const mRefs;
};

RefBase和LightRefBase一样提供了incStrong和decStrong成员函数来操作引用计数器；而RefBase和LightRefbase类最大的区别就是它不像LightRefBase那么简单，只提供一个引用计数器，而是提供了一个强引用计数器和一个弱引用计数器。这两种计数器的功能是由weakref_impl类的变量mRefs提供的。

weakref_impl类

RefBase类的成员变量mRefs的类型为weakref_impl指针，这个类的代码在RefBase.cpp文件内，里面的代码看似很复杂，其实细心了解下里面有一个DEBUG_REFS宏，这个宏里面的代码只有在Debug版本下才会去实现，否则为空，所以基本可以不用看。

class RefBase::weakref_impl : public RefBase::weakref_type
{
public:volatile int32_t    mStrong;volatile int32_t    mWeak;RefBase* const      mBase;volatile int32_t    mFlags;#if !DEBUG_REFSweakref_impl(RefBase* base): mStrong(INITIAL_STRONG_VALUE), mWeak(0), mBase(base), mFlags(0){}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) { }#elseweakref_impl(RefBase* base): mStrong(INITIAL_STRONG_VALUE), mWeak(0), mBase(base), mFlags(0), mStrongRefs(NULL), mWeakRefs(NULL), mTrackEnabled(!!DEBUG_REFS_ENABLED_BY_DEFAULT), mRetain(false){}~weakref_impl(){bool dumpStack = false;if (!mRetain && mStrongRefs != NULL) {dumpStack = true;ALOGE("Strong references remain:");ref_entry* refs = mStrongRefs;while (refs) {char inc = refs->ref >= 0 ? '+' : '-';ALOGD("\t%c ID %p (ref %d):", inc, refs->id, refs->ref);
#if DEBUG_REFS_CALLSTACK_ENABLEDrefs->stack.log(LOG_TAG);
#endifrefs = refs->next;}}if (!mRetain && mWeakRefs != NULL) {dumpStack = true;ALOGE("Weak references remain!");ref_entry* refs = mWeakRefs;while (refs) {char inc = refs->ref >= 0 ? '+' : '-';ALOGD("\t%c ID %p (ref %d):", inc, refs->id, refs->ref);
#if DEBUG_REFS_CALLSTACK_ENABLEDrefs->stack.log(LOG_TAG);
#endifrefs = refs->next;}}if (dumpStack) {ALOGE("above errors at:");CallStack stack(LOG_TAG);}}void addStrongRef(const void* id) {//ALOGD_IF(mTrackEnabled,//        "addStrongRef: RefBase=%p, id=%p", mBase, id);addRef(&mStrongRefs, id, mStrong);}void removeStrongRef(const void* id) {//ALOGD_IF(mTrackEnabled,//        "removeStrongRef: RefBase=%p, id=%p", mBase, id);if (!mRetain) {removeRef(&mStrongRefs, id);} else {addRef(&mStrongRefs, id, -mStrong);}}………..void addWeakRef(const void* id) {addRef(&mWeakRefs, id, mWeak);}void removeWeakRef(const void* id) {if (!mRetain) {removeRef(&mWeakRefs, id);} else {addRef(&mWeakRefs, id, -mWeak);}}……………private:struct ref_entry{ref_entry* next;const void* id;
#if DEBUG_REFS_CALLSTACK_ENABLEDCallStack stack;
#endifint32_t ref;};void addRef(ref_entry** refs, const void* id, int32_t mRef){if (mTrackEnabled) {AutoMutex _l(mMutex);ref_entry* ref = new ref_entry;// Reference count at the time of the snapshot, but before the// update.  Positive value means we increment, negative--we// decrement the reference count.ref->ref = mRef;ref->id = id;
#if DEBUG_REFS_CALLSTACK_ENABLEDref->stack.update(2);
#endifref->next = *refs;*refs = ref;}}void removeRef(ref_entry** refs, const void* id){if (mTrackEnabled) {AutoMutex _l(mMutex);ref_entry* const head = *refs;ref_entry* ref = head;while (ref != NULL) {if (ref->id == id) {*refs = ref->next;delete ref;return;}refs = &ref->next;ref = *refs;}ALOGE("RefBase: removing id %p on RefBase %p""(weakref_type %p) that doesn't exist!",id, mBase, this);ref = head;while (ref) {char inc = ref->ref >= 0 ? '+' : '-';ALOGD("\t%c ID %p (ref %d):", inc, ref->id, ref->ref);ref = ref->next;}CallStack stack(LOG_TAG);}}…..…..mutable Mutex mMutex;ref_entry* mStrongRefs;ref_entry* mWeakRefs;bool mTrackEnabled;// Collect stack traces on addref and removeref, instead of deleting the stack references// on removeref that match the address ones.bool mRetain;#endif
};

Weakref_impl类是weakref_base的子类，这个是接口与实现分离的思想。

RefBase的incStrong函数

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

这里的other就是实际的对象，这个对象可以是继承了LightRefBase的对象也可以是继承了RefBase的对象，因为这里主要分析RefBase对象，所以这里以及下面的内容都假设other对象是继承了RefBase的内容的，下面看RefBase的incStrong函数

void RefBase::incStrong(const void* id) const
{weakref_impl* const refs = mRefs;refs->incWeak(id);refs->addStrongRef(id);const int32_t c = android_atomic_inc(&refs->mStrong);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;}android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);refs->mBase->onFirstRef();
}

其中mRefs实在RefBase的构造函数中创建的

RefBase::RefBase(): mRefs(new weakref_impl(this))
{
}

重新回到incStrong函数中，我们会发现这个函数中其实只是做了三件事情：

增加弱引用计数 refs->incWeak(id);
增加强引用计数 const int32_t c = android_atomic_inc(&refs->mStrong);
如果发现是第一次调用对象的incStrong，那么就会修正mStrong引用计数，然后调用这个对象的onFirstRef函数

android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
refs->mBase->onFirstRef()

在调用weakref_impl的构造函数的时候会将mStrong的值初始化为INITIAL_STRONG_VALUE=1<<28；那么在执行加1操作后，mStrong就等于1<<28+1；返回的值c等于加1前的值，即1<<28；所以第一次调用incStrong后需要对mStrong的值进行修正，加上-INITIAL_STRONG_VALUE正好。

现在回头看增加弱引用计数的代码，通过调用weakref_impl的incWeak来对弱引用计数进行加1操作，而weakref_impl类的incWeak则是直接从父类weakref_base中继承来的。

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 = android_atomic_inc(&impl->mWeak);ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}

上面代码增加弱引用计数主要是执行android_atomic_inc方法来完成的。

RefBase的decStrong函数

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

上面的代码调用RefBase类的decStrong函数

void RefBase::decStrong(const void* id) const
{weakref_impl* const refs = mRefs;refs->removeStrongRef(id);const int32_t c = android_atomic_dec(&refs->mStrong);
#if PRINT_REFSALOGD("decStrong of %p from %p: cnt=%d\n", this, id, c);
#endifALOG_ASSERT(c >= 1, "decStrong() called on %p too many times", refs);if (c == 1) {refs->mBase->onLastStrongRef(id);if ((refs->mFlags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {delete this;}}refs->decWeak(id);
}

上面代码先将强引用计数减1，如果发现返回值为1的话，就代表调用此时强引用计数值已经为0了，那么就调用onLastStrongRef函数，这个函数在RefBase中的实现也是为空的，一般是留给子类来实现。然后判断mask是否为OBJECT_LISTTIME_STRONG，如果是，代表该对象的生命周期受强引用计数值控制，当这个对象的强引用计数值为0时，就将这个对象delete掉。

对弱引用计数值的操作则是调用decWeak.

void RefBase::weakref_type::decWeak(const void* id)
{weakref_impl* const impl = static_cast<weakref_impl*>(this);impl->removeWeakRef(id);const int32_t c = android_atomic_dec(&impl->mWeak);ALOG_ASSERT(c >= 1, "decWeak called on %p too many times", this);if (c != 1) return;// 如果对象受强引用控制if ((impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_STRONG) {// This is the regular lifetime case. The object is destroyed// when the last strong reference goes away. Since weakref_impl// outlive the object, it is not destroyed in the dtor, and// we'll have to do it here.// 如果对象没有被强引用过if (impl->mStrong == INITIAL_STRONG_VALUE) {// Special case: we never had a strong reference, so we need to// destroy the object now.// 删除impl该对象delete impl->mBase;} else {// ALOGV("Freeing refs %p of old RefBase %p\n", this, impl->mBase);// 这种时对象被强引用过了，所以只需要直接删除impl即可delete impl;}} else {// less common case: lifetime is OBJECT_LIFETIME_{WEAK|FOREVER}impl->mBase->onLastWeakRef(id);if ((impl->mFlags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_WEAK) {// this is the OBJECT_LIFETIME_WEAK case. The last weak-reference// is gone, we can destroy the object.delete impl->mBase;}}
}

在这个函数中，当弱引用计数不为1则直接return，如果为1那么减1后就为0了，需要对对象进行delete操作。而弱引用计数为1又分为两种情况：

第一种情况为对象的生命周期只受强引用控制，而当强引用计数为初始值的时候，就要删除impl->mBase就是删除实际的对象。而RefBase被删除的时候就会调用析构函数，而在析构函数中决定是否需要将mRefs删除。

RefBase::~RefBase()
{if (mRefs->mStrong == INITIAL_STRONG_VALUE) {// we never acquired a strong (and/or weak) reference on this object.delete mRefs;} else {// life-time of this object is extended to WEAK or FOREVER, in// which case weakref_impl doesn't out-live the object and we// can free it now.if ((mRefs->mFlags & OBJECT_LIFETIME_MASK) != OBJECT_LIFETIME_STRONG) {// It's possible that the weak count is not 0 if the object// re-acquired a weak reference in its destructorif (mRefs->mWeak == 0) {delete mRefs;}}}// for debugging purposes, clear this.const_cast(mRefs) = NULL;
}

当时如果强引用指数不为初始值的时候，就直接调用delete impl，那是因为在decStrong函数中就已经将实际的对象delete掉了。

第二种情况，当对象的生命周期不受强引用控制时，先调用onLastWeakRef函数，然后如果对象的生命周期时由弱引用控制，就直接删除RefBase对象，当然在RefBase的析构函数中也会删除mRefs对象。

弱指针

wp类

template <typename T>
class wp
{
public:typedef typename RefBase::weakref_type weakref_type;inline wp() : m_ptr(0) { }wp(T* other);wp(const wp<T>& other);wp(const sp<T>& other);template<typename U> wp(U* other);template<typename U> wp(const sp<U>& other);template<typename U> 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;
};

与强指针类相比，他们都有一个成员变量m_ptr指向目标对象，但是弱指针还有一个额外的成员变量m_refs，他的类型时weakref_type指针，下面我们分析弱指针的构造函数时再看看他是如何初始化的。这里我们需要关注的是构造函数和析构函数。

wp类的构造函数

先看看wp类的构造函数

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

这里调用RefBase的createWeak函数，返回值为m_refs。

在RefBase的createWeak函数中，直接调用weakref_impl的incWeak函数，这个函数之前分析过了就是增加弱引用计数的，同时返回mRefs给调用函数。

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

接下分析析构函数，这里直接调用weakref_impl的decWeak函数，前面分析过这个函数了。在这个函数里面弱引用次数减1，然后决定是否需要delete impl等。

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

升级为强指针

分析到这里，弱指针还没有分析完成，这里面还有一个非常重要的特性还没有分析，那就是将弱指针升级为强指针的操作，是直接调用其中的promote函数

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

http://blog4jimmy.com/2018/01/324.html