Android 智能指针详解

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Android 智能指针详解

源码基于：Android N

0. 前言

上一篇关于 sp的博文，通过 source code 来解释了sp的功能，sp被称为强指针，但是强引用更贴切点。那么弱引用wp到底有什么功能？与sp有什么区别？这一篇通过 source code 来解释。

1. 源码解析

system/core/include/utils/RefBase.htemplate <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;
};

1.1 通过定义与sp比较

都是模板类
都有7个构造函数和6个赋值重载函数
因为多了成员变量m_refs，所以，比较的运算符进行了重载，sp中没有m_refs，所以直接用宏COMPARE_WEAK
都有指针T* m_ptr
wp多了一个promote函数，为了向sp转换
wp并没有sp中重载运算符 * 和运算符 ->

1.2 构造函数和析构函数

template<typename T>
wp<T>::wp(T* other): m_ptr(other)
{if (other) m_refs = other->createWeak(this);
}template<typename T>
wp<T>::wp(const wp<T>& other): m_ptr(other.m_ptr), m_refs(other.m_refs)
{if (m_ptr) m_refs->incWeak(this);
}template<typename T>
wp<T>::wp(const sp<T>& other): m_ptr(other.m_ptr)
{if (m_ptr) {m_refs = m_ptr->createWeak(this);}
}template<typename T> template<typename U>
wp<T>::wp(U* other): m_ptr(other)
{if (other) m_refs = other->createWeak(this);
}template<typename T> template<typename U>
wp<T>::wp(const wp<U>& other): m_ptr(other.m_ptr)
{if (m_ptr) {m_refs = other.m_refs;m_refs->incWeak(this);}
}template<typename T> template<typename U>
wp<T>::wp(const sp<U>& other): m_ptr(other.m_ptr)
{if (m_ptr) {m_refs = m_ptr->createWeak(this);}
}template<typename T>
wp<T>::~wp()
{if (m_ptr) m_refs->decWeak(this);
}

从构造函数可以看出来几点与sp不同的地方：

并不是像 sp那样通过incStrong 和decStrong方式控制计数，而是通过createWeak 、incWeak、decWeak来控制，具体这几个函数是做什么的？与sp的两种函数有什么区别？后面解释。
并没有像 sp那样有移动构造函数，为什么呢？

1.3 赋值运算符的重载

template<typename T>
wp<T>& wp<T>::operator = (T* other)
{weakref_type* newRefs =other ? other->createWeak(this) : 0;//wp 中指针other通过createWeak创建m_refsif (m_ptr) m_refs->decWeak(this); //原来的m_refs需要decWeakm_ptr = other;m_refs = newRefs;return *this;
}template<typename T>
wp<T>& wp<T>::operator = (const wp<T>& other)
{weakref_type* otherRefs(other.m_refs);T* otherPtr(other.m_ptr);if (otherPtr) otherRefs->incWeak(this);if (m_ptr) m_refs->decWeak(this);m_ptr = otherPtr;m_refs = otherRefs;return *this;
}template<typename T>
wp<T>& wp<T>::operator = (const sp<T>& other)
{weakref_type* newRefs =other != NULL ? other->createWeak(this) : 0;//同形参为T* other，需要createWeakT* otherPtr(other.m_ptr);if (m_ptr) m_refs->decWeak(this);m_ptr = otherPtr;m_refs = newRefs;return *this;
}template<typename T> template<typename U>
wp<T>& wp<T>::operator = (U* other)
{weakref_type* newRefs =other ? other->createWeak(this) : 0;if (m_ptr) m_refs->decWeak(this);m_ptr = other;m_refs = newRefs;return *this;
}template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const wp<U>& other)
{weakref_type* otherRefs(other.m_refs);U* otherPtr(other.m_ptr);if (otherPtr) otherRefs->incWeak(this);if (m_ptr) m_refs->decWeak(this);m_ptr = otherPtr;m_refs = otherRefs;return *this;
}template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const sp<U>& other)
{weakref_type* newRefs =other != NULL ? other->createWeak(this) : 0;U* otherPtr(other.m_ptr);if (m_ptr) m_refs->decWeak(this);m_ptr = otherPtr;m_refs = newRefs;return *this;
}

通过赋值运算符重载看到，一般的指针在使用wp时，需要createWeak，如果已经是wp引用，直接incWeak

1.4 promote()

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;
}

2. 遗留问题

wp 中m_refs 到底是什么意思？
构造或者是赋值的时候，为什么指针用的是createWeak，而m_refs 用的是incWeak 和decWeak，又有什么作用？
wp称为弱指针也可以，确切的看并没有看到指针使用的地方，那wp如何使用？
sp的博文中遗留的问题decStrong 和incStrong的具体实现是什么？为什么控制指针的计数？

这些问题需要看下RefBase 就全然明白。