经常会在android的framework代码中发现sp和wp这样的指针,平时看的时候都把他当成一个普通的指针封装过掉了,这几天终于忍不住了,想深入了解一下。
相关的代码:
frameworks/base/include/utils/RefBase.h
frameworks/base/libs/utils/RefBase.cpp
sp和wp都是一个模板类,看一下sp类的定义:
- template <typename T>
- class sp
- {
- public:
- typedef typename RefBase::weakref_type weakref_type;
- inline sp() : m_ptr(0) { }
- sp(T* other);
- sp(const sp& other);
- ~sp();
- ......
- private:
-
- sp(T* p, weakref_type* refs);
-
- T* m_ptr;
- };
可以看到他确实封转了一个原生指针T* m_ptr. 再看一下其中一个构造函数和析构函数:
- template
- sp::sp(T* other)
- : m_ptr(other)
- {
- if (other) other->incStrong(this);
- }
-
- template
- sp::~sp()
- {
- if (m_ptr) m_ptr->decStrong(this);
- }
咋一看好奇怪,因为在构造函数中调用了incStrong(),在析构函数中调用的decStrong(),显然是管理引用计数的函数,但是sp类的中并没有定义这两个函数,这两个函数是在RefBase类中定义的,由此可以得出结论:
要想使用sp或者wp, T必需要继承RefBase类才行。
RefBase的静态关系如下:
其中weakref_type是RefBase的内嵌类,weakref_impl则是weakref_type的子类,RefBase的大部分工作都是交由weakref_impl类来完成,通过RefBase的成员变量weakref_impl* const mRefs。查看其中一个sp的构造函数:
- template
- sp::sp(T* other)
- : m_ptr(other)
- {
- if (other) other->incStrong(this);
- }
建立sp的动态关系如下:
sp
--> RefBase : incStrong()
-->weakref_impl : addStrongRef()
-->android_atomic_inc(&refs->mStrong)
可见当一个普通指针变成一个sp指针后,将会由RefBase类维护该指针的引用计数,当引用为零时则自动释放该指针指向的内存:
- 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 (c == 1) {
- const_cast(this)->onLastStrongRef(id);
- if ((refs->mFlags&OBJECT_LIFETIME_WEAK) != OBJECT_LIFETIME_WEAK) {
- delete this;
- }
- }
- refs->removeWeakRef(id);
- refs->decWeak(id);
- }
wp是怎么一回事?wp其实是弱指针的意思,wp类型不能直接对类型T进行操作,要想对T进行某种操作,必需把wp升级为sp指针,使用promote()来实现升级:
wp weakp= new T();
sp t = weakp.promote();
wp可能会在弱引用计数不为0的情况下被销毁,执行如下代码:
- class WPTest : public RefBase {
- public:
- WPTest(){
- LOGD("WPTest constructor");
- }
- virtual ~WPTest() {
- LOGD("WPTest destructor");
- }
-
- virtual void onFirstRef() {
- LOGD("first weak ptr ref callback");
- }
-
- virtual void onLastStrongRef(const void* id) {
- LOGD("last strong ptr ref callback");
- }
-
- virtual void onLastWeakRef(const void* id) {
- LOGD("last weak ptr ref callback");
- }
- };
-
-
- int main()
- {
- WPTest *T = new WPTest();
- {
- wp weakp(T);
-
- {
- LOGD("promote to strong ptr.../n");
-
- sp strongp = weakp.promote();
-
- LOGD("strong ptr's lifetime is just about to finish .../n");
- }
-
- LOGD("weak ptr's lifetime is just about to finish .../n");
- }
-
- LOGD("weak ptr is out of scope./n");
-
- return 0;
- }
程序打印的结果是:
D/sp-wp-sample( 225): WPTest constructor
D/sp-wp-sample( 225): promote to strong ptr...
D/sp-wp-sample( 225): first weak ptr ref callback
D/sp-wp-sample( 225): strong ptr's lifetime is just about to finish ...
D/sp-wp-sample( 225): last strong ptr ref callback
D/sp-wp-sample( 225): WPTest destructor
D/sp-wp-sample( 225): weak ptr's lifetime is just about to finish ...
D/sp-wp-sample( 225): weak ptr is out of scope.
由此可见虽然wp的生命周期还没有结束,但是因为升级为sp后,sp的强引用计数为0,导致WPTest 被销毁,当强引用为0而弱引用不为0时,WPTest 销毁时,基类RefBase的mRefs指向的weakref_impl类并没有释放,从而保证了弱引用可以继续起作用,这点可以从RefBase的析构函数中看出来:
- RefBase::~RefBase()
- {
-
- if (mRefs->mWeak == 0) {
-
- delete mRefs;
- }
- }
不过也可以改变这一行为,我们修改一下WPTest的构造函数:
- WPTest(){
- LOGD("WPTest constructor");
- extendObjectLifetime(OBJECT_LIFETIME_WEAK);
- }
这时的打印结果是:
D/sp-wp-sample( 217): WPTest constructor
D/sp-wp-sample( 217): promote to strong ptr...
D/sp-wp-sample( 217): first weak ptr ref callbac
D/sp-wp-sample( 217): strong ptr's lifetime is j
D/sp-wp-sample( 217): last strong ptr ref callba
D/sp-wp-sample( 217): weak ptr's lifetime is j
D/sp-wp-sample( 217): last weak ptr ref callback
D/sp-wp-sample( 217): WPTest destructor
D/sp-wp-sample( 217): weak ptr is out of scope.
可以看出现在只有当强引用和弱引用的计数都为0时,WPTest对象才会被销毁。
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