scoped_refptr

/*
一、本文中包括以下几个类:
	RefCountedBase					
		-->为子类增加引用计数,基类的基类,无线程安全
	RefCountedThreadSafeBase		
		-->为子类增加引用计数,基类的基类,有线程安全	
	RefCounted:public RefCountedBase
		-->为子类增加引用计数,基类，线程安全
	RefCountedThreadSafe:public RefCountedThreadSafeBase 
		-->为子类增加引用计数,基类，线程安全
	DefaultRefCountedThreadSafeTraits
		-->为RefCountedThreadSafe提供析构支持
	RefCountedData : public base::RefCounted< base::RefCountedData<T> >
		-->为其它数据增加引用计数
	scoped_refptr
		-->自身有作用域的、其对象带引用计数的 智能指针。出作用域就析构,
		释放自己对对象的引用。
*/



#ifndef BASE_MEMORY_REF_COUNTED_H_
#define BASE_MEMORY_REF_COUNTED_H_

#include <cassert>

#include "base/atomic_ref_count.h"
#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/threading/thread_collision_warner.h"

namespace base {

namespace subtle {

class BASE_EXPORT RefCountedBase {
 public://仅有一个引用时返回true
  bool HasOneRef() const { return ref_count_ == 1; }
 protected:
  RefCountedBase();
  ~RefCountedBase();

  void AddRef() const;//增加引用计数
  // Returns true if the object should self-delete.
  bool Release() const;//减少引用计数
 private:
  mutable int ref_count_;
#ifndef NDEBUG
  mutable bool in_dtor_;
#endif

  DFAKE_MUTEX(add_release_);

  DISALLOW_COPY_AND_ASSIGN(RefCountedBase);
};

class BASE_EXPORT RefCountedThreadSafeBase {
 public:
  bool HasOneRef() const;//仅有一个引用时返回true

 protected:
  RefCountedThreadSafeBase();
  ~RefCountedThreadSafeBase();

  void AddRef() const;//增加引用计数,原子地+=1
  // Returns true if the object should self-delete.
  bool Release() const;//增加引用计数,原子地-=1

 private:
  mutable AtomicRefCount ref_count_;
#ifndef NDEBUG
  mutable bool in_dtor_;
#endif

  DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafeBase);
};

}  // namespace subtle
/*
	对父类接口进行包装，在release时释放自己的内存
*/
template <class T>
class RefCounted : public subtle::RefCountedBase {
 public:
  RefCounted() {}

  void AddRef() const {//直接调用父类的
    subtle::RefCountedBase::AddRef();
  }

  void Release() const {
    if (subtle::RefCountedBase::Release()) {
		//释放内存,引用为0时父类Release返回True
		//会释放内存
      delete static_cast<const T*>(this);
    }
  }

 protected:
  ~RefCounted() {}

 private:
  DISALLOW_COPY_AND_ASSIGN(RefCounted<T>);
};

// Forward declaration.
template <class T, typename Traits> class RefCountedThreadSafe;

/*
	在多线程情况下，用户可通过Traits来确定对象release时的操作,而
	下面这个结构是默认的操作，它将调用RefCountedThreadSafe的
	DeleteInternal来删除RefCountedThreadSafe子类对象，这个有点绕。
 */
template<typename T>
struct DefaultRefCountedThreadSafeTraits {
  static void Destruct(const T* x) { 
    RefCountedThreadSafe<T,
                         DefaultRefCountedThreadSafeTraits>::DeleteInternal(x);
  }
};
 
template <class T, typename Traits = DefaultRefCountedThreadSafeTraits<T> >
class RefCountedThreadSafe : public subtle::RefCountedThreadSafeBase {
 public:
  RefCountedThreadSafe() {}

  void AddRef() const {
	//直接调用父类的
    subtle::RefCountedThreadSafeBase::AddRef();
  }

  void Release() const {
	  //先调用父类的,当引用为0时父类Release函数会返回True
	  //这样的话，会调用释放内存的函数,同时用户可以通过
	  //Traits来参与进来，决定内存怎么释放。
    if (subtle::RefCountedThreadSafeBase::Release()) {
      Traits::Destruct(static_cast<const T*>(this));
    }
  }

 protected:
  ~RefCountedThreadSafe() {}

 private:
  friend struct DefaultRefCountedThreadSafeTraits<T>;
  static void DeleteInternal(const T* x) { delete x; }

  DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafe);
};

//
// A wrapper for some piece of data so we can place other things in
// scoped_refptrs<>.
// 给一些其它数据增加计数引用
template<typename T>
class RefCountedData : public base::RefCounted< base::RefCountedData<T> > {
 public:
  RefCountedData() : data() {}
  RefCountedData(const T& in_value) : data(in_value) {}

  T data;

 private:
  friend class base::RefCounted<base::RefCountedData<T> >;
  ~RefCountedData() {}
};

}  // namespace base
/*
	scoped_refptr智能指针	
 */
template <class T>
class scoped_refptr {
 public:
  typedef T element_type;
  scoped_refptr() : ptr_(NULL) {//构造
  }
  scoped_refptr(T* p) : ptr_(p) {//构造
    if (ptr_)
      ptr_->AddRef();//子类必须是能引用计数的
  }
  //拷贝构造
  scoped_refptr(const scoped_refptr<T>& r) : ptr_(r.ptr_) {	
    if (ptr_)
      ptr_->AddRef();
  }
  //拷贝构造
  template <typename U>
  scoped_refptr(const scoped_refptr<U>& r) : ptr_(r.get()) {
    if (ptr_)
      ptr_->AddRef();
  }
  //析构,释放本指针的引用,导致对象引用-=1
  ~scoped_refptr() {
    if (ptr_)
      ptr_->Release();
  }
  //返回对象
  T* get() const { return ptr_; }
  operator T*() const { return ptr_; }
  T* operator->() const {
    assert(ptr_ != NULL);
    return ptr_;
  }

  // Release a pointer.
  // The return value is the current pointer held by this object.
  // If this object holds a NULL pointer, the return value is NULL.
  // After this operation, this object will hold a NULL pointer,
  // and will not own the object any more.
  // 本智能指针对象不再拥有object对象指针，将它传给其它智能指针对象
  T* release() WARN_UNUSED_RESULT {
    T* retVal = ptr_;
    ptr_ = NULL;
    return retVal;
  }
  //赋值,接受对象的指针,放弃本智能指针原来拥有的对象,致使原对象引用-=1
  //重新拥有新的对象
  scoped_refptr<T>& operator=(T* p) {
    // AddRef first so that self assignment should work
    if (p)
      p->AddRef();
    T* old_ptr = ptr_;
    ptr_ = p;
    if (old_ptr)
      old_ptr->Release();
    return *this;
  }
  //赋值,复制,将r指针所指的内容赋值给本智能指针
  scoped_refptr<T>& operator=(const scoped_refptr<T>& r) {
    return *this = r.ptr_;
  }
  //赋值,复制,将r指针所指的内容赋值给本智能指针
  template <typename U>
  scoped_refptr<T>& operator=(const scoped_refptr<U>& r) {
    return *this = r.get();
  }
  //swap内部函数
  void swap(T** pp) {
    T* p = ptr_;
    ptr_ = *pp;
    *pp = p;
  }
  //交换两个智能指针内容
  void swap(scoped_refptr<T>& r) {
    swap(&r.ptr_);
  }
  //那个对象,带引用计数的
 protected:
  T* ptr_;
};

// Handy utility for creating a scoped_refptr<T> out of a T* explicitly without
// having to retype all the template arguments
// 返回对象指针的智能指针对象
template <typename T>
scoped_refptr<T> make_scoped_refptr(T* t) {
  return scoped_refptr<T>(t);
}

#endif  // BASE_MEMORY_REF_COUNTED_H_