WebRTC中的引用计数指针scoped_refptr

WebRTC为了避免使用C++11(截止68版本),没有使用std::shared_ptr,造了一个scoped_refptr的轮子来替代.

scoped_refptr(rtc_base/scoped_ref_ptr.h)实现计数指针

template <class T>
class scoped_refptr {
......
};

1. 和常造的shared_ptr的轮子在原理上类似,都是利用C++析构函数,判断引用计数情况.适时释放持有的资源.
2. 不同之处在于scoped_refptr把引用计数留给了资源对象来实现

节选部分实现 : 
...
scoped_refptr(const scoped_refptr<T>& r) : ptr_(r.ptr_) {
    if (ptr_)
      ptr_->AddRef();
  }

~scoped_refptr() {
    if (ptr_)
      ptr_->Release();
  }
...
protected:
  T* ptr_;

持有的对象 在AddRef和Release接口中实现计数

举例此类对象 : RefCountedBase(api/refcountedbase.h)
AddRef增加引用,Release判断引用计数,作出资源释放.

class RefCountedBase {
 public:
  RefCountedBase() = default;

  void AddRef() const { ref_count_.IncRef(); }
  RefCountReleaseStatus Release() const {
    const auto status = ref_count_.DecRef();
    if (status == RefCountReleaseStatus::kDroppedLastRef) {
      delete this;
    }
    return status;
  }

 protected:
  virtual ~RefCountedBase() = default;

 private:
  mutable webrtc::webrtc_impl::RefCounter ref_count_{0};

  RTC_DISALLOW_COPY_AND_ASSIGN(RefCountedBase);
};