typename in C++

typename as we know in C++ is used in template.
The difference between typename and class is in the nested type.
Take the class sp in RefBase.h of Android Framework as example:


template <typename T>
class sp
{
public:
    typedef typename RefBase::weakref_type weakref_type;
    .........
}



class RefBase
{
public:
           .........
         class weakref_type
         {
          .........
          }
}
--------------------------
Notice that " typedef typename RefBase::weakref_type weakref_type; "
Compiler will  not know that if weakref_type is a variable of RefBase.  Compiler will know that weakref_type is a type or class of RefBase by giving typename .