[webkit-dev] Why Page::setMainFrame use PassRefPtr type parameter?

[pattin.shieh]

Look at this:void Page::setMainFrame(PassRefPtr<Frame> mainFrame).
Why use PassRefPtr? Infact,change to following will work same:
void Page::setMainFrame(Frame *mainFrame)

1).Look into the function source code:
   m_mainFrame = mainFrame;   --> call "operator=(T* optr)",reference count
+1
It mean that if change to "Frame *mainFrame" ,will call mainFrame->ref();

2).Using "PassRefPtr<Frame> mainFrame":
   m_mainFrame = mainFrame; --> call "operator=(const PassRefPtr<T>&
o)",reference count no change

But,reference count was changed when the function is called:
     page->setMainFrame(this);  --> call "PassRefPtr(T* ptr) : m_ptr(ptr) {
if (ptr) ptr->ref(); }", reference count +1

So,The "PassRefPtr<Frame> mainFrame"  and the "Frame *mainFrame" cause same
reference count.
But use "PassRefPtr<Frame>" will call new operation!
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