Why could not display the string?

From:
"Lee Tow" <fbjlt@pub3.fz.fj.cn>
Newsgroups:
microsoft.public.vc.atl
Date:
Thu, 21 Jun 2007 15:32:59 +0800
Message-ID:
<OlEeWa9sHHA.508@TK2MSFTNGP02.phx.gbl>
Hello all:
   First I create a component using atl,look:
STDMETHODIMP CTpsz::PrtStr(wchar_t **str)
{
 // TODO: Add your implementation code here
 ULONG cb=60;
 wchar_t *tmp;
 tmp=(wchar_t*)CoTaskMemAlloc(cb);
 tmp=L"Hello World!";
 str=&tmp;
 return S_OK;
}
and the CLSID is CLSID_Tpsz,interface ID is IID_ITpsz;
then I want to get the string and display on client,look:
void main()
{
    HRESULT hr;
    hr=CoInitialize(NULL);
    if(FAILED(hr))
    {
      MessageBox(NULL,L"Initial COM Fail",L"Caption",MB_OK);
      return;
    }

    ITpsz *pITpsz=NULL;

hr=CoCreateInstance(CLSID_Tpsz,NULL,CLSCTX_INPROC_SERVER,IID_ITpsz,(void**)&
pITpsz);
    if(FAILED(hr))
    {
      MessageBox(NULL,L"Create object fail",L"Caption",MB_OK);
      return;
    }

    wchar_t *str=NULL;
    hr=pITpsz->PrtStr(&str);
    if(FAILED(hr))
    {
       MessageBox(NULL,L"Get Inteface fail",L"Caption",MB_OK);
    }
    MessageBox(NULL,str,L"Caption",MB_OK);
    CoTaskMemFree(str);
    pITpsz->Release();
    CoUninitialize();
}

but I display nothing,why?how to solve?Thanks very much.

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