Why could not display the string?

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.