BSTR as an ole automation parameter (freeing, allocating) !!!


When a BSTR is passed to an ole automation function call.

Case 1 >> HRESULT _stdcall MyFunction([in] BSTR strng );

   After using strng, must i free the BSTR by SysFreeString() ?

Case 2  >> HRESULT _stdcall MyFunction([int, out] BSTR * strng );

   After using strng, i want to change strng value (as an out parameter),
   must i free the BSTR by SysFreeString() to recreate one with
   another value ?
   What are the steps to initialize the strng to another value ?

Case 3 >> HRESULT _stdcall MyFunction([out, retval] BSTR * strng );

   To pass an empty string, what can i do ? When the function
   receives a pointer to a BSTR, can i let the pointer untouched ?
   what does the value pointed by the pointer ?

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marsAuthor Commented:
In the Case 3
  What IS the value pointed by the pointer ?
   The rules are as follows:
Case I) The client is responsible for allocating and calling SysFreeString (not the server)

Case II) The client is responsible for allocating and calling SysFreeString.  The server can call SysFreeString and then reallocate.  BUT, whatever the client gets back (potentially different from what it sent) must call SysFreeString on the return value.

Case III) The server is responsible for allocating and the client is responsible for calling SysFreeString

Incidentally, these rules also apply to memory allocated via CoTaskMemAlloc and CoTaskMemFree.


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   I forgot to answer the last part of your question.  You can definitely allocate 0-length BSTRs as in
BSTR bstr = ::SysAllocString(L"");

You get a "honest-to-goodness" BSTR, it's just that the 4-byte prefix to the pointer has the length of 0.  
Oh, and how you treat the 0-length BSTR follows the same rules as for greater than 0 length BSTR.
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