Windows SDK String: What are the rules for BSTR allocation and deallocation?
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    Windows SDK String: What are the rules for BSTR allocation and deallocation?

    Q: What are the rules for BSTR allocation and deallocation ?

    A:
    • Case 1: BSTR passed by value

      ODL/IDL method signature:
      Code:
      [id(1), helpstring("method SomeMethodRule1")] HRESULT SomeMethodRule1([in] BSTR bstrParam);
      Client side rules:
      Client allocates and deallocates BSTRs

      Client side example code:
      Code:
      {
        BSTR bstrParam = SysAllocString(L"Test");
        if(SUCCEEDED(pTestBstr->SomeMethodRule1(bstrParam)))
        {
          //use bstrParam
        }
        //done.. Now free..
        SysFreeString(bstrParam);
      }
      Server side rules:
      None

      Server side example code:
      Code:
      STDMETHODIMP CBSTRTest::SomeMethodRule1(BSTR bstrParam)
      {
        // TODO: Add your implementation code here
        MessageBox(NULL,bstrParam,_T("MethodRule1"),MB_OK);
      
        return S_OK;
      }
    • Case 2: BSTR passed by reference

      ODL/IDL method signature:
      Code:
      [id(2), helpstring("method SomeMethodRule2")] HRESULT SomeMethodRule2([in,out] BSTR* bstrParam);
      Client side rules:
      Client allocates and deallocates BSTRs

      Client side example code:
      Code:
      {
        BSTR bstrParam = SysAllocString(L"Test");
        if(SUCCEEDED(pTestBstr->SomeMethodRule2(&bstrParam)))
        {
          //use bstrParam
          MessageBox(NULL,bstrParam,_T("New value"),MB_OK);
        }
        //done.. Now free..
        SysFreeString(bstrParam);
      }
      Server side rules:
      Server can replace the BSTR with a new one, hence returning the new one to the client

      Server side example code:
      Code:
      STDMETHODIMP CBSTRTest::SomeMethodRule2(BSTR *bstrParam)
      {
        // TODO: Add your implementation code here
        MessageBox(NULL,*bstrParam,_T("MethodRule2"),MB_OK);
      
        //now reallocate if required
        SysReAllocString(bstrParam,L"newValueRule2");
      
        //or modify existing one if required
        return S_OK;
      }
    • Case 3: BSTR returned from server

      ODL/IDL method signature:
      Code:
      [id(3), helpstring("method SomeMethodRule3")] HRESULT SomeMethodRule3([out] BSTR* bstrParam);
      Client side rules:
      Client does not allocate. Client deallocates.

      Client side example code:
      Code:
      {
        BSTR bstrParam;
        if(SUCCEEDED(pTestBstr->SomeMethodRule3(&bstrParam)))
        {
          //use bstrParam
          MessageBox(NULL,bstrParam,_T("New value"),MB_OK);
      
          //Done.. Now free..
          SysFreeString(bstrParam);
        }
      }
      Server side rules:
      Server allocates BSTR.

      Server side example code:
      Code:
      STDMETHODIMP CBSTRTest::SomeMethodRule3(BSTR *bstrParam)
      {
        // TODO: Add your implementation code here
        *bstrParam = SysAllocString(L"newValueRule3");
          
        return S_OK;
      }


    Attached Files Attached Files
    Last edited by Andreas Masur; November 13th, 2005 at 07:25 AM.

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