Q250847: BUG: ATL C4305 Error when VARIANT Used as a Parameter to Event

Article: Q250847
Product(s): Microsoft C Compiler
Version(s): winnt:6.0
Operating System(s): 
Keyword(s): kbATL kbCOMt kbConnPts kbVC600bug kbATL300bug kbDSupport kbGrpDSMFCATL
Last Modified: 26-JUN-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
- The Microsoft Active Template Library (ATL) 3.0 
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SYMPTOMS
========

The ATL Connection Point wizard generates a proxy class that implements a
connection point for an out going interface. If an out going interface has an
event that takes an argument of type VARIANT, then the wizard generates the
wrong code in the out going call (Fire method) for that event. The Visual C++
compiler gives these two warnings when you call this fire method in your code

  warning C4305: 'argument' : truncation from 'struct tagVARIANT *' to 'bool'

  warning C4800: 'struct tagVARIANT *' : forcing value to bool 'true' or
  'false' (performance warning)

CAUSE
=====

Implementing the Connection Point wizard generates the wrong code in the Fire_
method for the event that takes VARIANT as parameter. For Event1, with VARIANT
parameter Param1, it generates this code

  pVars[0] = &Param1;

instead of this

  pVars[0] = Param1;

in Fire_Event1 method of the proxy class.

RESOLUTION
==========

To work around this problem manually remove the "&" before the Param1.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Create a new ATL DLL/EXE project.

2. Insert a simple object with Connection Point support.

3. Add a method named Event1 to source(event) interface that takes a VARIANT as
  parameter

4. Compile the project.

5. Right-click the ATL class and click Implement Connection Point to bring up
  the Connection Point wizard.

6. Select the event interface and click OK.

7. Create a test method on the ATL object's incoming interface with VARIANT as a
  parameter and call it "Fire_Event1".

  STDMETHODIMP CTestQ::Test(VARIANT var)
  {
  	Fire_Event1(var);
  	return S_OK;
  }

8. Compile your application to see the warnings C4305 and C4800.

REFERENCES
==========

Adding Connection Points to an ATL Object

(c) Microsoft Corporation 2000, All Rights Reserved.
Contributions by Sreedhar Pelluru, Microsoft Corporation


Additional query words: event VARIANT C4305 C4800

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Keywords          : kbATL kbCOMt kbConnPts kbVC600bug kbATL300bug kbDSupport kbGrpDSMFCATL 
Technology        : kbVCsearch kbAudDeveloper kbATLsearch kbATL300 kbVC32bitSearch kbVCPE600 kbVCEE600 kbVCLE600
Version           : winnt:6.0
Issue type        : kbbug
Solution Type     : kbpending

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