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Q192099: FIX: STATIC_DOWNCAST and DYNAMIC_DOWNCAST

Article: Q192099
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,5.0
Operating System(s): 
Keyword(s): kberrmsg kbdocerr kbLangCPP kbMFC kbDocs kbVC400bug kbVC500bug kbVC600fix kbBug kbGrpDS
Last Modified: 27-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, version 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SYMPTOMS
========

The following provides supplemental information to the documentation on
STATIC_DOWNCAST:

     STATIC_DOWNCAST( class_name, pobject )

Parameters:

  

     class_name
   

  The name of the class you want to cast to.

   
     pobject
   

  The pointer to be cast to a pointer to a class_name object.

Remarks:

  This macro casts pobject to a pointer to a class_name object. pobject must
  either be NULL, or point to an object of a class which is derived directly,
  or indirectly, from class_name. In builds of your application with the _DEBUG
  preprocessor symbol defined, the macro will ASSERT if pobject is not NULL and
  if it points to an object that is not a "kind of" the class_name (see
  definition for the CObject::IsKindOf). In non- _DEBUG builds, the macro
  performs the cast without any type checking.

  class_name must be derived from CObject and use the DECLARE_DYNAMIC and
  IMPLEMENT_DYNAMIC, the DECLARE_DYNCREATE and IMPLEMENT_DYNCREATE, or the
  DECLARE_SERIAL and IMPLEMENT_SERIAL macros explained in the Help topic
  "CObject Class: Deriving a Class from Cobject."

  For example, you might cast a pointer to CYourDocument, called pYourDoc, to a
  pointer to CDocument using the following expression:

        CDocument* pDoc = STATIC_DOWNCAST(CDocument, pYourDoc);

  If pYourDoc does not point to an object derived directly or indirectly from
  CDocument, the macro will ASSERT.

  Similarly, the documentation on DYNAMIC_DOWNCAST should be supplemented by
  saying that class_name must be derived from CObject and use the
  DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC, the DECLARE_DYNCREATE and
  IMPLEMENT_DYNCREATE, or the DECLARE_SERIAL and IMPLEMENT_SERIAL macros.

  If you leave out the pair of DECLARE_... and IMPLEMENT_... macros from your
  code, trying to cast the class pointer using STATIC_DOWNCAST or
  DYNAMIC_DOWNCAST gives compiler errors like the following:

  error C2039: 'classCMyClass' : is not a member of 'CMyClass'

  error C2065: 'classCMyClass' : undeclared identifier

STATUS
======



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

Following is additional information on related subjects:

1. The documentation on CObject::IsKindOf should also include:

  This function works only for classes declared with the
  DECLARE_DYNAMIC, DECLARE_DYNCREATE, or DECLARE_SERIAL macros.

2. The documentation on DECLARE_DYNCREATE should also include:

  The DECLARE_DYNCREATE macro includes all the functionality of
  DECLARE_DYNAMIC.

3. You may also want to see the topics on the static_cast and dynamic_cast
  operators in the Visual C++ documentation. Please note that the
  STATIC_DOWNCAST and DYNAMIC_DOWNCAST macros suggest an action that is
  contrary to the common usage of the term "downcast". The term "downcast" is
  commonly used to signify the movement of an object down a class hierarchy,
  from a given class to a class derived from it. This may be confusing when you
  consider that the STATIC_DOWNCAST and DYNAMIC_DOWNCAST macros perform the
  action of "upcasting". The term "upcast" is commonly used to signify the
  movement of an object up the class hierarchy, from a derived class to a class
  it is derived from.

REFERENCES
==========

  Q151070 FIX: DYNAMIC_DOWNCAST & STATIC_DOWNCAST Causes Stack Overflow

Additional query words: DECLARE_DYNAMIC IMPLEMENT_DYNAMIC DECLARE_DYNCREATE IMPLEMENT_DYNCREATE DECLARE_SERIAL IMPLEMENT_SERIAL IsKindOf

======================================================================
Keywords          : kberrmsg kbdocerr kbLangCPP kbMFC kbDocs kbVC400bug kbVC500bug kbVC600fix kbBug kbGrpDSMFCATL kbISS 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC500 kbVC32bitSearch kbVC500Search
Version           : winnt:4.0,5.0
Issue type        : kbbug
Solution Type     : kbfix

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