Q116172: BUG: Based Ptr. Init Fails at Global Scope in CPP File

Article: Q116172
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,4.1,4.2
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), used with:
   - Microsoft C/C++ for MS-DOS 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 4.1, 4.2 
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SYMPTOMS
========

When attempting to compile an application written in C++ that contains a based
pointer initialized at global scope, the compiler incorrectly returns the error
message

  error C2440 : 'initializing' : cannot convert from 'int __far *' to 'int
  __based(xxxx) *'

where xxxx is the segment on which the pointer is based.

This error does not occur for based pointers that are initialized at function
scope.

RESOLUTION
==========

Sample Code 1
-------------

With the 16-bit compilers listed above, this error can be eliminated by
typecasting the constant that is used to initialize the based pointer to a type
"_based(void) *". The following code demonstrates how to generate the compiler
error as well as the workaround:

  /* Compile options needed:   none
  */ 

  // Base Pointer
  int *BasePtr;

  // This line compiles fine (no initialization)
  int _based(BasePtr) *test1;

  // This line generates an C2440 error
  int _based(BasePtr) *test2=0;

  // This line demonstrates the 16 bit workaround
  int _based(BasePtr) *test3 = (int _based(void) *)0;

  void main(void)
  {
  // Initialization at file scope does not generate C2440

     int _based(BasePtr) *test4 = 0;
  }

Sample Code 2
-------------

In the 32-bit compiler for Windows NT, version 8.0, the workaround given for the
16-bit compilers does not work. The "_based(void) *" cast generates the
following two error messages if you are using the 32-bit compiler:

  error C2493: illegal form of __based error C2440: 'initializing' : cannot
  convert from 'int *' to 'int __based(BasePtr) *'

In this case, the file scope based pointer cannot be declared and initialized in
one step. An appropriate workaround would be to declare the pointer at file
scope and initialize the pointer inside of a function. The following code
demonstrates how to generate the compiler error as well as the workaround:

  /* Compile options needed: none
  */ 
  // Base Pointer
  int *BasePtr;

  // This line compiles fine (no initialization).
  int _based(BasePtr) *test1;

  // This line generates C2440 error.
  int _based(BasePtr) *test2=0;

  // This line generates C2440 and C2493 errors.
  int _based(BasePtr) *test3 = (int _based(void) *)0;

  void main(void)
  {
  // 32-bit workaround: initialize the based pointer at function scope.

     test1 = 0;

  // Initialization at file scope does not generate C2440 or C2493.

     int _based(BasePtr) *test4 = 0;
  }

STATUS
======

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

Additional query words: kbVC400bug 8.00 8.00c 9.00

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,2.0,4.0,4.1,4.2
Issue type        : kbbug

=============================================================================

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