Q62097: INFO: Line Continuation Evaluated Before Inline Comment

Article: Q62097
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 27-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C for OS/2 
- 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, 5.0, 6.0 
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SUMMARY
=======

To maintain ANSI compliance, the above C versions evaluate line continuation
characters "\" before inline comment tokens "//". This is a change from C
version 5.1 behavior, which ignores line continuation characters within inline
comments. This order of evaluation may results in the following error in the
sample below:

  error C2065: 'INT2' : undeclared identifier

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

Section 2.1.1.2 of the ANSI Draft proposal (dated December 7, 1988) states the
order in which source code translation is to take place. The second and third
steps are paraphrased below.

  2. Each instance of a new-line character and an immediately preceding
  backslash character is deleted, splicing physical source lines to form
  logical source lines. ...

  3. The source file is decomposed into preprocessing tokens and sequences of
  white-space characters... Each comment is replaced by one space character.

As this indicates, the line concatenation should be performed before any and all
evaluation of preprocessing tokens and comments. The change in interpretation
can cause problems with programs that depend on the C 5.1 interpretation. The
problem usually manifests itself in sections of code being ignored. This can be
seen in the following program, which behaves differently under the different
compilers.

Sample Code
-----------

  #include <stdio.h>

  #define INT1 1          //    Inline comment    \ 
  #define INT2 2    //  This line is ignored  \ 

  void main(void)
  {
     printf("%d %d \n",INT1,INT2);
  }

Additional query words: 8.00 8.00c 9.00

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Keywords          : kbcode kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC16bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbinfo

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