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Q39312: Commenting Out Blocks of Code and Comments for Debugging

Article: Q39312
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 29-DEC-1988

The C language does not support the use of nested comments. This might
appear to be a limitation in cases where it is necessary,  perhaps for
debugging purposes, to comment out a block of code which  contains
comments. However, this situation can be overcome by using the
much more powerful preprocessor commands #if and #endif.

The #if preprocessor command evaluates a constant expression to either
true (which has the value 1) or false (which has the value 0) to
perform conditional compilation. If the expression equates to true,
the code segment will be compiled. If it equates to false, it will be
ignored by the compiler. Therefore, if the syntax below is used,
the enclosed block of code will be forever ignored by the compiler
giving a convenient method of effectively commenting out a block of
code.

Since the expression in the #if can be any constant expression, you
can make complicated tests.  You can even use the /Dconst=value
switch on the CL command line to set a preprocessor symbol which will
include or exclude the debugging code when you compile.  This
technique is described more fully on pages 75-77 of the Microsoft C
Optimizing Compiler User's Guide.

The following is a simple example:

#define OUT 0

#if OUT       /* will always equate to "false" */

   /* code and comments that you wish to remove are here */

#endif

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