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─────C/C++ Compiler─────────────────────────────────────────────────────────
 
     Syntax:  /Didentifier[=|#[value]]
 
     Use the /D option to define constants or macros for your source
     file. The <identifier> is the name of the constant or macro. No
     space can separate /D and the <identifier>. The <value> can be
     either a string or a number. If the <value> is a string and
     includes spaces, enclose it in quotes. If you leave out both the
     equal sign and the <value>, the identifier is assumed to be
     defined, and its value is set to 1. For example, entering /DSET
     defines a macro named SET with a value of 1. Note that the
     <identifier> argument is case sensitive. For example, the /D
     option above would have no effect on a constant named set that is
     defined in the source file.
 
     Note that with Microsoft C/C++ you can substitute a number sign
     (#) for an equal sign (=) when setting <identifier> to a value.
     From either the DOS command line or from a batch file, you cannot
     set an environment variable, such as CL, to a string that contains
     an equal sign. Environment variables do, however, accept the
     number sign. Now that the CL driver allows the substitution of #
     for = with the /D option, you can use the CL environment variable
     to define preprocessor constants:
 
          SET CL="/DTEST#0"
 
     Use the /D option in combination with either the #if or #ifdef
     directive to compile source files conditionally.
 
     Using the /D option with a keyword, identifier, or a numeric
     constant and appending an equal sign followed by a space replaces
     the keyword, identifier, or a numeric constant with no text in the
     source file. For example, use the following command to remove all
     occurrences of RELEASE from TEST.C:
 
          CL /DRELEASE= TEST.C
 
     Similarly, use the following command to remove all occurrences of
     the keyword __far in TEST.C:
 
          CL /D__far=  TEST.C
 
     Defining macros and constants with the /D option has the same
     effect as using a #define preprocessor directive at the beginning
     of your source file. The identifier is defined until either an
     #undef directive in the source file removes the definition or the
     compiler reaches the end of the file.
 
     If an identifier defined in a /D option is also defined within the
     source file, CL uses the definition on the command line until it
     encounters the redefinition of the identifier in the source file.
 
     Example
 
          #if !defined(RELEASE)<R>
              _nheapchk();<R>
          #endif
 
     This code fragment calls a function to check the near heap unless
     the constant RELEASE is defined. While developing the program, you
     can leave RELEASE undefined and perform heap checking to find bugs.
     Assuming the program name is BIG.C, you would compile with the
     following command:
 
          CL BIG.C
 
     After you have found all of the bugs in the program, you can define
     RELEASE in a /D option so the program runs faster, as follows:
 
          CL /DRELEASE BIG.C
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