/D   (Preprocessor Definitions)

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This option defines symbols or constants for your source file.

Command Line Project Settings Description
/Dname[= | # [{string | number}] ] Preprocessor Definitions With one exception, defining symbols and constants with this option has the same effect as using a #define preprocessor directive at the beginning of your source file. If you use /D, quotes on the command line will be stripped whereas they are retained in a #define statement.

The constant is defined until either an #undef directive in the source file removes the definition, or the compiler reaches the end of the file.

To find this option in the development environment, click Settings on the Project menu. Then click the C/C++ tab, and click Preprocessor in the Category box.

Note   The behavior of the Preprocessor Definitions text box differs from the behavior of the /D command-line option; you cannot use either an equal sign (=) or a number sign (#) to assign a value to symbols typed in the text box.

Use the constants created by the compiler and this option in combination with either the #if or #ifdef directive to compile source files conditionally.

You can redefine a keyword, identifier, or numeric constant that has been defined in a source file. If a constant defined in a /D option is also defined within the source file, CL uses the definition on the command line until it encounters a redefinition in the source file. 

You can undefine a previous definition. To do so, use the /D option with a keyword, identifier, or numeric constant, and append an equal sign (=) followed by a space.

You cannot set the CL environment variable to a string that contains an equal sign. To use /D with the CL environment variable, specify a number sign instead of an equal sign:

SET CL "/DTEST#0"

Examples

The following command defines the symbol DEBUG in TEST.C:

CL /DDEBUG  TEST.C

The following command removes all occurrences of the keyword __far in TEST.C:

CL /D__far=  TEST.C

See Also   Undefine Symbols (/U, /u)