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(cpp-295.info)#error Directive

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The `#error' and `#warning' Directives
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   The directive `#error' causes the preprocessor to report a fatal
error.  The rest of the line that follows `#error' is used as the error
message.  The line must consist of complete tokens.

   You would use `#error' inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a Vax, you might write

     #ifdef __vax__
     #error "Won't work on Vaxen.  See comments at get_last_object."
     #endif

Note: Nonstandard Predefined, for why this works.

   If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with `#error'.  For example,

     #if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
         || HASH_TABLE_SIZE % 5 == 0
     #error HASH_TABLE_SIZE should not be divisible by a small prime
     #endif

   The directive `#warning' is like the directive `#error', but causes
the preprocessor to issue a warning and continue preprocessing.  The
rest of the line that follows `#warning' is used as the warning message.

   You might use `#warning' in obsolete header files, with a message
directing the user to the header file which should be used instead.