Generating an error if checked boolean macro is not defined

The accepted answer of using function-macros is good, but if you want to keep normal macros - and still use the value of FOO if defined and generate an error otherwise you could do:

#if FOO / defined(FOO)
#else
#endif

If FOO is not defined it will trigger integer division by zero.

Colleagues (hi Hartmut, Kurt) who maintained a large code base which was extensively configured with #defines ran exactly into the same problem. A simple mis-spelling, possibly in a make file, could result in subtle errors which were hard to track down. Their solution: Use function macros! In

#if SOME_COND()
 // ...
#endif

the compiler complains if SOME_COND() is not defined, as opposed to a simple SOME_COND which will be replaced by 0 if undefined. I like it because it can be used to transport several values without cluttering the code up with additional #ifdefs.