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Turns out the preprocessor is actually an organically grown pseudo language (as opposed to a properly designed language feature) inside C, which later got standardized through an incredibly complex set of rules and definitions.

This can be used to explain much of C. :)

An impressive application of the C preprocessor is the Boost.Preprocessor library [1]. It basically implements a primitive (and bounded) version of Lisp inside the preprocessor.

I used it once to write DSLs to define data structures, where a few lines of code were expanded to hundreds of lines of templates, (de)serialization code, etc...

[1] http://www.boostpro.com/mplbook/preprocessor.html

This reminds me of one of the assignments in cppgm.org, where you get to implement all those features. http://www.cppgm.org/pa4.html

i like how one of the assignments is "read the C++ spec" o_O

C preprocessor internals