3.9. Directives Within Macro Arguments
Occasionally it is convenient to use preprocessor directives within
the arguments of a macro. The C and C++ standards declare that
behavior in these cases is undefined.
Versions of CPP prior to 3.2 would reject such constructs with an
error message. This was the only syntactic difference between normal
functions and function-like macros, so it seemed attractive to remove
this limitation, and people would often be surprised that they could
not use macros in this way. Moreover, sometimes people would use
conditional compilation in the argument list to a normal library
function like printf, only to find that after a library upgrade
printf had changed to be a function-like macro, and their code
would no longer compile. So from version 3.2 we changed CPP to
successfully process arbitrary directives within macro arguments in
exactly the same way as it would have processed the directive were the
function-like macro invocation not present.
If, within a macro invocation, that macro is redefined, then the new
definition takes effect in time for argument pre-expansion, but the
original definition is still used for argument replacement. Here is a
#define f(x) x x
#define f 2
which expands to
with the semantics described above.