Is it possible for you to make the macro defined to some special value
rather than being undefined ?
Yes, but I have many cases like this. This time it will look like this (the ifdef DEFINE_FOO is not needed): // my default values #define FOO1 foo1 #define FOO2 foo2 ... #define FOO42 foo42 // optional overriding values # define FOO1_OVERRIDE (BOOST_PP_NIL) ... # define FOO23_OVERRIDE (BOOST_PP_NIL) // The only macro I would like to have to write # define FOO24_OVERRIDE (myFoo24, BOOST_PP_NIL) # define FOO25_OVERRIDE (BOOST_PP_NIL) ... # define FOO42_OVERRIDE (BOOST_PP_NIL) And it turns out to be simpler to write: #define FOO1_OVERRIDE FOO1 ... #define FOO23_OVERRIDE FOO23 #define FOO24_OVERRIDE myFoo24 #define FOO25_OVERRIDE FOO25 ... #define FOO42_OVERRIDE FOO42 Thanks for your help.