This seems to work even with x = -1.0:
BOOST_PP_SEQ_CAT( (-1.0) (123) (e) (-10) ) // -1.0123e- 10
But I'm not sure how this actually works inside Boost.Preprocessor...
Unfortunately neither of the ideas offered work for me, I tried: #define C_(x, y) BOOST_JOIN(x, y) #define C2_(x, y) BOOST_PP_SEQ_CAT((x)(e)(y)) int main() { float f = C_(5.0, e-001); float g = C2_(5.0, -001); } And the preprocessor produced: int main() { float f = 5.0e -001; float g = 5.0e- 001; } Note the whitespace inserted before or after the "-". The strange thing is, if I invoke: C_(5.0, 0e-001) Then I get: 5.00e-001 And no whitespace anymore! This leads to me to wonder if GCC is doing something special when it recognizes that the token might be a valid number.... or else that this is just blind luck? Either way, I'd really like to find something that's actually guaranteed to work, rather than just happens to :-( Thanks, John.