On Fri, Dec 16, 2011 at 10:23 AM, John Maddock <boost.regex@virgin.net>wrote:
Folks we have a problem: rather late in the day I've discovered that our Math.Constants code doesn't compile on anything except VC++, and the problem is preprocessor token pasting. So I'm hoping we have some experts around here that can help!
The issue is we currently construct the constants from a macro invocation such as:
BOOST_DEFINE_MATH_CONSTANT(**half, 5.**000000000000000000000000000000**000000, 000000000000000000000000000000**000000000000000000000000000000**00000000000000, -01);
But the "-01" part (the exponent) isn't a valid pp-token, so trying to token-paste it fails on GCC (compiler error).
Does anyone know if there's a way to overcome this?
Maybe include the "e" in that 4th argument? E.g., "e-01". If you actually need to recover the "-01" without the e, I'm pretty sure you can do something like #define remove_e( X ) BOOST_PP_CAT( remove_e_, X ) #define remove_e_e #define remove_e_E // prefer "E" instead? Weak explanation for why I believe this will work below... As an alternative, I've considered something like:
BOOST_DEFINE_MATH_CONSTANT(ln_**two, 6.**931471805599453094172321214581**765680e-001, "6.**931471805599453094172321214581**765680755001343602552541206800** 094933936219696947156058633269**96418687542001481021e-001");
But this would still require token pasting of the
6.**931471805599453094172321214581**765680e-001
part. But is this now a valid pp-token if the "-" is internal to the token? It works on the compilers I've tried, but I don't want to be skewered by this again down the line....
Truly hating the preprocessor yours, John.
From my experience, only the characters on which the preprocessor is slabbing glue for the token pasting matters, and anything you can use in a C++ identifier is fine there. I know Lorenzo's done some crazy stuff with token pasting with things that looks like "const bind & identifier", e.g., detecting the presence of the "const" and "bind" keywords, and I presume
the internal "&" doesn't mess things up. I'm not an expert in the technicalities of the preprocessor, however. - Jeff