On Thu, 05 Dec 2002 15:43:27 -0500, David Abrahams <[EMAIL PROTECTED]> wrote:
>Hmm. Well, if it's non-conforming we probably shouldn't use it. If >it's just a vc bug, we could do something like > > #ifndef BOOST_STRICT_CONFIG > # ifdef BOOST_MSVC > # define BOOST_WORKAROUND(symbol, test) BOOST_DEFINED_##symbol && symbol test > # define BOOST_DEFINED_BOOST_MSVC BOOST_MSVC > # define BOOST_DEFINED_BOOST_DINKUMWARE_STDLIB BOOST_DINKUMWARE_STDLIB > ... > # else > # define BOOST_WORKAROUND(symbol, test) (defined(symbol) && symbol test) > #else > # define BOOST_WORKAROUND(symbol, test) 0 > #endif The C++ standard isn't clear enough about it. It says (emphasys mine): Preprocessing directives of the forms # if constant-expression new-line groupopt # elif constant-expression new-line groupopt check whether the controlling constant expression evaluates to nonzero. 4 Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined >>is generated<< as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined. The issue is what "is generated" means. Strictly speaking, your code doesn't "generate" it; the token is already lexed in phase 3 and macro expansion just replaces the macro name with a sequence of tokens that contains 'defined'. A situation where it is really "generated" would be e.g. when it is the result of ## operator. Well, as I said the standard doesn't make it clear which case it refers to, however the C (not C++) committee recently responded to a similar issue with this: http://wwwold.dkuug.dk/JTC1/SC22/WG14/www/docs/dr_258.htm It seems to me that it doesn't leave much room for doubts for C (note especially the suggested technical corrigendum). And since we don't have more precise information for C++ I think such a construct is best avoided. Just my opinion? Genny. _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
