-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Serguei KOLOS Sent: Tuesday, February 21, 2006 12:42 AM To: boost@lists.boost.org Subject: [boost] problem with BOOST_PP_IS_EMPTY macro
Hello
Using the BOOST_PP_IS_EMPTY macro with string literal as parameter causes compiler warning: warning: pasting "BOOST_PP_IS_EMPTY_DEF_" and ""AA"" does not give a valid preprocessing token
That macro is 1) not a library interface (so why are you using it?) and 2) is not a general purpose emptiness detector (there is no such thing). There are very specific constraints on what the input is supposed to be.
#include <boost/preprocessor/if.hpp> #include <boost/preprocessor/tuple.hpp> #include <boost/preprocessor/facilities/is_empty.hpp>
#define MY_BOOST_PP_IS_EMPTY(x) MY_BOOST_PP_IS_EMPTY_I(MY_BOOST_PP_IS_EMPTY_HELPER x)
First, this will fail if 'x' is a parenthesized expression. I.e. if 'x' is () or (a) or (a, b), etc.. It will also fail if 'x' ends with a function-like macro name. Second, passing nothing--as in TEST_CASE_1( ) and TEST_CASE_2( )--is results in undefined behavior in current C++ (which will give you warnings on many preprocessors). Third, even with placemarkers from C99 (well-defined semantics for empty arguments), you still cannot generally test emptiness. Trust me, there is *no* way to do it. The best that you can do is to contrain the input to a subset of non-pathological input. The macros in the library, IS_EMPTY, IS_1, and IS_EMPTY_OR_1 are all designed for a very specific purpose: testing flags such as: #define CONFIG_A #define CONFIG_B 1 #define CONFIG_C 0 IS_EMPTY(CONFIG_A) // 1 IS_EMPTY(CONFIG_B) // 0 IS_EMPTY(CONFIG_C) // 0 IS_1(CONFIG_A) // 0 IS_1(CONFIG_B) // 1 IS_1(CONFIG_C) // 0 IS_EMPTY_OR_1(CONFIG_A) // 1 IS_EMPTY_OR_1(CONFIG_B) // 1 IS_EMPTY_OR_1(CONFIG_C) // 0 In other words, they are for testing simply flags. Valid input is either 0 or 1, or a macro expansion that results in 0 or 1 or nothing. Nothing else is even supposed to work. But, as I said above, these are not library interfaces. You can use them, but make sure that you use them correctly. Regarding emptiness detection in general, it can't be done without well-defined semantics for token-pasting arbitrary tokens (i.e. where the result is not a single valid preprocessing token). With variadics and placemarkers (i.e. C99 facilities) you can get awfully close--the contraint is that you cannot have a function-like macro name as the end of the input--but that is as close as you can get. Regards, Paul Mensonides