----- Mensaje original ----- De: Marius Lazer <Marius.Lazer@us.icap.com> Fecha: Jueves, Julio 12, 2007 7:22 pm Asunto: Re: [boost] multi-index warnings Para: boost@lists.boost.org
Joaquín,
Here it is (as succinct as I could). All is running on Solaris 10.
Thanks, Marius [...]
Hello Marius, OK, I've been able to reproduce the problem and after analyzing it I think that, unless I'm grossly mistaken, the problem lies inside Boost.Hash --which is not to say this lib has any error, it's just an effect of this overzealous -Wsign-promo switch. Let me explain my findings: I've been able to reproduce the warnings using exclusively Boost.Hash: ***code1*** // warns with Boost 1.34, clean compile with Boost 1.31 #include <boost/functional/hash/hash.hpp> int main(){} ***code1*** This warns with Boost 1.34 and does not warn wih Boost 1.33.1, under GCC 3.4.4 for Cygwin, always using the -Wsign-promo switch. This is in contradiction with some previous reports by you, my guess is that you forgot to include the switch (could you confirm this?) According to GCC docs, -Wsign-promo "warn[s] when overload resolution chooses a promotion from unsigned or enumeral type to a signed type over a conversion to an unsigned type of the same size.[...]" The relevant portion of Boost 1.34 Boost.Hash can be isolated and simplified to more easily show the issue: ***code2*** #include <cstddef> std::size_t hash_value(int){return 0;} std::size_t hash_value(long int){return 0;} template<typename> struct hash; template<> struct hash<bool> { std::size_t operator()(bool x)const{return hash_value(x);} }; int main(){} ***code2*** This emits the same kind of warnings you've seen: -Wsign-promo informs about the int overload having been chosen in preference to the long int overload (you can try yourself). As for why Boost.Hash 1.33.1 does not produce warnings, the reason is that in Boost 1.33.1 hash<T> is defined (roughly) as: template<typename> struct hash { std::size_t operator()(T x)const{return hash_value(t);} }; whereas in Boost 1.34 the class template hash is left undefined and a number of full specializations (like bool and other integral types) are explicitly defined, in the spirit of the code2 snippet. Matter of fact, the following will emit the warnings *both* with Boost 1.33.1 and 1.34: ***code3*** // warns with Boost 1.33.1 and 1.34 #include <boost/functional/hash/hash.hpp> int main() { boost::hash<bool> h; h(true); } ***code3*** Again, I'd be grateful if you can confirm all of this in your own environment. What to do about this? Of course if you can skip the -Wsign-promo switch no warnings will arise. If this is not an option, the author of Boost.Hash should have to be convinced to fix this, possibly by providing more overloads of hash_value (for bool and the rest of offending types) so as to avoid integral promotions altogether. Hope this helps, please come back if your experiments do not agree with this analysis. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo