Brian McNamara wrote:
More generally, I would really like a Boost library which lets me generate custom error messages portably. ... lam2.cpp:94: instantiated from here lambda.hpp:1137: no method `boost::fcpp::lambda_impl:: YouCannotPassTheSameLambdaVarTo_lambda_MoreThanOnce<true>::go' ... What I would really love is if there were a boost library which enabled me to dispatch messages along the lines of
BOOST_CUSTOM_ERROR( b, msg ) BOOST_CUSTOM_WARNING( b, msg )
It probably should be a class which can be used like this: mpl::apply_if< ..., identity<int>, COMPILE_TIME_ERROR<Required_property_not_found> > ; Though I don't have a use case for this yet. If we want insantiation of COMPILE_TIME_ERROR to generate an error message, then error must be inside type declaration, since member functions are not instantiated. So the simplest version can be: template<class ErrorMessage> class COMPILE_TIME_ERROR { typedef typename ErrorMessage::oops type; }; class Life_is_hard {}; Which actually produces *very* nice output: sa.cpp: In instantiation of `COMPILE_TIME_ERROR<Life_is_hard>': /home/ghost/Work/boost/boost/mpl/apply_if.hpp:39: instantiated from `boost::mpl::apply_if<boost::mpl::int_<0>, boost::mpl::identity<int>, COMPILE_TIME_ERROR<Life_is_hard> >' sa.cpp:20: instantiated from here .... Which has "COMPILE_TIME_ERROR" in the very first line (other variants I've tried mention COMPILE_TIME_ERROR on the second/third/... line). But if some compiler generates good message only if error is inside function body we're out of luck. I attach my test if somebody is interested in trying on other compilers, and in case mailman will strip it, it's also at http://zigzag.cs.msu.su:7813/sa.cpp
where in both cases "b" is a compile-time boolean, "msg" is the name of a type, and the behavior is
if the boolean is true force this compiler to generate a short diagnostic with typename "msg" featured prominently
In the long run, there really needs to be language support for meta-programs to generate legible diagnostics, but in the meantime, I would love a stopgap solution along the lines above. I think it would "only" involve the work of finding the best "trick" for each compiler brand/version. Perhaps a little concerted effort would discover one trick which worked well across many compilers.
Possibly. Useing an non-existent method is likely to work.
Is there interest in this?
Sure. In particular, I'd like to see a number of static assertions added to BGL, to make error messages less confusing. - Volodya