On 1/25/2012 6:42 PM, Jeremiah Willcock wrote:
At least on GCC 4.6 in C++0x (C++11) mode, BOOST_RESULT_OF_USE_DECLTYPE is not set by default, preventing classes such as boost::transform_iterator from working on built-in lambda expressions. Is there a reason that the use of decltype is not on by default for compilers that support it, perhaps with a flag to force emulation mode (like Boost.Move supports)?
It's not the default because when we made it the default, all heck broke loose. Partly it was because of buggy libraries that returned a different type from operator() than their nested result<> template declared. Partly, it was due to a bug in the specification of decltype that was only resolved in closing minutes with the adoption of N3276[*]. And partly it is because of continuing non-compliance of compilers that don't property implement N3276. We now have a boost feature macro for detecting this particular form of non-compliance (BOOST_NO_DECLTYPE_N3276), so we can try conditionally enabling decltype-based result_of again. But we'd still need to fix the broken libraries. And let's not kid ourselves ... if it can break boost libraries, it can break end-user code too. The potential for downstream havoc is huge. That said, I still believe it's the right thing to do. [*] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3276.pdf -- Eric Niebler BoostPro Computing http://www.boostpro.com