On 16/08/2014 04:59 a.m., John Maddock wrote:

I have a couple of pull requests to add support for the final keyword in Boost.Config and an is_final trait to type traits, so my questions are:

1) Are these generally useful? 2) Do we need BOOST_NO_CXX11_FINAL_CLASS and BOOST_NO_CXX11_FINAL_OVERRIDE or will a single BOOST_NO_CXX11_FINAL do? 3) The trait appears to be unimplementable via SFINAE (anyone care to prove me wrong?) which means it's only usable on GCC and lookalikes which have an __is_final intrinsic. Should we still add this to Boost or is the answer "use GCC's native features" given that a Boost version effectively adds nothing?

For completeness, let me state my motivation for those pull request. I'm doing some work on Fusion sequences, and I want to apply the Empty Base Optimization. Since C++11, this optimization has to be restricted to non final classes, so the usual predicate is `std::is_empty<T>::value && !std::is_final<T>::value`. I believe this is the entire motivation behind the addition of the `is_final` trait to the standard. I don't yet know if those Fusion changes will end upstream, but it's likely that other libraries doing EBO will want to restrict it to non-final classes. By adding the corresponding trait, the user still has a chance to specialize it for its own final types if the compiler can't detect that. Note that MSVC will sooner or later have to implement `std::is_final`, so "use GCC's native features" does not sound compelling. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com