2014-06-06 15:17 GMT+04:00 Peter Dimov <lists@pdimov.com>:
Antony Polukhin wrote:
May be it would be better to move boost/mpl/bool.hpp, boost/mpl/bool_fwd.hpp, boost/mpl/integral_c.hpp, mpl/or.hpp, mpl/identity.hpp, mpl/and.hpp into the Core library?
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We might duplicate these as boost/core/something.hpp and boost::core::something, but they will not be quite the same as their mpl counterparts, and will not magically make a module that includes a boost/mpl/ header drop its dependency to MPL.
Yes, I what to do something close to "duplicate them in boost::core::". And no, they won't differ from boost::mpl:: at all. For example if we take a look at the <boost/mpl/identity.hpp> we'll see that it depends on #include <boost/mpl/aux_/lambda_support.hpp> and has macros inside it (BOOST_MPL_AUX_LAMBDA_SUPPORT). This is a MPL only related macro and moving <boost/mpl/identity.hpp> into core with that macro would be a bad idea. But if we investigate further, we'll see that BOOST_MPL_AUX_LAMBDA_SUPPORT macro expands to nothing on modern compilers. For antique-unsupported compilers it does some work, but we do not support those any more. It means that we can move identity.hpp into boost/core/identity.hpp: namespace boost { namespace core { template<typename T = na> struct identity { typedef T type; }; template<typename T = na> struct make_identity { typedef identity<T> type; }; }} And modify <boost/mpl/identity.hpp>: namespace boost { namespace mpl { using boost::core::identity; using boost::make_identity; BOOST_MPL_AUX_NA_SPEC_NO_ETI(1, identity) BOOST_MPL_AUX_NA_SPEC_NO_ETI(1, make_identity) }} Now some libraries could drop dependency to MPL by inclusion of boost/core/identity.hpp As John said, let's not get carried away. Module names, header directories,
namespaces should match. boost/, boost/detail/, boost/pending/ headers that do not obviously belong to a specific module can arguably be considered fair game.
Totally agree. But some parts of the MPL look more like Core parts for Boost (and C++ Standard library), rather than MPL parts. It's strange to see that implicit_cast/algorithm/sync "uses meta-programming library", however they only use identity<>/bool_. I know that the idea is arguable, but there's something wrong with having dependency to MPL in each Boost library. -- Best regards, Antony Polukhin