boost/hana/at.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::at` and `boost::hana::at_c`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_AT_HPP
  11 #define BOOST_HANA_AT_HPP
  12
  13 #include <boost/hana/fwd/at.hpp>
  14
  15 #include <boost/hana/concept/integral_constant.hpp>
  16 #include <boost/hana/concept/iterable.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/integral_constant.hpp>
  20
  21 #include <cstddef>
  22
  23
  24 BOOST_HANA_NAMESPACE_BEGIN
  25     //! @cond
  26     template <typename Xs, typename N>
  27     constexpr decltype(auto) at_t::operator()(Xs&& xs, N const& n) const {
  28         using It = typename hana::tag_of<Xs>::type;
  29         using At = BOOST_HANA_DISPATCH_IF(at_impl<It>,
  30             hana::Iterable<It>::value
  31         );
  32
  33     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  34         static_assert(hana::Iterable<It>::value,
  35         "hana::at(xs, n) requires 'xs' to be an Iterable");
  36
  37         static_assert(hana::IntegralConstant<N>::value,
  38         "hana::at(xs, n) requires 'n' to be an IntegralConstant");
  39     #endif
  40
  41         return At::apply(static_cast<Xs&&>(xs), n);
  42     }
  43     //! @endcond
  44
  45     template <typename It, bool condition>
  46     struct at_impl<It, when<condition>> : default_ {
  47         template <typename ...Args>
  48         static constexpr auto apply(Args&& ...) = delete;
  49     };
  50
  51     template <std::size_t n, typename Xs>
  52     constexpr decltype(auto) at_c(Xs&& xs) {
  53         return hana::at(static_cast<Xs&&>(xs), hana::size_t<n>{});
  54     }
  55 BOOST_HANA_NAMESPACE_END
  56
  57 #endif // !BOOST_HANA_AT_HPP