ceph/src/boost/boost/hana/eval.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::eval`.
   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_EVAL_HPP
  11 #define BOOST_HANA_EVAL_HPP
  12
  13 #include <boost/hana/fwd/eval.hpp>
  14
  15 #include <boost/hana/config.hpp>
  16 #include <boost/hana/core/dispatch.hpp>
  17 #include <boost/hana/detail/wrong.hpp>
  18 #include <boost/hana/functional/id.hpp>
  19
  20
  21 BOOST_HANA_NAMESPACE_BEGIN
  22     //! @cond
  23     template <typename Expr>
  24     constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
  25         return eval_impl<typename hana::tag_of<Expr>::type>::apply(
  26             static_cast<Expr&&>(expr)
  27         );
  28     }
  29     //! @endcond
  30
  31     template <typename T, bool condition>
  32     struct eval_impl<T, when<condition>> : default_ {
  33         template <typename Expr>
  34         static constexpr auto eval_helper(Expr&& expr, int)
  35             -> decltype(static_cast<Expr&&>(expr)())
  36         { return static_cast<Expr&&>(expr)(); }
  37
  38         template <typename Expr>
  39         static constexpr auto eval_helper(Expr&& expr, long)
  40             -> decltype(static_cast<Expr&&>(expr)(hana::id))
  41         { return static_cast<Expr&&>(expr)(hana::id); }
  42
  43         template <typename Expr>
  44         static constexpr auto eval_helper(Expr&&, ...) {
  45             static_assert(detail::wrong<Expr>{},
  46             "hana::eval(expr) requires the expression to be a hana::lazy, "
  47             "a nullary Callable or a unary Callable that may be "
  48             "called with hana::id");
  49         }
  50
  51         template <typename Expr>
  52         static constexpr decltype(auto) apply(Expr&& expr)
  53         { return eval_helper(static_cast<Expr&&>(expr), int{}); }
  54     };
  55 BOOST_HANA_NAMESPACE_END
  56
  57 #endif // !BOOST_HANA_EVAL_HPP