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3rdparty/boost_1_81_0/boost/hana/eval.hpp 1.77 KB
63e88f80   Hu Chunming   提交三方库
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  /*!
  @file
  Defines `boost::hana::eval`.
  
  Copyright Louis Dionne 2013-2022
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   */
  
  #ifndef BOOST_HANA_EVAL_HPP
  #define BOOST_HANA_EVAL_HPP
  
  #include <boost/hana/fwd/eval.hpp>
  
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/dispatch.hpp>
  #include <boost/hana/detail/wrong.hpp>
  #include <boost/hana/functional/id.hpp>
  
  
  namespace boost { namespace hana {
      //! @cond
      template <typename Expr>
      constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
          return eval_impl<typename hana::tag_of<Expr>::type>::apply(
              static_cast<Expr&&>(expr)
          );
      }
      //! @endcond
  
      template <typename T, bool condition>
      struct eval_impl<T, when<condition>> : default_ {
          template <typename Expr>
          static constexpr auto eval_helper(Expr&& expr, int)
              -> decltype(static_cast<Expr&&>(expr)())
          { return static_cast<Expr&&>(expr)(); }
  
          template <typename Expr>
          static constexpr auto eval_helper(Expr&& expr, long)
              -> decltype(static_cast<Expr&&>(expr)(hana::id))
          { return static_cast<Expr&&>(expr)(hana::id); }
  
          template <typename Expr>
          static constexpr auto eval_helper(Expr&&, ...) {
              static_assert(detail::wrong<Expr>{},
              "hana::eval(expr) requires the expression to be a hana::lazy, "
              "a nullary Callable or a unary Callable that may be "
              "called with hana::id");
          }
  
          template <typename Expr>
          static constexpr decltype(auto) apply(Expr&& expr)
          { return eval_helper(static_cast<Expr&&>(expr), int{}); }
      };
  }} // end namespace boost::hana
  
  #endif // !BOOST_HANA_EVAL_HPP