ceph/src/boost/boost/hana/fwd/is_empty.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::is_empty`.
   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_FWD_IS_EMPTY_HPP
  11 #define BOOST_HANA_FWD_IS_EMPTY_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Returns whether the iterable is empty.
  19     //! @ingroup group-Iterable
  20     //!
  21     //! Given an `Iterable` `xs`, `is_empty` returns whether `xs` contains
  22     //! no more elements. In other words, it returns whether trying to
  23     //! extract the tail of `xs` would be an error. In the current version
  24     //! of the library, `is_empty` must return an `IntegralConstant` holding
  25     //! a value convertible to `bool`. This is because only compile-time
  26     //! `Iterable`s are supported right now.
  27     //!
  28     //!
  29     //! Example
  30     //! -------
  31     //! @include example/is_empty.cpp
  32 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  33     constexpr auto is_empty = [](auto const& xs) {
  34         return tag-dispatched;
  35     };
  36 #else
  37     template <typename It, typename = void>
  38     struct is_empty_impl : is_empty_impl<It, when<true>> { };
  39
  40     struct is_empty_t {
  41         template <typename Xs>
  42         constexpr auto operator()(Xs const& xs) const;
  43     };
  44
  45     constexpr is_empty_t is_empty{};
  46 #endif
  47 BOOST_HANA_NAMESPACE_END
  48
  49 #endif // !BOOST_HANA_FWD_IS_EMPTY_HPP