ceph/src/boost/libs/hana/include/boost/hana/fwd/front.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::front`.
   4
   5 @copyright Louis Dionne 2013-2016
   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_FRONT_HPP
  11 #define BOOST_HANA_FWD_FRONT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Returns the first element of a non-empty iterable.
  19     //! @ingroup group-Iterable
  20     //!
  21     //! Given a non-empty Iterable `xs` with a linearization of `[x1, ..., xN]`,
  22     //! `front(xs)` is equal to `x1`. If `xs` is empty, it is an error to
  23     //! use this function. Equivalently, `front(xs)` must be equivalent to
  24     //! `at_c<0>(xs)`, and that regardless of the value category of `xs`
  25     //! (`front` must respect the reference semantics of `at`).
  26     //!
  27     //!
  28     //! Example
  29     //! -------
  30     //! @include example/front.cpp
  31 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  32     constexpr auto front = [](auto&& xs) -> decltype(auto) {
  33         return tag-dispatched;
  34     };
  35 #else
  36     template <typename It, typename = void>
  37     struct front_impl : front_impl<It, when<true>> { };
  38
  39     struct front_t {
  40         template <typename Xs>
  41         constexpr decltype(auto) operator()(Xs&& xs) const;
  42     };
  43
  44     constexpr front_t front{};
  45 #endif
  46 BOOST_HANA_NAMESPACE_END
  47
  48 #endif // !BOOST_HANA_FWD_FRONT_HPP