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

   1 /*!
   2 @file
   3 Forward declares `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_FWD_AT_HPP
  11 #define BOOST_HANA_FWD_AT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16 #include <cstddef>
  17
  18
  19 BOOST_HANA_NAMESPACE_BEGIN
  20     //! Returns the `n`th element of an iterable.
  21     //! @ingroup group-Iterable
  22     //!
  23     //! Given an `Iterable` and an `IntegralConstant` index, `at` returns the
  24     //! element located at the index in the linearization of the iterable.
  25     //! Specifically, given an iterable `xs` with a linearization of
  26     //! `[x1, ..., xN]`, `at(xs, k)` is equivalent to `xk`.
  27     //!
  28     //! If the `Iterable` actually stores the elements it contains, `at` is
  29     //! required to return a lvalue reference, a lvalue reference to const
  30     //! or a rvalue reference to the matching element, where the type of
  31     //! reference must match that of the iterable passed to `at`. If the
  32     //! `Iterable` does not store the elements it contains (i.e. it generates
  33     //! them on demand), this requirement is dropped.
  34     //!
  35     //!
  36     //! @param xs
  37     //! The iterable in which an element is retrieved. The iterable must
  38     //! contain at least `n + 1` elements.
  39     //!
  40     //! @param n
  41     //! A non-negative `IntegralConstant` representing the 0-based index of
  42     //! the element to return. It is an error to call `at` with an index that
  43     //! out of bounds of the iterable.
  44     //!
  45     //!
  46     //! Example
  47     //! -------
  48     //! @include example/at.cpp
  49 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  50     constexpr auto at = [](auto&& xs, auto const& n) -> decltype(auto) {
  51         return tag-dispatched;
  52     };
  53 #else
  54     template <typename It, typename = void>
  55     struct at_impl : at_impl<It, when<true>> { };
  56
  57     struct at_t {
  58         template <typename Xs, typename N>
  59         constexpr decltype(auto) operator()(Xs&& xs, N const& n) const;
  60     };
  61
  62     constexpr at_t at{};
  63 #endif
  64
  65     //! Equivalent to `at`; provided for convenience.
  66     //! @ingroup group-Iterable
  67     //!
  68     //!
  69     //! @note
  70     //! `hana::at_c<n>` is an overloaded function, not a function object.
  71     //! Hence, it can't be passed to higher-order algorithms. This is done
  72     //! for compile-time performance reasons.
  73     //!
  74     //!
  75     //! Example
  76     //! -------
  77     //! @include example/at_c.cpp
  78 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  79     template <std::size_t n>
  80     constexpr auto at_c = [](auto&& xs) {
  81         return hana::at(forwarded(xs), hana::size_c<n>);
  82     };
  83 #else
  84     template <std::size_t n, typename Xs>
  85     constexpr decltype(auto) at_c(Xs&& xs);
  86 #endif
  87 BOOST_HANA_NAMESPACE_END
  88
  89 #endif // !BOOST_HANA_FWD_AT_HPP