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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::length`.
   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_LENGTH_HPP
  11 #define BOOST_HANA_FWD_LENGTH_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Return the number of elements in a foldable structure.
  19     //! @ingroup group-Foldable
  20     //!
  21     //! Given a `Foldable` `xs`, `length(xs)` must return an object of an
  22     //! unsigned integral type, or an `IntegralConstant` holding such an
  23     //! object, which represents the number of elements in the structure.
  24     //!
  25     //! @note
  26     //! Since only compile-time `Foldable`s are supported in the library
  27     //! right now, `length` must always return an `IntegralConstant`.
  28     //!
  29     //!
  30     //! Example
  31     //! -------
  32     //! @include example/length.cpp
  33 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  34     constexpr auto length = [](auto const& xs) {
  35         return tag-dispatched;
  36     };
  37 #else
  38     template <typename T, typename = void>
  39     struct length_impl : length_impl<T, when<true>> { };
  40
  41     struct length_t {
  42         template <typename Xs>
  43         constexpr auto operator()(Xs const& xs) const;
  44     };
  45
  46     constexpr length_t length{};
  47 #endif
  48 BOOST_HANA_NAMESPACE_END
  49
  50 #endif // !BOOST_HANA_FWD_LENGTH_HPP