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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::at_key`.
   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_AT_KEY_HPP
  11 #define BOOST_HANA_FWD_AT_KEY_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 value associated to the given key in a structure, or fail.
  19     //! @ingroup group-Searchable
  20     //!
  21     //! Given a `key` and a `Searchable` structure, `at_key` returns the first
  22     //! value whose key is equal to the given `key`, and fails at compile-time
  23     //! if no such key exists. This requires the `key` to be compile-time
  24     //! `Comparable`, exactly like for `find`. `at_key` satisfies the following:
  25     //! @code
  26     //!     at_key(xs, key) == find(xs, key).value()
  27     //! @endcode
  28     //!
  29     //! If the `Searchable` actually stores the elements it contains, `at_key`
  30     //! is required to return a lvalue reference, a lvalue reference to const
  31     //! or a rvalue reference to the found value, where the type of reference
  32     //! must match that of the structure passed to `at_key`. If the `Searchable`
  33     //! does not store the elements it contains (i.e. it generates them on
  34     //! demand), this requirement is dropped.
  35     //!
  36     //!
  37     //! @param xs
  38     //! The structure to be searched.
  39     //!
  40     //! @param key
  41     //! A key to be searched for in the structure. The key has to be
  42     //! `Comparable` with the other keys of the structure. In the current
  43     //! version of the library, the comparison of `key` with any other key
  44     //! of the structure must return a compile-time `Logical`.
  45     //!
  46     //!
  47     //! Example
  48     //! -------
  49     //! @include example/at_key.cpp
  50 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  51     constexpr auto at_key = [](auto&& xs, auto const& key) -> decltype(auto) {
  52         return tag-dispatched;
  53     };
  54 #else
  55     template <typename S, typename = void>
  56     struct at_key_impl : at_key_impl<S, when<true>> { };
  57
  58     struct at_key_t {
  59         template <typename Xs, typename Key>
  60         constexpr decltype(auto) operator()(Xs&& xs, Key const& key) const;
  61     };
  62
  63     constexpr at_key_t at_key{};
  64 #endif
  65 BOOST_HANA_NAMESPACE_END
  66
  67 #endif // !BOOST_HANA_FWD_AT_KEY_HPP