hana/fwd/find.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::find`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_FIND_HPP
0011 #define BOOST_HANA_FWD_FIND_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Finds the value associated to the given key in a structure.
0019     //! @ingroup group-Searchable
0020     //!
0021     //! Given a `key` and a `Searchable` structure, `find` returns the `just`
0022     //! the first value whose key is equal to the given `key`, or `nothing` if
0023     //! there is no such key. Comparison is done with `equal`. `find` satisfies
0024     //! the following:
0025     //! @code
0026     //!     find(xs, key) == find_if(xs, equal.to(key))
0027     //! @endcode
0028     //!
0029     //!
0030     //! @param xs
0031     //! The structure to be searched.
0032     //!
0033     //! @param key
0034     //! A key to be searched for in the structure. The key has to be
0035     //! `Comparable` with the other keys of the structure. In the current
0036     //! version of the library, the comparison of `key` with any other key
0037     //! of the structure must return a compile-time `Logical`.
0038     //!
0039     //!
0040     //! Example
0041     //! -------
0042     //! @include example/find.cpp
0043 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0044     constexpr auto find = [](auto&& xs, auto const& key) {
0045         return tag-dispatched;
0046     };
0047 #else
0048     template <typename S, typename = void>
0049     struct find_impl : find_impl<S, when<true>> { };
0050
0051     struct find_t {
0052         template <typename Xs, typename Key>
0053         constexpr auto operator()(Xs&& xs, Key const& key) const;
0054     };
0055
0056     BOOST_HANA_INLINE_VARIABLE constexpr find_t find{};
0057 #endif
0058 }} // end namespace boost::hana
0059
0060 #endif // !BOOST_HANA_FWD_FIND_HPP