hana/fwd/find_if.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::find_if`.
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_IF_HPP
0011 #define BOOST_HANA_FWD_FIND_IF_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 first key satisfying a predicate.
0019     //! @ingroup group-Searchable
0020     //!
0021     //! Given a `Searchable` structure `xs` and a predicate `pred`,
0022     //! `find_if(xs, pred)` returns `just` the first element whose key
0023     //! satisfies the predicate, or `nothing` if there is no such element.
0024     //!
0025     //!
0026     //! @param xs
0027     //! The structure to be searched.
0028     //!
0029     //! @param predicate
0030     //! A function called as `predicate(k)`, where `k` is a key of the
0031     //! structure, and returning whether `k` is the key of the element
0032     //! being searched for. In the current version of the library, the
0033     //! predicate has to return an `IntegralConstant` holding a value
0034     //! that can be converted to `bool`.
0035     //!
0036     //!
0037     //! Example
0038     //! -------
0039     //! @include example/find_if.cpp
0040 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0041     constexpr auto find_if = [](auto&& xs, auto&& predicate) {
0042         return tag-dispatched;
0043     };
0044 #else
0045     template <typename S, typename = void>
0046     struct find_if_impl : find_if_impl<S, when<true>> { };
0047
0048     struct find_if_t {
0049         template <typename Xs, typename Pred>
0050         constexpr auto operator()(Xs&& xs, Pred&& pred) const;
0051     };
0052
0053     BOOST_HANA_INLINE_VARIABLE constexpr find_if_t find_if{};
0054 #endif
0055 }} // end namespace boost::hana
0056
0057 #endif // !BOOST_HANA_FWD_FIND_IF_HPP