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Diffstat (limited to 'boost/hana/fwd/index_if.hpp')
| -rw-r--r-- | boost/hana/fwd/index_if.hpp | 60 |
1 files changed, 60 insertions, 0 deletions
diff --git a/boost/hana/fwd/index_if.hpp b/boost/hana/fwd/index_if.hpp new file mode 100644 index 0000000000..59f98f59fa --- /dev/null +++ b/boost/hana/fwd/index_if.hpp @@ -0,0 +1,60 @@ +/*! +@file +Forward declares `boost::hana::index_if`. + +@copyright Louis Dionne 2013-2017 +@copyright Jason Rice 2017 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_INDEX_IF_HPP +#define BOOST_HANA_FWD_INDEX_IF_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Finds the value associated to the first key satisfying a predicate. + //! @ingroup group-Iterable + //! + //! Given an `Iterable` structure `xs` and a predicate `pred`, + //! `index_if(xs, pred)` returns a `hana::optional` containing an `IntegralConstant` + //! of the index of the first element that satisfies the predicate or nothing + //! if no element satisfies the predicate. + //! + //! + //! @param xs + //! The structure to be searched. + //! + //! @param predicate + //! A function called as `predicate(x)`, where `x` is an element of the + //! `Iterable` structure and returning whether `x` is the element being + //! searched for. In the current version of the library, the predicate + //! has to return an `IntegralConstant` holding a value that can be + //! converted to `bool`. + //! + //! + //! Example + //! ------- + //! @include example/index_if.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto index_if = [](auto&& xs, auto&& predicate) { + return tag-dispatched; + }; +#else + template <typename S, typename = void> + struct index_if_impl : index_if_impl<S, when<true>> { }; + + struct index_if_t { + template <typename Xs, typename Pred> + constexpr auto operator()(Xs&& xs, Pred&& pred) const; + }; + + constexpr index_if_t index_if{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_INDEX_IF_HPP + |