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/*!
@file
Forward declares `boost::hana::index_if`.
Copyright Louis Dionne 2013-2022
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>
namespace boost { namespace hana {
//! 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;
};
BOOST_HANA_INLINE_VARIABLE constexpr index_if_t index_if{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_INDEX_IF_HPP
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