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/*!
@file
Forward declares `boost::hana::at_key`.
@copyright Louis Dionne 2013-2017
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_AT_KEY_HPP
#define BOOST_HANA_FWD_AT_KEY_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns the value associated to the given key in a structure, or fail.
//! @ingroup group-Searchable
//!
//! Given a `key` and a `Searchable` structure, `at_key` returns the first
//! value whose key is equal to the given `key`, and fails at compile-time
//! if no such key exists. This requires the `key` to be compile-time
//! `Comparable`, exactly like for `find`. `at_key` satisfies the following:
//! @code
//! at_key(xs, key) == find(xs, key).value()
//! @endcode
//!
//! If the `Searchable` actually stores the elements it contains, `at_key`
//! is required to return a lvalue reference, a lvalue reference to const
//! or a rvalue reference to the found value, where the type of reference
//! must match that of the structure passed to `at_key`. If the `Searchable`
//! does not store the elements it contains (i.e. it generates them on
//! demand), this requirement is dropped.
//!
//!
//! @param xs
//! The structure to be searched.
//!
//! @param key
//! A key to be searched for in the structure. The key has to be
//! `Comparable` with the other keys of the structure. In the current
//! version of the library, the comparison of `key` with any other key
//! of the structure must return a compile-time `Logical`.
//!
//!
//! Example
//! -------
//! @include example/at_key.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto at_key = [](auto&& xs, auto const& key) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename S, typename = void>
struct at_key_impl : at_key_impl<S, when<true>> { };
struct at_key_t {
template <typename Xs, typename Key>
constexpr decltype(auto) operator()(Xs&& xs, Key const& key) const;
};
constexpr at_key_t at_key{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_AT_KEY_HPP
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