68 lines
2.3 KiB
C++
68 lines
2.3 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::at_key`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_AT_KEY_HPP
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#define BOOST_HANA_FWD_AT_KEY_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Returns the value associated to the given key in a structure, or fail.
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//! @ingroup group-Searchable
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//!
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//! Given a `key` and a `Searchable` structure, `at_key` returns the first
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//! value whose key is equal to the given `key`, and fails at compile-time
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//! if no such key exists. This requires the `key` to be compile-time
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//! `Comparable`, exactly like for `find`. `at_key` satisfies the following:
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//! @code
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//! at_key(xs, key) == find(xs, key).value()
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//! @endcode
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//!
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//! If the `Searchable` actually stores the elements it contains, `at_key`
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//! is required to return a lvalue reference, a lvalue reference to const
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//! or a rvalue reference to the found value, where the type of reference
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//! must match that of the structure passed to `at_key`. If the `Searchable`
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//! does not store the elements it contains (i.e. it generates them on
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//! demand), this requirement is dropped.
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//!
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//!
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//! @param xs
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//! The structure to be searched.
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//!
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//! @param key
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//! A key to be searched for in the structure. The key has to be
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//! `Comparable` with the other keys of the structure. In the current
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//! version of the library, the comparison of `key` with any other key
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//! of the structure must return a compile-time `Logical`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/at_key.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto at_key = [](auto&& xs, auto const& key) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename S, typename = void>
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struct at_key_impl : at_key_impl<S, when<true>> { };
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struct at_key_t {
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template <typename Xs, typename Key>
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constexpr decltype(auto) operator()(Xs&& xs, Key const& key) const;
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};
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constexpr at_key_t at_key{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_AT_KEY_HPP
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