58 lines
1.7 KiB
C++
58 lines
1.7 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::count`.
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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_COUNT_HPP
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#define BOOST_HANA_FWD_COUNT_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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//! Return the number of elements in the structure that compare equal to
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//! a given value.
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//! @ingroup group-Foldable
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//!
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//! Given a Foldable structure `xs` and a value `value`, `count` returns
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//! an unsigned integral, or a Constant thereof, representing the number
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//! of elements of `xs` that compare equal to `value`. For this method to
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//! be well-defined, all the elements of the structure must be Comparable
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//! with the given value.
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//!
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//!
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//! @param xs
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//! The structure whose elements are counted.
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//!
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//! @param value
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//! A value compared with each element in the structure. Elements
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//! that compare equal to this value are counted, others are not.
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//!
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//!
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//! Example
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//! -------
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//! @include example/count.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto count = [](auto&& xs, auto&& value) {
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return tag-dispatched;
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};
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#else
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template <typename T, typename = void>
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struct count_impl : count_impl<T, when<true>> { };
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struct count_t {
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template <typename Xs, typename Value>
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constexpr auto operator()(Xs&& xs, Value&& value) const;
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};
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constexpr count_t count{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_COUNT_HPP
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