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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::count`.
Copyright Louis Dionne 2013-2022
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_COUNT_HPP
#define BOOST_HANA_FWD_COUNT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Return the number of elements in the structure that compare equal to
//! a given value.
//! @ingroup group-Foldable
//!
//! Given a Foldable structure `xs` and a value `value`, `count` returns
//! an unsigned integral, or a Constant thereof, representing the number
//! of elements of `xs` that compare equal to `value`. For this method to
//! be well-defined, all the elements of the structure must be Comparable
//! with the given value.
//!
//!
//! @param xs
//! The structure whose elements are counted.
//!
//! @param value
//! A value compared with each element in the structure. Elements
//! that compare equal to this value are counted, others are not.
//!
//!
//! Example
//! -------
//! @include example/count.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto count = [](auto&& xs, auto&& value) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct count_impl : count_impl<T, when<true>> { };
struct count_t {
template <typename Xs, typename Value>
constexpr auto operator()(Xs&& xs, Value&& value) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr count_t count{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_COUNT_HPP
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