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51 lines
1.4 KiB
C++
51 lines
1.4 KiB
C++
/*!
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@file
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Forward declares `boost::hana::length`.
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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_LENGTH_HPP
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#define BOOST_HANA_FWD_LENGTH_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 a foldable structure.
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//! @ingroup group-Foldable
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//!
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//! Given a `Foldable` `xs`, `length(xs)` must return an object of an
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//! unsigned integral type, or an `IntegralConstant` holding such an
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//! object, which represents the number of elements in the structure.
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//!
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//! @note
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//! Since only compile-time `Foldable`s are supported in the library
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//! right now, `length` must always return an `IntegralConstant`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/length.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto length = [](auto const& xs) {
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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 length_impl : length_impl<T, when<true>> { };
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struct length_t {
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template <typename Xs>
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constexpr auto operator()(Xs const& xs) const;
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};
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constexpr length_t length{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_LENGTH_HPP
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