50 lines
1.5 KiB
C++
50 lines
1.5 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::is_empty`.
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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_IS_EMPTY_HPP
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#define BOOST_HANA_FWD_IS_EMPTY_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 whether the iterable is empty.
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//! @ingroup group-Iterable
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//!
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//! Given an `Iterable` `xs`, `is_empty` returns whether `xs` contains
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//! no more elements. In other words, it returns whether trying to
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//! extract the tail of `xs` would be an error. In the current version
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//! of the library, `is_empty` must return an `IntegralConstant` holding
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//! a value convertible to `bool`. This is because only compile-time
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//! `Iterable`s are supported right now.
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//!
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//!
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//! Example
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//! -------
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//! @include example/is_empty.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto is_empty = [](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 It, typename = void>
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struct is_empty_impl : is_empty_impl<It, when<true>> { };
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struct is_empty_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 is_empty_t is_empty{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_IS_EMPTY_HPP
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