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::if_`.
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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_IF_HPP
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#define BOOST_HANA_FWD_IF_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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//! Conditionally return one of two values based on a condition.
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//! @ingroup group-Logical
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//!
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//! Specifically, `then` is returned iff `cond` is true-valued, and
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//! `else_` is returned otherwise. Note that some `Logical` models may
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//! allow `then` and `else_` to have different types, while others may
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//! require both values to have the same type.
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//!
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//!
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//! @param cond
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//! The condition determining which of the two values is returned.
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//!
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//! @param then
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//! The value returned when `cond` is true-valued.
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//!
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//! @param else_
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//! The value returned when `cond` is false-valued.
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//!
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//!
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//! Example
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//! -------
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//! @include example/if.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename L, typename = void>
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struct if_impl : if_impl<L, when<true>> { };
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struct if_t {
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template <typename Cond, typename Then, typename Else>
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constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const;
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};
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constexpr if_t if_{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_IF_HPP
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