57 lines
2.1 KiB
C++
57 lines
2.1 KiB
C++
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/*!
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@file
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Defines `BOOST_HANA_DISPATCH_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_DETAIL_DISPATCH_IF_HPP
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#define BOOST_HANA_DETAIL_DISPATCH_IF_HPP
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#include <boost/hana/config.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN
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struct deleted_implementation {
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template <typename ...T>
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static constexpr auto apply(T&& ...) = delete;
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};
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//! @ingroup group-details
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//! Dispatch to the given implementation method only when a condition is
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//! satisfied.
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//!
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//! If the condition is satisfied, this macro is equivalent to the type
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//! `IMPL`. Otherwise, it is equivalent to a type with a deleted static
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//! function named `apply`. When a tag-dispatching error happens, the
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//! condition should be false and the deleted static function `apply`
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//! will prevent the compiler from generating too much garbage.
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//!
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//! @note
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//! When `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined, the
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//! condition is always ignored and this macro expands to the
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//! implementation only.
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//!
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//! @remark
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//! This must be implemented as a macro, because we don't want the
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//! condition to be evaluated at all when
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//! `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined.
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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#define BOOST_HANA_DISPATCH_IF(IMPL, ...) \
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::std::conditional_t< \
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(__VA_ARGS__), \
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IMPL, \
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::boost::hana::deleted_implementation \
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> \
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/**/
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#else
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#define BOOST_HANA_DISPATCH_IF(IMPL, ...) IMPL
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_DETAIL_DISPATCH_IF_HPP
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