90 lines
2.3 KiB
C++
90 lines
2.3 KiB
C++
/* Copyright 2017 Joaquin M Lopez Munoz.
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* Distributed under the Boost Software License, Version 1.0.
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* (See accompanying file LICENSE_1_0.txt or copy at
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* http://www.boost.org/LICENSE_1_0.txt)
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*
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* See http://www.boost.org/libs/poly_collection for library home page.
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*/
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#ifndef BOOST_POLY_COLLECTION_DETAIL_IS_EQUALITY_COMPARABLE_HPP
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#define BOOST_POLY_COLLECTION_DETAIL_IS_EQUALITY_COMPARABLE_HPP
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#if defined(_MSC_VER)
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#pragma once
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#endif
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#include <boost/config.hpp>
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#include <type_traits>
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#if !defined(BOOST_NO_SFINAE_EXPR)
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#include <utility>
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#else
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#include <boost/poly_collection/detail/is_likely_stateless_lambda.hpp>
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#include <boost/type_traits/has_equal_to.hpp>
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#endif
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namespace boost{
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namespace poly_collection{
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namespace detail{
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#if !defined(BOOST_NO_SFINAE_EXPR)
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/* trivial, expression SFINAE-based implementation */
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template<typename T,typename=void>
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struct is_equality_comparable:std::false_type{};
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template<typename T>
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struct is_equality_comparable<
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T,
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typename std::enable_if<
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std::is_convertible<
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decltype(std::declval<T>()==std::declval<T>()),bool
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>::value
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>::type
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>:std::true_type{};
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#else
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/* boost::has_equal_to does a decent job without using expression SFINAE,
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* but it produces a compile error when the type T being checked is
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* convertible to an equality-comparable type Q. Exotic as it may seem,
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* this is exactly the situation with the very important case of stateless
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* lambda expressions, which are convertible to an equality-comparable
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* function pointer with the same signature. We take explicit care of
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* stateless lambdas then.
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*/
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template<typename T,typename=void>
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struct is_equality_comparable:std::integral_constant<
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bool,
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has_equal_to<T,T,bool>::value
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>{};
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template<typename T>
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struct is_equality_comparable<
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T,
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typename std::enable_if<is_likely_stateless_lambda<T>::value>::type
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>:
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#if !defined(BOOST_MSVC)
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std::true_type{};
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#else
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/* To complicate things further, in VS stateless lambdas are convertible not
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* only to regular function pointers, but also to other call conventions
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* such as __stdcall, __fastcall, etc., which makes equality comparison
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* ambiguous.
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*/
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std::false_type{};
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#endif
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#endif
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} /* namespace poly_collection::detail */
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} /* namespace poly_collection */
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} /* namespace boost */
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#endif
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