forked from boostorg/iterator
84 lines
2.0 KiB
C++
84 lines
2.0 KiB
C++
// (C) Copyright David Abrahams 2002.
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// (C) Copyright Jeremy Siek 2002.
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// (C) Copyright Thomas Witt 2002.
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// Distributed under the Boost Software License, Version 1.0. (See
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// 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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#ifndef BOOST_ENABLE_IF_23022003THW_HPP
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#define BOOST_ENABLE_IF_23022003THW_HPP
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#include <boost/config.hpp>
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#include <boost/iterator/detail/config_def.hpp>
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#if defined(BOOST_NO_SFINAE) || defined(BOOST_NO_IS_CONVERTIBLE)
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#include <boost/type_traits/type_identity.hpp>
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#endif
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//
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// Boost iterators uses its own enable_if cause we need
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// special semantics for deficient compilers.
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// 23/02/03 thw
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//
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namespace boost
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{
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namespace iterators
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{
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//
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// Base machinery for all kinds of enable if
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//
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template<bool>
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struct enabled
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{
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template<typename T>
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struct base
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{
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typedef T type;
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};
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};
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//
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// For compilers that don't support "Substitution Failure Is Not An Error"
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// enable_if falls back to always enabled. See comments
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// on operator implementation for consequences.
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//
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template<>
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struct enabled<false>
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{
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template<typename T>
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struct base
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{
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#ifdef BOOST_NO_SFINAE
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typedef T type;
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// This way to do it would give a nice error message containing
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// invalid overload, but has the big disadvantage that
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// there is no reference to user code in the error message.
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//
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// struct invalid_overload;
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// typedef invalid_overload type;
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//
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#endif
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};
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};
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template <class Cond,
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class Return>
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struct enable_if
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# if !defined(BOOST_NO_SFINAE) && !defined(BOOST_NO_IS_CONVERTIBLE)
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: enabled<(Cond::value)>::template base<Return>
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# else
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: boost::type_identity<Return>
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# endif
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{
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};
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} // namespace iterators
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} // namespace boost
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#include <boost/iterator/detail/config_undef.hpp>
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#endif // BOOST_ENABLE_IF_23022003THW_HPP
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