68 lines
2.4 KiB
C++
68 lines
2.4 KiB
C++
// Copyright 2019 Hans Dembinski
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//
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE_1_0.txt
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// or copy at http://www.boost.org/LICENSE_1_0.txt)
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#ifndef BOOST_HISTOGRAM_DETAIL_VARIANT_PROXY_HPP
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#define BOOST_HISTOGRAM_DETAIL_VARIANT_PROXY_HPP
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#include <boost/core/nvp.hpp>
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#include <boost/histogram/axis/traits.hpp> // variant_access
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#include <boost/histogram/detail/static_if.hpp>
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#include <boost/mp11/algorithm.hpp> // mp_with_index, mp_find, mp_at
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#include <boost/mp11/list.hpp> // mp_size
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#include <boost/throw_exception.hpp>
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#include <stdexcept>
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namespace boost {
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namespace histogram {
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namespace detail {
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// This is a workaround to remain backward compatible in the serialization format. The
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// proxy uses only the public interface of axis::variant for serialization and works
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// independently of the underlying variant implementation.
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template <class Variant>
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struct variant_proxy {
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Variant& variant;
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template <class Archive>
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void serialize(Archive& ar, unsigned /* version */) {
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detail::static_if_c<Archive::is_loading::value>(
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[this](auto& ar) { // loading
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int which = 0;
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ar >> make_nvp("which", which);
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constexpr unsigned N = mp11::mp_size<Variant>::value;
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if (which < 0 || static_cast<unsigned>(which) >= N)
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// throw if which >= N, can happen if type was removed from variant
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BOOST_THROW_EXCEPTION(
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std::runtime_error("variant has fewer types than stored version"));
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mp11::mp_with_index<N>(static_cast<unsigned>(which), [&ar, this](auto i) {
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using T = mp11::mp_at_c<Variant, i>;
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T value;
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ar >> make_nvp("value", value);
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this->variant = std::move(value);
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T* new_address = variant_access::template get_if<T>(&this->variant);
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ar.reset_object_address(new_address, &value);
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});
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},
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[this](auto& ar) { // saving
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visit(
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[&ar](const auto& value) {
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using T = std::decay_t<decltype(value)>;
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const int which = static_cast<int>(mp11::mp_find<Variant, T>::value);
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ar << make_nvp("which", which);
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ar << make_nvp("value", value);
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},
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this->variant);
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},
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ar);
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}
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};
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} // namespace detail
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} // namespace histogram
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} // namespace boost
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#endif
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