boost/histogram/detail/variant_proxy.hpp

   1 // Copyright 2019 Hans Dembinski
   2 //
   3 // Distributed under the Boost Software License, Version 1.0.
   4 // (See accompanying file LICENSE_1_0.txt
   5 // or copy at http://www.boost.org/LICENSE_1_0.txt)
   6
   7 #ifndef BOOST_HISTOGRAM_DETAIL_VARIANT_PROXY_HPP
   8 #define BOOST_HISTOGRAM_DETAIL_VARIANT_PROXY_HPP
   9
  10 #include <boost/core/nvp.hpp>
  11 #include <boost/histogram/axis/traits.hpp> // variant_access
  12 #include <boost/histogram/detail/static_if.hpp>
  13 #include <boost/mp11/algorithm.hpp> // mp_with_index, mp_find, mp_at
  14 #include <boost/mp11/list.hpp>      // mp_size
  15 #include <boost/throw_exception.hpp>
  16 #include <stdexcept>
  17
  18 namespace boost {
  19 namespace histogram {
  20 namespace detail {
  21
  22 // This is a workaround to remain backward compatible in the serialization format. The
  23 // proxy uses only the public interface of axis::variant for serialization and works
  24 // independently of the underlying variant implementation.
  25 template <class Variant>
  26 struct variant_proxy {
  27   Variant& variant;
  28
  29   template <class Archive>
  30   void serialize(Archive& ar, unsigned /* version */) {
  31     detail::static_if_c<Archive::is_loading::value>(
  32         [this](auto& ar) { // loading
  33           int which = 0;
  34           ar >> make_nvp("which", which);
  35           constexpr unsigned N = mp11::mp_size<Variant>::value;
  36           if (which < 0 || static_cast<unsigned>(which) >= N)
  37             // throw if which >= N, can happen if type was removed from variant
  38             BOOST_THROW_EXCEPTION(
  39                 std::runtime_error("variant has fewer types than stored version"));
  40           mp11::mp_with_index<N>(static_cast<unsigned>(which), [&ar, this](auto i) {
  41             using T = mp11::mp_at_c<Variant, i>;
  42             T value;
  43             ar >> make_nvp("value", value);
  44             this->variant = std::move(value);
  45             T* new_address = variant_access::template get_if<T>(&this->variant);
  46             ar.reset_object_address(new_address, &value);
  47           });
  48         },
  49         [this](auto& ar) { // saving
  50           visit(
  51               [&ar](const auto& value) {
  52                 using T = std::decay_t<decltype(value)>;
  53                 const int which = static_cast<int>(mp11::mp_find<Variant, T>::value);
  54                 ar << make_nvp("which", which);
  55                 ar << make_nvp("value", value);
  56               },
  57               this->variant);
  58         },
  59         ar);
  60   }
  61 };
  62
  63 } // namespace detail
  64 } // namespace histogram
  65 } // namespace boost
  66
  67 #endif