Sorry, email went out before intended... Maybe easier to troubleshoot than the partial code I posted earlier, I've modifed Peter's Godbolt sample from [1] to replicate (I think) what I need. Short and long links below. I'm not familiar with Godbolt, hopefully those links are correct. I'd appreciate some insights on this issue. Thanks, --DD
https://godbolt.org/z/3xhjnMn5G https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAM1QDsCBlZAQwBtMQBGAFlJvoCqAZ0wAFAB4gA5AAYppAFZdSrZrVDIApACYAQjt2kR7ZATx1KmWugDCqVgFcAtrRDcZpK%2BgAyeWpgA5ZwAjTGIQAFYANlIAB1QhQnNaO0cXNw94xLM6X38gp1Dw6KNMExzaBgJmYgJU51d3UvLkqpqCPMCQsMiYoWra%2BvSm/vbOgqLegEojVAdiZA4pHQBmP2RHLABqTRWbADcavDUCXexNGQBBVfXNzB29837iTGYnM4vrq5vaDYdt3Y2YKoBIEAD0TlinE4ADoELFYh8rg5EuotrQ3pghLFmIstsDQSAQJDobt9N8rgRMJDVFTARtmEIhDCWVsACqkLYMpksmFba4rc5XfroImHYjHeiAtm8s5bJzMADWmAA%2BuLJQQIFsRUTEgAvVUELZ%2BLDiTnXbRRHRRXlbZhbKafTQAdnJly2Hq2LwI81o8tiKoA7oQECqTZhxID9ZhUFReRA2Y7BVrw2adhF9JbNBEACIQZgOIhbACSjtdTqunqrXswPuIfp1IHVJ2lsuT2oIopAfhVsVUizD1gjgOLZ05jZoxEDNXQgIF2C19qmtqTbs%2BVZdOdXTudOYrlypNOYdL23OZrLZSMuBaLfkIaqOJy1jejKqNqfHnbFj6lexlLI%2BS0tn2R1vnLStPW9X1/SDENB1NKM8ANWN40TM4UyHNNs0zKJ9mzPMb1QEsyzXCDqxrOs/X2GFqT7XFMBHdDtwpd1PU3Zjrl3fcwR43i%2BP4gTBKE4SRNEsSeM%2BHitipfp92eBxTH5Hc3Q9fZUDwdBjVoGgIFA1iXRUz1GzQQsHhsQEdm0bQLW0Ml1x2LiwL3FZSMueTFLXcDWLUjStJ0vSPQM%2ByPWMuYjUBCydD0KK7Igzcd2c1zPj8I0FT8XTlPsl8kMNLTTQeHMtk4WKvlYwjgIK%2BUlVVZtfxsS5OX0ZNUw4rKvybPBsggbD8PzQtUGta0tlQKYHN0YaYT8fyyQcnNORAkr7MIoa2AIGRKrvAgHwlJ8PGA1c7VYNbJu01AMpcpb%2BpWo7OA22h71qzVOHmg7VthKaztaxzSqkGZWGkCJ5FcWR5FQaRzOivRtTmBZ7lWTh5AIaQ5CmGZFRAFZuBhThtGdFZnUtGQZG0GQoiiAAOFRpG4eQnC4InSGBuRSDBqR5CEEAPCRkGZjgWAkDQSE8HYMgKAgQXYmFnpgAATm0PhhapYgOYgYJkfkYI/BqABPaQEdIQWnCsAgAHlaFYXWQdILAFXUdh1etvAXlMPB9ixB2I0wZBCyWZmUrKB3WDwYJiB1uwsD1xGJTpqQEZmfhGBYe2eD4OgCGEMRJCtpRntUdQUAMAwVGDjnYAxI2QAJfpSDd8JY%2BdOOZlQWIKg5qQAFoRV2HMtEh3ROGdLZ25N7R2bKL2KksawhlcZ6vHGbpwmerIkjoGflBXioF8KHpnuMCfWgGOp7AaZR95dug2lqbfJj3o/17vsY/C6Hel5mIQYcWLg/oBoGHdZ8Q5MojtyiNwLYwBkDIC2DLGE2gtgQFwIQEglkVjPS2HYIWIsUGcFGhDTMuhEbq1RqQdGVkYQRBluTFYEQrJWRlpwcmlCqZSBpozf%2B0h2ac1INzFGpA%2BaIBAGFWIhZyCUAllLJenh8BEEkQnJgbAOApwThnCQDtAyh1iJHH%2BUhAZsKtqzE2hZhFGljFsQBwDQHgMgdA2B8CMGSywfDKYhCeZozcCsWBzpuC4wiAPCmXj8bMNYUzUGHCjBcJ4b9bRo89HM1Zi4lGMxa6JAsNwIAA%3D Just in case, here's the code (I modified to add init_variant and the last two lines in main, and this does not compile) inline: #include <variant> #include <iostream> #include <boost/mp11.hpp> using namespace boost::mp11; template<class... T, class... A> std::variant<T...> make_variant( std::size_t index, A&&... a ) { return mp_with_index<sizeof...(T)>( index, [&](auto I){ return std::variant<T...>( std::in_place_index<I>, std::forward<A>( a )... ); }); } template<class... T> auto init_variant( std::size_t index, std::variant<T...>& v) { return mp_with_index<sizeof...(T)>( index, [&v](auto I){ return v.emplace<I>(); }); } //////////////////////////////////////////////////////////////////////////////// // test struct A { void info() { std::cout << "A"; } }; struct B { void info() { std::cout << "B"; } }; int main() { std::size_t index = 1; auto v = make_variant<A, B>(index); std::visit([](auto&& o) { o.info(); }, v); auto& alt0 = init_variant(0, v); alt0.info(); auto& alt1 = init_variant(1, v); alt1.info(); } On Wed, Jun 30, 2021 at 3:27 PM Dominique Devienne <ddevie...@gmail.com> wrote: > Hi. I'm modernizing some code, to use std::variant instead of ad-hoc > peudo-variant structs. > These structs need to be encoded/decoded via avro::codec_traits > specializations, but given > that std::variant is variadic, I'm struggling a bit. I've done my research > and found peter's [1] and also looked at [2], which I thought might work, > but does not, and I don't understand the errors I'm getting with MSVC 2019 > in C++17 mode. > > The code is below, with also a subset of the compiler errors I'm getting. > > > [1] > https://www.reddit.com/r/cpp/comments/f8cbzs/creating_stdvariant_based_on_index_at_runtime/filw8g7?utm_source=share&utm_medium=web2x&context=3 > [2] > https://www.boost.org/doc/libs/develop/libs/mp11/doc/html/mp11.html#mp_with_indexni_f > > template <> struct codec_traits<std::monostate> { > static void encode(Encoder& e, const std::monostate&) { > e.encodeNull(); > } > > static void decode(Decoder& d, std::monostate&) { > d.decodeNull(); > } > }; > > template <typename... Ts> struct codec_traits<std::variant<Ts...>> { > static void encode(Encoder& e, const std::variant<Ts...>& var) { > e.encodeUnionIndex(var.index()); > std::visit( > [&e](const auto& alternative) { > avro::encode(e, alternative); > }, > var > ); > } > > static void decode(Decoder& d, std::variant<Ts...>& var) { > const size_t index = d.decodeUnionIndex(); > if (index >= sizeof...(Ts)) { > throw avro::Exception("Invalid Union index"); > } > auto& alternative = > boost::mp11::mp_with_index<sizeof...(Ts)>( > index, > [&var](auto I) { > return var.emplace<I>(); // I is mp_size_t<index>{} > here > } > ) > ; > avro::decode(d, alternative); > } > }; > > > 1>Messages.cpp > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): > error C2220: the following warning is treated as an error > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): > warning C4239: nonstandard extension used: 'initializing': conversion from > 'std::monostate' to 'std::monostate &' > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): > message : A non-const reference may only be bound to an lvalue > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): > message : while compiling class template member function 'void > avro::codec_traits<T>::decode(avro::Decoder > &,std::variant<std::monostate,int64_t,double> &)' > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] > 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(341): > message : see reference to function template instantiation 'void > avro::codec_traits<T>::decode(avro::Decoder > &,std::variant<std::monostate,int64_t,double> &)' being compiled > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] > 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(333): > message : see reference to class template instantiation > 'avro::codec_traits<T>' being compiled > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\codegen/Messages.i.ipp(1527): > message : see reference to function template instantiation 'void > avro::encode<pdgm::eml::etp12::Datatypes::IndexValue_item_v>(avro::Encoder > &,const T &)' being compiled > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] > > > 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(87,1): > error C2440: 'return': cannot convert from '__int64' to 'std::monostate' > 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(87,1): > message : No constructor could take the source type, or constructor > overload resolution was ambiguous > 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(371): > message : see reference to function template instantiation 'std::monostate > boost::mp11::detail::mp_with_index_impl_<3>::call<0,_Ty>(size_t,F &&)' > being compiled > 1> with > 1> [ > 1> > > _Ty=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f>, > 1> > > F=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f> > 1> ] > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): > message : see reference to function template instantiation 'std::monostate > boost::mp11::mp_with_index<3,avro::codec_traits<T>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f>>(size_t,F > &&)' being compiled > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v, > 1> > > F=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f> > 1> ] > 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): > message : while compiling class template member function 'void > avro::codec_traits<T>::decode(avro::Decoder > &,std::variant<std::monostate,int64_t,double> &)' > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] > 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(341): > message : see reference to function template instantiation 'void > avro::codec_traits<T>::decode(avro::Decoder > &,std::variant<std::monostate,int64_t,double> &)' being compiled > 1> with > 1> [ > 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v > 1> ] >
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