Re: [MP11] Initialize the Nth alternative of an std::variant based on a runtime index

From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2021-07-01 09:44:01

Solved, by moving the decode() inside the lambda, and adding the `.template
` for GCC (MSVC was happy w/o it...)
A miraculous inspiration I guess. Unusual the ML is silent like this. Must
have been a question asked the wrong way. --DD

    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");
        }
        boost::mp11::mp_with_index<sizeof...(Ts)>(
            index, [&](auto Idx) {
                constexpr size_t alternative_index = Idx;
                avro::decode(d, var.template emplace<alternative_index>());
            }
        );
    }

On Wed, Jun 30, 2021 at 3:27 PM Dominique Devienne <ddevienne_at_[hidden]>
> 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.
>>
>