Better diagnostic for value_to errors on described structures

[sdebionne]

Given the following use case that comes from the documentation:

#include <iostream>
#include <string>

#include <boost/json/src.hpp>
#include <boost/describe.hpp>
#include <boost/exception/diagnostic_information.hpp>

struct money
{
    std::string currency;
    double value;
};

BOOST_DESCRIBE_STRUCT(money, (), (currency, value))

struct test
{
    double pi;
    bool happy;
    money object;
};

BOOST_DESCRIBE_STRUCT(test, (), (pi, happy, object))

int main()
{
    boost::json::value jv = {
        { "pi", 3.141 },
        { "happy", true },
        {"object", {
            //{ "currency", "USD" },
            { "value", 42.99 }
        }}
        };

    try {
        test res = boost::json::value_to<test>(jv);
    }
    catch (std::exception const& ex) {
        std::cerr << boost::diagnostic_information(ex, true) << std::endl;
    }

    return 0;
}

The missing currency member gives the following exception:

Dynamic exception type: boost::detail::with_throw_location<boost::system::system_error>
std::exception::what: unknown name [boost.json:39 at /nobackup/lid00lima21/debionne/miniconda3/envs/lima2/include/boost/json/detail/value_to.hpp:371 in function 'operator()']

It would be interesting to have an information where the exception occurred in the json value during value_to conversion. Ideally something like errinfo_json_path="object.concurreny".

[kiwixz]

I also miss that feature very much but I don't know if that's possible with only the error code, so I worry this will get wontfix-ed...

FYI I implemented it with exceptions in my boost::pfr tag_invoke (which I use instead of describing structs):

template <typename T, typename Context>
requires IsJsonAggregate<T>::value
T tag_invoke(boost::json::value_to_tag<T>, const boost::json::value& json, const Context& ctx) {
    const boost::json::object& obj = json.as_object();

    T r;

    boost::pfr::for_each_field(r, [&](auto& field, auto i) {
        std::string_view name = boost::pfr::get_name<i, T>();

        auto it = obj.find(name);
        if (it == obj.end())
            throw Exception{"Missing field {}", name};

        try {
            field = boost::json::value_to<std::decay_t<decltype(field)>>(it->value(), ctx);
        }
        catch (const std::exception& ex) {
            throw Exception{"Could not parse {}: {}", name, ex.what()};
        }
    });

    return r;
}

[grisumbras]

A contributor is working on something in this area, and I also have some ideas. But most likely such feature will require customisation, potentially defining a macro.

[sdebionne]

With #1070 and #1075, is there anything that we could with? Like testing?