Non-throwing conversions for Eigen::Matrix4d

[nigels-com]

Boost 1.81.0
Visual C++ 2019 (64-bit)

I'm trying to follow the documentation for using the tag_invoke method of adding support for Eigen dense matrix types.

I've managed to get things working including appropriate error checking, but lacking confidence that I'm doing it the intended or correct way. The ip_address example discussed in the documentation uses: return make_error_code( std::errc::invalid_argument ); which is resulting in a compilation error for me:

XXX\EigenJson.cpp(25,62): error C2440: 'return': cannot convert from 'std::error_code' to 'boost::system::result<Eigen::Matrix<double,4,4,0,4,4>,boost::system::error_code>' [XXXX\xxx.vcxproj]

There isn't much documentation of boost::json::result_for (or relevant links) but result_from_errno does seem to work.

In particular it seems unfortunate to collect the error_code ec for the to_number<double>() and not have a way to set that error as the result.

Is the documentation just a bit stale w.r.t boost 1.81.0? Am I missing something obvious? Tell me I'm doing it wrong. :)

result_for<Eigen::Matrix4d, value>::type
tag_invoke(const try_value_to_tag<Eigen::Matrix4d>&, const value& jv)
{
    if (!jv.is_array())
    {
        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
        return result_from_errno<Eigen::Matrix4d>(EINVAL, &loc);
    }

    constexpr size_t size = 16;
    const auto& a = jv.as_array();

    if (a.size() != size)
    {
        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
        return result_from_errno<Eigen::Matrix4d>(EINVAL, &loc);
    }

    Eigen::Matrix4d v;
    for (size_t i = 0; i < size; ++i)
    {
        error_code ec;
        v.data()[i] = a[i].to_number<double>(ec);
        if (ec.failed())
        {
            BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
            return result_from_errno<Eigen::Matrix4d>(EINVAL, &loc);
        }
    }
    return v;
}

[pdimov]

In particular it seems unfortunate to collect the error_code ec for the to_number<double>() and not have a way to set that error as the result.

if( ec.failed() ) return ec; should work.

[pdimov]

return make_error_code( std::errc::invalid_argument ); also works for me: https://godbolt.org/z/cbTPcdefs

[pdimov]

It doesn't seem to work when I use the actual Eigen matrix, though. https://godbolt.org/z/9n9xY33Pn :-)

I'll open an issue in Boost.System about that. The reason for it not working is that the Eigen matrix has a constructor taking any T, so result considers the conversion ambiguous, but this constructor is explicit, so it shouldn't be in play for implicit conversions.

[nigels-com]

Indeed, Eigen does tend to be complex to deal with.

    for (size_t i = 0; i < size; ++i)
    {
        error_code ec;
        v.data()[i] = a[i].to_number<double>(ec);
        if (ec.failed())
        {
            return ec;
        }
    }

XXX\EigenJson.cpp(45,22): error C2440: 'return': cannot c
onvert from 'boost::json::error_code' to 'boost::system::result<Eigen::Matrix<double,4,4,0,4,4>,boost::system::error_co
de>' [XXXX\spatial_math.vcxproj]

[pdimov]

This is a manifestation of the same issue. It will be fixed in the next release, but in the meantime, you can work around it by explicitly requesting error construction of the result by replacing return ec; with return { boost::system::in_place_error, ec };

This also works for the make_error_code case: https://godbolt.org/z/xqjv849Gs

[nigels-com]

Thanks for the workaround, appreciated.

result_for<Eigen::Matrix4d, value>::type
tag_invoke(const try_value_to_tag<Eigen::Matrix4d>&, const value& jv)
{
    const auto a = jv.if_array();
    if (!a)
    {
        return { boost::system::in_place_error, make_error_code(boost::system::errc::invalid_argument) };
    }

    constexpr size_t size = 16;
    if (a->size() != size)
    {
        return { boost::system::in_place_error, make_error_code(boost::system::errc::invalid_argument) };
    }

    Eigen::Matrix4d v;
    for (size_t i = 0; i < size; ++i)
    {
        error_code ec;
        v.data()[i] = (*a)[i].to_number<double>(ec);
        if (ec.failed())
        {
            return { boost::system::in_place_error, ec };
        }
    }
    return v;
}

[pdimov]

You are welcome.

Note that your previous

        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
        return result_from_errno<Eigen::Matrix4d>(EINVAL, &loc);

was actually better, because it added a source location to the returned error code. If you later print r.error().what(), or invoke r.value() which will throw system_error, the message in this case will contain the source file/line of the BOOST_CURRENT_LOCATION. This is convenient if you want to know what exactly failed, because invalid_argument by itself doesn't say much.

See e.g. https://godbolt.org/z/76a631Pfo.

You're not required to use invalid_argument either; you can return e.g. boost::json::error::not_array in the first check, and boost::json::error::size_mismatch in the second: https://godbolt.org/z/1eh9W8eW3

[pdimov]

(And if you want both custom error codes and source locations, https://godbolt.org/z/vf38enaPd.)

[nigels-com]

Happy to close this issue now, thanks for the advice.

[grisumbras]

Wait, this issue should be closed now, right @nigels-com ?

[nigels-com]

Yes, I think so. Thanks.