Better control over error_info introspection and diagnostic information

[sdebionne]

We use Boost.Exception in our project but we lack a way to exercise better control over the diagnostic formatting. This has already been discussed before in a related issue #55.

In our client/server project, we need to 'serialize' the exceptions (e.g. to JSON) while retaining as much information as possible for the client to display. The client being responsible to format and present the error message.

I managed to hack the library's diagnostic_information to return a boost::json::value (instead of a std::string) but I would prefer a solution that is part of the library.

I know this library is kind of superseded by LEAF, but we are not ready to switch (mostly because of the awkward try catch syntax).

What do you think about introducing a customization point (visitor pattern?) to introspect an exception and its error_infos to get diagnostic information in a custom data structure?

[zajo]

I'd rather have code to serialize to json. How does your json serialization work?

[sdebionne]

I use Boost.JSON. Diagnostic information are collected in a 'boost::json::value'. Serialisation to a string is performed using Boost.JSON's serialize function, but this should be outside the scope of Boost.Exception (so if someone wants to serialise to say CBOR instead, this is still possible).
I am not sure if Boost.Exception wants to have a dependency on Boost.JSON, so my proposal was to provide a generic visiting mechanism instead.

[zajo]

I am not familiar with Boost.JSON, it may be possible to define serialization to JSON where the dependency to Boost.JSON exists only if the code that is serializing to JSON is instantiated. I'd be totally fine if only serialization to JSON is implemented. This could be implemented along the lines of how op<< is used if available.

Either way, if you can sketch up a solution I'll take a look.

[sdebionne]

Here are two patches that implement a proof of concept:

exception.patch
throw_exception.patch

A new test can be build with ./b2 libs/exception/test//diagnostic_information_visitor_test. The other tests are broken since I did not manage to isolate the JSON part.

[zajo]

Thank you, I'll take a look soon.

[zajo]

It seems like the .patch files you posted are corrupt. Could you check please?

[sdebionne]

Sorry about that, I ended up pushing my modifications in my forks instead:

https://github.com/boostorg/exception/compare/develop...sdebionne:exception:diagnostic-information-json?expand=1

https://github.com/boostorg/throw_exception/compare/develop...sdebionne:throw_exception:diagnostic-information-json?expand=1