Make switchType more comfortable to use

[franzpoeschel]

This PR:

• moves the switchType function templates to a separate header DatasetHelpers.hpp
• adds new switchType versions for ADIOS2, this allows to throw out lots of annoying redirection SFINAE helper structs
• makes switchType less annoying to use:
  1. The return type needs not be given as a template parameter explicitly any more
  2. Throwing errors upon detecting an unknown datatype is now done automatically if the switchType action struct has an errorMsg member

TODO:

• Apply changes in JSON backend
• Cleanup

This is slightly API breaking since the template is no longer exposed to users. Users may still include the header explicitly if they need it.

[ax3l]

Looks great, minor wish about the macro-usage inline & some cleanup suggestions :)

[ax3l]

Cool, looks nice to use :)

You can const those members, I guess.

Suggested change

	std::string errorMsg = "JSON: writeDataset";
	std::string const errorMsg = "JSON: writeDataset";

[franzpoeschel]

Hm, to be fair I'm not a huge fan of making non-static members const since that automatically deletes any operator= members. Since there are places where I store instances of these structs inside other structs (the ADIOS2 backend for example), I would prefer not to have some inconspicuous member silently delete those operators..

[ax3l]

Oh I see, agreed.

[ax3l]

Thank you, looks great!

We should refactor some Python code where we are not yet using switchType, I think :)

[ax3l]

Ups, we get an ICC error in WarpX:

[ 12%] Building CXX object CMakeFiles/openPMD.dir/src/Format.cpp.o
/tmp/ci-mtwgsg2YYE/src/include/openPMD/DatatypeHelpers.hpp(86): error: static assertion failed with "[switchType] Action needs either an errorMsg member of type std::string or operator()<unsigned>() overloads."
          static_assert(
          ^
          detected during:
            instantiation of "ReturnType openPMD::detail::CallUndefinedDatatype<n, ReturnType, Action, Placeholder, Args...>::call(Action, Args &&...) [with n=1000, ReturnType=openPMD::Datatype, Action=openPMD::detail::BasicDatatype, Placeholder=void, Args=<>]" at line 259
            instantiation of "auto openPMD::switchType(openPMD::Datatype, Action, Args &&...)->decltype((<expression>)) [with Action=openPMD::detail::BasicDatatype, Args=<>]" at line 390 of "/tmp/ci-mtwgsg2YYE/src/src/Datatype.cpp"
/tmp/ci-mtwgsg2YYE/src/include/openPMD/DatatypeHelpers.hpp(91): error: class "openPMD::detail::BasicDatatype" has no member "errorMsg"
              "[" + std::string( action.errorMsg ) + "] Unknown Datatype." );
                                        ^
          detected during:
            instantiation of "ReturnType openPMD::detail::CallUndefinedDatatype<n, ReturnType, Action, Placeholder, Args...>::call(Action, Args &&...) [with n=1000, ReturnType=openPMD::Datatype, Action=openPMD::detail::BasicDatatype, Placeholder=void, Args=<>]" at line 259
            instantiation of "auto openPMD::switchType(openPMD::Datatype, Action, Args &&...)->decltype((<expression>)) [with Action=openPMD::detail::BasicDatatype, Args=<>]" at line 390 of "/tmp/ci-mtwgsg2YYE/src/src/Datatype.cpp"
/tmp/ci-mtwgsg2YYE/src/include/openPMD/DatatypeHelpers.hpp(86): error: static assertion failed with "[switchType] Action needs either an errorMsg member of type std::string or operator()<unsigned>() overloads."
          static_assert(
          ^
          detected during:
            instantiation of "ReturnType openPMD::detail::CallUndefinedDatatype<n, ReturnType, Action, Placeholder, Args...>::call(Action, Args &&...) [with n=0, ReturnType=openPMD::Datatype, Action=openPMD::detail::BasicDatatype, Placeholder=void, Args=<>]" at line 267
            instantiation of "auto openPMD::switchType(openPMD::Datatype, Action, Args &&...)->decltype((<expression>)) [with Action=openPMD::detail::BasicDatatype, Args=<>]" at line 390 of "/tmp/ci-mtwgsg2YYE/src/src/Datatype.cpp"
/tmp/ci-mtwgsg2YYE/src/include/openPMD/DatatypeHelpers.hpp(91): error: class "openPMD::detail::BasicDatatype" has no member "errorMsg"
              "[" + std::string( action.errorMsg ) + "] Unknown Datatype." );
                                        ^
          detected during:
            instantiation of "ReturnType openPMD::detail::CallUndefinedDatatype<n, ReturnType, Action, Placeholder, Args...>::call(Action, Args &&...) [with n=0, ReturnType=openPMD::Datatype, Action=openPMD::detail::BasicDatatype, Placeholder=void, Args=<>]" at line 267
            instantiation of "auto openPMD::switchType(openPMD::Datatype, Action, Args &&...)->decltype((<expression>)) [with Action=openPMD::detail::BasicDatatype, Args=<>]" at line 390 of "/tmp/ci-mtwgsg2YYE/src/src/Datatype.cpp"
compilation aborted for /tmp/ci-mtwgsg2YYE/src/src/Datatype.cpp (code 2)