Macro for creating enum classes

[johnomotani]

After the discussion on #1835 I was thinking about using an enum class as an option. The way we are using enum classes at the moment has some boiler-plate to define toString and enumFromString functions. Using some magic copied from bout/macro_for_each.hxx, it seems to be possible to replace that with a macro so you can write something like

BOUT_ENUM_CLASS(myenum, foo, bar);

and get an enum class with possible values myenum::foo and myenum::bar, along with toString(myenum e) and myenumFromString(std::string s) functions. I've also put in overloads for Options::as<>() and ostream& operator<< so that the enum class can be loaded from options, e.g.

myenum e = options["myenum_option"].withDefault(myenum::foo);

I'd also like to be able to assign a string to the enum class, but it doesn't seem to be possible to override operator= for an enum class.

Before I think about this any more: is this a useful feature, or an abuse of macros?

[ZedThree]

I think this is probably essential! Worthwhile looking over some other implementations of compiletime enum reflection:

• https://github.com/aantron/better-enums
• https://github.com/Neargye/magic_enum
• https://github.com/quicknir/wise_enum
• https://github.com/therocode/meta_enum
• https://github.com/BlackMATov/enum.hpp

Some of these are C++17 only, but could be backported depending on what they use; some make an actual class rather than an enum, which might be painful for the derivative template machinery.
We might be able to use one of those wholesale, or else adapt to our needs.

I've almost finished adding fmt to BOUT++, which could also be useful here. It has fmt::basic_string_view which is an implementation of C++17's std::string_view that some of the above implementations use.

[bendudson]

Thanks @johnomotani I think this is both a wonderful abuse of macros, and a very useful feature. The overload of Options::as is very nice.

[d7919]

Can the work in progress label be removed?

[johnomotani]

Documentation of the new macro is definitely still needed.

@ZedThree - are there any useful changes to make right now from your earlier comment, or do you think it is easy enough to upgrade later?

[ZedThree]

std::string s -> const std::string& s

[ZedThree]

Adding the fmt stuff can be done later once I work out how to do it.

One thing to change now though is that identifiers beginning with an underscore followed by a capital are reserved, so maybe change those -- I suggest just prefixing everything with BOUT_.

[ZedThree]

Also, might need to use this trick to get _BOUT_ENUM_CLASS_MAP_ARGS working on Windows:
cfb7888

But you'll need a Windows system to test it on, so that can wait