[boost] [Better Enums] Maps between enums and other types

Subject: [boost] [Better Enums] Maps between enums and other types
From: Anton Bachin (antonbachin_at_[hidden])
Date: 2015-07-11 21:16:31

Hello,

In past exchanges on this list, several people expressed a desire for
bidirectional maps on enums. I've made a simple implementation [1] that relies
on constexpr functions containing a switch. The switch forces case
exhaustiveness checking. This method therefore requires C++14 to be truly
useful.

Usage is:

    ENUM(Channel, int, Red, Green, Blue)

    constexpr const char* describe(Channel channel)
    {
        switch(channel) {
            case Channel::Red:   return "the red channel";
            case Channel::Green: return "the green channel";
            case Channel::Blue:  return "the blue channel";
        }
    }

    constexpr auto descriptions = better_enums::make_map(describe);

    int main()
    {
        std::cout << descriptions.from_enum(Channel::Red) << std::endl;
        std::cout << descriptions.to_enum("the green channel") << std::endl;
    }

This prints

    the red channel
    Green

In principle, make_map should enumerate the function by passing every possible
value of Channel, and use the results to build up some lookup data structure. I
am not doing that right now because I haven't found a compile-time data
structure with fast enough generation to be practical.

I suppose with reflective information, there are other ways to check for
exhaustiveness besides using a switch. The implementation is currently
inefficient (it uses a linear scan to do the reverse mapping). There is also no
support for multimaps or partial maps. I could probably support the latter
easily by having the function return an option type. I am not sure multimaps are
a good idea at all. The only point of them would be to do a reverse mapping from
multiple values to the same enum.

Interested to hear any opinions.

Anton

[1] http://aantron.github.io/better-enums/tutorial/Maps.html