Getting enum from value

[Kreikey via Digitalmars-d-learn]

On Saturday, 5 August 2017 at 20:11:27 UTC, Matthew Remmel wrote:
> On Saturday, 5 August 2017 at 18:26:10 UTC, Kreikey wrote:
>> On Saturday, 5 August 2017 at 15:33:57 UTC, Matthew Remmel
>
> I'm annoyed that I didn't think of trying to cast it. That 
> works great if the value exists in the enum. It does something 
> weird if the value doesn't though. This is my test.d file:
>
> import std.stdio;
>
> enum Foo {
>     A = "AV",
>     B = "BV"
> }
>
> void main() {
>     Foo k = cast(Foo)"BV"; // Works and prints correctly
>
>     k = cast(Foo)"CV";
>     writeln("Type: ", typeid(k));  // Type: test.Foo
>     writeln("Value: ", k);       // Value: cast(Foo)CV
> }
> --------
> The output shows the type being the Foo enum but the value is 
> 'cast(Foo)CV'. I would of expected an error or exception to be 
> thrown if it wasn't able to cast into an actual enum member. Is 
> this something with how the enums are implemented under the 
> hood?

That was my first post on this forum, so I'm glad it was at least 
a little bit useful :-D
I think the reasoning for no error on bad casts is that casting 
is a blunt instrument that assumes the programmer knows what he's 
doing, and it breaks the type system. So you'd want to use one of 
the aforementioned solutions if you're set on using enums in this 
way. You might also consider using associative arrays, but it's 
also a bit cumbersome. There's no way to get around searching:

   capitals = [
     "Indiana" : "Indianapolis",
     "Illinois" : "Chicago",
     "Ohio" : "Columbus"
       ];
   auto r = capitals.byKeyValue.find!((a, b) => a.value == 
b)("Chicago");
   if (!r.empty) {
     writeln(capitals[r.front.key]);
   } else {
     writeln("not found");
   }

You could also define another associative array statesByCapital 
with the key : value orders reversed, and then you could also do 
statesByCapitol["Chicago"]. Of course then you'd have to keep 
things in sync if things change. But I discovered a neat trick 
you could use to generate such a two way mapping. You could 
define one array string[] capitals, and another array string[] 
states. Then you could do:

auto capitalsByState = assocArray(zip(states, capitals));
auto statesByCapital = assocArray(zip(capitals, states));

If your data doesn't change for the lifetime of the program, that 
looks like a nice way to do it.