Re: [Boost][Proposal] library for enhanced switch syntax.

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2019-11-11 09:27:54

On Sun, 10 Nov 2019 at 14:16, Phil Endecott via Boost
<boost_at_[hidden]> wrote:
>
> Specifically thinking about strings, there are numerous ways
> that it could be done.  Starting with

Even for integers there are multiple ways it could be done.
It's up to the compiler to decide what is best.

> (3) character-by-character comparison using nested switches:
>
> switch (s[0]) {
>   case 'a': switch (s[1]) {
>               case 'a': switch (s[2]) {
>                           case 0: f(); break;
>                         }
>               case 'b': switch (s[2]) {
>                           case 0: g(); break;
>                         }
>             }
>   case 'x': switch (s[1]) {
>               case 'y': switch (s[2]) {
>                           case 0: h(); break;
>                         }
>             }
> }

That is probably the "natural" implementation that one would expect.

> (5) hashing, and then checking:
>
> switch (hash(s)) {
>   case "aa"HASH: if (s=="aa") f(); break;
>   case "ab"HASH: if (s=="ab") g(); break;
>   case "xy"HASH: if (s=="xy") h(); break;
> }

That code doesn't really need to check, if your hashes collide the
compiler will reject the code.