The new invariant.

[Andrei Alexandrescu (See Website For Email)]

Chris Nicholson-Sauls wrote:
> A couple of quick questions about the upcoming 'invariant' type 
> constructor.  Given a class Foo, will there be a way to mark the class 
> as having only invariant instances?  Ie can I declare it as 'invariant 
> class Foo {}' much as 'scope class Foo {}' would make it RAII-only?

Yes.

> Second, say I want to make an associative array of invariant Foo 
> instances.  How would I declare this?  (Let's use int as the key type, 
> for simplicity.)
> invariant Foo[int] pool ; // reads to me as the AA type is invariant, 
> which is useless
> invariant(Foo)[int] pool ; // perhaps?  like with the new const()?

The latter.

> This would be quite nifty for one of my projects, if it works like I 
> expect.
> 
> private static invariant(Symbol)[invariant(char[])] pool ;

That too. Possibly we'll also alias invariant(char[]) to string.

> It'd be beautiful.  Though I still worry a little about the current 
> invariant{} contract being broken.

We're looking into it.


Andrei