local class instance (at module-level)

[Alex]

On Thursday, 14 March 2019 at 11:05:22 UTC, spir wrote:
> I desperately try to declare/define/initialise a simple class 
> instance at module-level. This is a special (conceptually 
> static and immutable) instance used as a "marker", that just 
> should exist and be accessible by methods of this class and/or 
> other classes defined in the same module. (Thus I don't care if 
> TLS.) I use it as a remplacement for null, to avoid semantic 
> confusion and unhelpful segfaults in case of bug.
>
> I have tried a number of options and never manage to do it, 
> including:
> * [C/auto/static immutable c0] = new C(0) ;
> * C c0 ; c0.i = 0 ;
> * defining a static this()
> * more...
>
> The most confusing error is:
> Error: variable `_base.c0` is a thread-local class and cannot 
> have a static initializer. Use `static this()` to initialize 
> instead.
>
> I also could not find any information --for a while, 
> repeatedly, since I can go on developing in the meantime, using 
> null instead. I'm bluffed and confused, since there is nothing 
> weird in that, is there? (the compiler can just allocate it in 
> static mem and take the address)
>
> Reduced test case:
> ===========================
> class C {
>     uint i ;
>     this (uint i) {
>         this.i = i ;
>     }
> }
>
> // error
> auto c0 = new C(0) ;
>
> void main () {
>     // ok
>     auto c0 = new C(0) ;
> }
> ===========================
>
> I would enjoy an explanation (or a pointer to) in addition to a 
> solution.
>
> Thank you,
> diniz
>
> PS: I take the opportnity to ask if I can count on the compiler 
> to intern literal strings (which my code may use in several 
> places, including loops), esp. "", or should I declare and use 
> (for instance):
> static immutable s0 = "" ;

Basically, this works for me:

´´´
class C {
     uint i ;
     this (uint i) {
         this.i = i ;
     }
}

C c0;

static this()
{
     c0 = new C(0);
}

void main () {
     assert(!(c0 is null));
}
´´´