Run-time initialised static variables

[dekevin]

On Tuesday, 6 February 2018 at 23:21:36 UTC, Jonathan M Davis 
wrote:
> On Tuesday, February 06, 2018 23:03:07 dekevin via 
> Digitalmars-d-learn wrote:
>> Hello everyone,
>> I just ran into the problem, that I need a static variable, 
>> where
>> the initialisation code for that variable is only accessible
>> during run-time (since part of the initialisation code will be
>> dynamically linked).
>>
>> Is there a way to do this in D?
>>
>> To be a bit more concrete, this is where I have the problem 
>> (where ℚ uses GMP, which is dynamically linked):
>>
>> struct ℚInf {
>>     ℚ qval;
>>     immutable static ℚInf zero = ℚInf(0,1);
>>     this(long num, long den) {
>>          qval = ℚ(num,den); //this initialisation requires
>> dynamically linked code
>>      }
>> }
>
> So, you want qval to be static and initialized at runtime? Then 
> use a static constructor. e.g.
>
> struct QInf
> {
>     ...
>     static this()
>     {
>         qval = ...;
>     }
>     ...
> }
>
> That doesn't work if you're dealing with a static local 
> variable, but it works for static members of structs and 
> classes, and it works for module-level variables.
>
> And if you want to make qval immutable, then use a shared 
> static constructor. e.g.
>
> struct QInf
> {
>     ...
>     shared static this()
>     {
>         qval = ...;
>     }
>     ...
> }
>
> You can currently initialize immutable static variables with 
> non-shared static constructors, but that's a bug and will 
> result in the immutable variable being reinitialized whenever a 
> new thread is created.
>
> - Jonathan M Davis

Thanks a lot! I will change all my initialisations to static 
constructors now.
The only additional problem I have, is that ℚInf has a disabled 
default constructor.
Is there a way to allow shared static constructors, but not the 
default constructor?
struct ℚInf {
      ℚ qval;
      immutable static ℚInf zero;
     @disable this();
      shared static this() {
           zero.qval = ℚ(0);
      }
      this(long num, long den) {
           qval = ℚ(num,den); //this initialisation requires
  dynamically linked code
       }
}

Best,
Kevin