Is there way to construct an "argument pack" from a non-static array (like the switch below)?  I need to transport a variadic call through a void*.

switch (a.length) {
  case 1: foo(a[1]); break;
  case 2: foo(a[1], a[2]); break;
  case 3: foo(a[1], a[2], a[3]); break;
...
}

On Monday, 2 May 2016 at 18:22:52 UTC, Erik Smith wrote:
> Is there way to construct an "argument pack" from a non-static array (like the switch below)?  I need to transport a variadic call through a void*.
>
> switch (a.length) {
>   case 1: foo(a[1]); break;
>   case 2: foo(a[1], a[2]); break;
>   case 3: foo(a[1], a[2], a[3]); break;
> ...
> }

Yes there is.
Using a string mixin.

On Monday, 2 May 2016 at 18:56:59 UTC, Stefan Koch wrote:
> On Monday, 2 May 2016 at 18:22:52 UTC, Erik Smith wrote:
>> Is there way to construct an "argument pack" from a non-static array (like the switch below)?  I need to transport a variadic call through a void*.
>>
>> switch (a.length) {
>>   case 1: foo(a[1]); break;
>>   case 2: foo(a[1], a[2]); break;
>>   case 3: foo(a[1], a[2], a[3]); break;
>> ...
>> }
>
> Yes there is.
> Using a string mixin.

I'm not sure how to apply mixins here because they require compile time evaluation and a.length is runtime.  Ideas?

erik

On Monday, 2 May 2016 at 18:22:52 UTC, Erik Smith wrote:
> Is there way to construct an "argument pack" from a non-static array (like the switch below)?  I need to transport a variadic call through a void*.
>
> switch (a.length) {
>   case 1: foo(a[1]); break;
>   case 2: foo(a[1], a[2]); break;
>   case 3: foo(a[1], a[2], a[3]); break;
> ...
> }

I don't think it's possible to call a vararg function whose number of arguments is only known at runtime, for the same reasons it is impossible in C [1].

Your switch statement is probably the best you can do, other than rewriting the API to not use varargs (which, depending on what the function is doing, I would recommend). You can possibly use string mixins or static foreach to avoid repeating the case clauses.

[1] Populating a va_list: http://stackoverflow.com/questions/988290/populating-a-va-list

> I don't think it's possible to call a vararg function whose number of arguments is only known at runtime, for the same reasons it is impossible in C [1].
>
> Your switch statement is probably the best you can do, other than rewriting the API to not use varargs (which, depending on what the function is doing, I would recommend). You can possibly use string mixins or static foreach to avoid repeating the case clauses.
>
> [1] Populating a va_list: http://stackoverflow.com/questions/988290/populating-a-va-list

That was fast :).  The generation idea helps a little - thanks.  It works though and I can't use va_list in this case.

erik