Get class type parameters at compile time

[js.mdnq]

On Thursday, 13 December 2012 at 22:10:40 UTC, Philippe Sigaud
wrote:
> On Thu, Dec 13, 2012 at 11:56 AM, js.mdnq 
> <js_adddot+mdng at gmail.com> wrote:
>
>> I need to get the number of type parameters of a class at 
>> compile time:
>>
>>
>> class a(T1, T2, ...)
>> {
>>    static if (a.TypeInfo.NumParameters == 1)
>>       ....
>>    else
>>       ....
>>
>> }
>>
>> Is this possible?
>>
>
> Yes, it's possible:
>
> // Inside the class, it's easy:
> class Test(T...)
> {
>     enum num = T.length;// T is known here
> }
>
> // Outside the class, it's a bit tricky, but quite doable:
> template TemplateArity(Type)
> {
>     enum T = Type.stringof;
>     mixin("alias " ~ T ~ " U;");
>     static if (is(Type _ == U!Args, Args...))
>         enum TemplateArity = Args.length;
>     else
>         enum TemplateArity = -1;
> }
>
> void main()
> {
>     alias Test!(int, double, string) T;
>
>     assert(T.num == 3); // Internal knowledge
>     assert(TemplateArity!T == 3); // External deduction
> }

cool, I'll need it outside the class. Thanks.