Get class type parameters at compile time

[Philippe Sigaud]

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
}
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