Widening a type

[Denis Koroskin]

On Fri, 17 Apr 2009 06:11:00 +0400, Doctor J <nobody at nowhere.com> wrote:

> OK, here's one for you that sounds like it ought to be easy, but I don't  
> immediately see how to do it in a pretty way.
>
> Given a type parameter T of a template:
> If T is an integral type, I want to declare a variable 'widest' of type  
> ulong;
> If T is a floating-point type, I want to declare a variable 'widest' of  
> type double.
> And it has to be prettier than my solution.  :)
>
>         static if (is (T: ulong))
>             ulong widest = 0;
>         else if (is (T: double))
>             double widest = 0.0;
>         else
>             static assert (false, "Unimplemented type " ~ T.stringof) ;
>
> Now, I thought this sounds like a great job for a mixin:
>template Widen (T, alias varname)
> {
>     static if (is (T: ulong))
>         ulong varname = 0;
>     else if (is (T: double))
>         double varname = 0.0;
>     else
>         static assert (false, "Unimplemented type " ~ T.stringof) ;
> }
>
> mixin Widen!(T, widest);
>
> ...but alas, "Declaration expected, not 'if'".
>
> Help?
>
>

I would avoid mixin in such situation and use template instead:

template Widen(T)
{
    static if (is(T : ulong)) {
        alias ulong Widen;
    } else static if (is(T : double)) {
        alias double Widen;
    } else {
        static assert (false, "Unimplemented type " ~ T.stringof) ;
    }
}

Widen!(T) widest;