Re: Widening a type

[Denis Koroskin]

On Fri, 17 Apr 2009 06:11:00 +0400, Doctor J <nob...@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;