Parameterized Structs

[Peter Lundgren]

== Quote from Ali Çehreli (acehreli at yahoo.com)'s article
> On 03/02/2011 08:56 PM, Peter Lundgren wrote:
> > Where can I go to learn about parameterized structs? I can't seem to find any
> > literature on the subject. In particular, what are you allowed to use as a
> > parameter? I would like to define a struct like so:
> >
> > struct MyStruct(T, T[] a) {
> >      ...
> > }
> >
> > but I receive the following error:
> >
> > Error: arithmetic/string type expected for value-parameter, not T[]
> >
> > Are arrays not allowed?
> Are you trying to parametrize by the type of the container or just
> trying to use an array of a specified type? (As opposed to say, a linked
> list of the specified type?)
> If the former, it's simple. And the simplest thing is to just use an
> array in the implementation:
> struct S(T)
> {
>      T[] a;
>      void foo(T element)
>      {
>          /* Just use like an array */
>          a ~= element;
>          a[0] = element;
>      }
> }
> void main()
> {
>      auto s = S!double();
>      s.foo(1.5);
> }
> If you want to use a different container of the specified T, then a
> second template parameter can be used. This one uses an array as the
> default one:
> class SomeContainer
> {}
> struct S(T, Cont = T[])
> {
>      Cont a;
>      void foo(T element)
>      {
>          /* This time the use must match the allowed container types */
>      }
> }
> void main()
> {
>      auto s = S!(double, SomeContainer)();
>      s.foo(1.5);
> }
> I would recommend pulling information out ;) of this page:
>    http://digitalmars.com/d/2.0/template.html
> "Template Alias Parameters" is very different after C++ and can be very
> powerful:
>    http://digitalmars.com/d/2.0/template.html#TemplateAliasParameter
> Ali

I'm using this for an alternative implementation of a string, if you will. Where T
is the type of a single character and a would be the alphabet (an array of allowed
characters). The rest of the implementation of the struct would, of course, depend
upon the provided alphabet.

I guess value parameters can't be arbitrary types. I can probably get by with
using a string for my alphabet just fine, it just seemed an arbitrary limitation.
Why accept only arrays of characters when the code will be the same for any type?