Class parameters that are either setable at run-time or compile-time

[Rikki Cattermole via Digitalmars-d-learn]

On 10/08/2014 10:47 p.m., Markus Mayr wrote:
> Hello,
>
> I am new to D. I am sorry if this question was already answered or if it
> is trivial. I am having a class. That class takes several parameters
> that may be known at either compile time or run time. I want users of my
> class to be able to pass the parameters either as template parameters to
> the class or to the constructor of that class. At the moment, I have got
> the following code:
>
>      class X(int a = some_dummy_value, int b = some_dummy_value)
>      {
>          static if (a == some_dummy_value) static immutable int a_ = a;
>          else immutable int a_;
>
>          // etc.
>
>          this(int a = some_dummy_value, int b = some_dummy_value) {
>              static if (a != some_dummy_value) this.a_ = a;
>              static if (b != some_dummy_value) this.b_ = b;
>          }
>      };
>
> This works, as far as I can tell. But there is one problem: users may
> forget to provide the value at compile time and run time. In that case,
> the members a_, b_ end up with some_dummy_values. Of course, I can
> notify the user at run time about this misbehaviour, but I would prefer
> to tell them at compile time.
>
> The only solution I am aware of is to use static if in order to provide
> one of four constructors depending on the compile time values of a, b, i.e.
> * none provided at compile time,
> * a provided at compile time,
> * b provided at compile time,
> * both provided at compile time.
>
> This is perfectly fine for one or two arguments, but I have gotten three
> of them (i.e. 8 constructors) and may want to add more later on. Since
> the number of constructors scales exponentially with the number of
> parameters, I have a feeling that this approach won't scale well.
>
> Is there any elegant solution to the problem?
>
> By the way, this is why I am doing this: Depending on the values of a
> and b, the class chooses how to implement some parts of its interface;
> if a or b are not provided, it may choose generic but slow fall back
> implementations. If there is any other solution to that problem, I also
> would like to know.
>
> Thanks a lot and I hope this is how this mailing list works!
>
> Best regards,
> Markus Mayr

Unless you need to change the defaults of the values being passed in, 
don't pass them by template parameters.
Please please please use constructors. They work at both compile time 
and runtime.

Remember every change in template parameters equals a new type.

http://dlang.org/class.html#constructors