Template for function or delegate (nothing else)

[Steven Schveighoffer]

On Wed, 09 Feb 2011 16:41:25 -0500, useo <useo at start.bg> wrote:

> == Auszug aus bearophile (bearophileHUGS at lycos.com)'s Artikel
>> useo:
>> > I just have a problem with my variables.
>> >
>> > For example... my class/template just looks like:
>> >
>> > class Example(T) if (is(T == delegate) || is(T == function))
>> > {
>> >    T callback;
>> >
>> >    void setCallback(T cb) {
>> >       callback = cb;
>> >    }
>> >
>> > }
>> >
>> > This means that I need variables like Example!(void function
> ())
>> > myVariable. But is there any possibility to use variables like
>> > Example myVariable?
>> D is not the SML language, templates are just placeholders. If
> you don't instantiate a template, you have only a symbol. Example
> is only assignable to an alias (and in past, to a typedef):
>> alias Example Foo;
>> > The template declaration only defines the type of
>> > a callback and perhaps one method-declaration nothing else. I
> already
>> > tried Example!(void*) because delegates and functions are void
>> > pointers but I always get an error. I hope there is any way
> to do
>> > this.
>> I don't yet understand what you are trying to do.
>> Other notes:
>> - What if your T is a functor (a callable class/struct/union
> instance that defined opCall)?
>> - sizeof of a function pointer is 1 CPU word, while a delegate
> is 2 CPU words (and a delegate clojure has stuff on the heap too,
> sometimes).
>> Bye,
>> bearophile
>
> Idea is the following:
>
> class Example(T) if (is(T == delegate) || is(T == function)) {
>
>    T callback;
>
>    void setCallback(T cb) {
>       callback = cb;
>    }
>
>    void opCall() {
>       callback();
>    }
>
> }
>
> other file:
>
> import example;
>
> private {
>
>    Example variable;
>
> }
>
> void setExampleVariable(Example ex) {
>    variable = ex;
> }
>
> void callCurrentExampleVariable() {
>    variable();
> }

I don't think you want templates.  What you want is a tagged union (and a  
struct is MUCH better suited for this):

// untested!

struct Example
{
    private
    {
       bool isDelegate;
       union
       {
          void function() fn;
          void delegate() dg;
       }
    }

    void setCallback(void function() f) { this.fn = f; isDelegate = false;}
    void setCallback(void delegate() d) { this.dg = d; isDelegate = true;}

    void opCall()
    {
       if(isDelegate)
          dg();
       else
          fn();
    }
}

-Steve