Templates and delegates trouble

[Steven Schveighoffer]

"Lars Kyllingstad" wrote
> Hello,
>
> I want to make a function (named 'call' in the example below) that calls 
> another function (F). I want to allow F to be an ordinary function, a 
> delegate, or a functor. I've been able to make it work with functions and 
> functors, but not with delegates. Any tips would be greatly appreciated.
>
> I have defined 'call' like this:
>
>     T call(alias F, T=real)(T arg)
>     {
>         return F(arg);
>     }
>
> I can now do like this:
>
>     call!(sqrt)(4.0);
>
> or
>
>     class Sqrt
>     {
>         real opCall(real x) { return sqrt(x); }
>     }
>
>     auto sqrt = new Sqrt;
>     call!(sqrt)(4.0);
>
> Both of these examples compile and the function returns 2.0 as expected. I 
> can not, however, do this:
>
>     class Sqrt
>     {
>         real eval(real x) { return sqrt(x); }
>     }
>
>     auto sqrt = new Sqrt;
>     call!(sqrt.eval)(4.0);
>
> To this, the compiler (GDC) says:
>
>     Error: need 'this' to access member eval
>
> So it would seem that even though eval is a method of the Sqrt class, F 
> aliases it as a function pointer. Is there any way of making this work?

You can't alias a delegate like that.

Try this:

auto sqrt = new Sqrt;
auto fn = &sqrt.eval;
call!(fn)(4.0);

What is happening is you are aliasing the eval function symbol, but not the 
sqrt instance to call it from.  Aliasing isn't exactly like a macro 
substitution.

This might also work (haven't tested it):

call!(&sqrt.eval)(4.0);

-Steve