Templates and delegates trouble
Steven Schveighoffer
schveiguy at yahoo.com
Tue Oct 7 07:55:06 PDT 2008
"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
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