templating operators

[Derek Parnell]

On Thu, 2 Mar 2006 18:01:07 -0800, Brad Roberts wrote:

> I'm trying to create a container that will allow arbitrary key/value pair 
> trees.  Box seems like a great start at supporting a 'holds anything' 
> container.  The next step is making a smooth associative array of them.  
> However, I don't seem to be able to create a nice templated opIndex or 
> opIndexAssign.
> 
> -------- test.d ---------
> import std.stdio;
> import std.boxer;
> 
> class ValueContainer
> {
>     Box[char[]] _objects;
> 
>     template opIndex(T)
>     {   
>         T opIndex(char[] key)
>         {   
>             return unbox!(T)(_objects[key]);
>         }
>     }
> 
>     int opIndexAssign(int newValue, char[] key)
>     {   
>         _objects[key] = box(newValue);
>         return 0;
>     }
> 
>     template getValue(T)
>     {   
>         T getValue(char[] key)
>         {   
>             return unbox!(T)(_objects[key]);
>         }
>     }
> 
>     template insertKeyAndValue(T)
>     {   
>         void insertKeyAndValue(char[] key, T newValue)
>         {   
>             _objects[key] = box(newValue);
>         }
>     }
> }
> 
> int main()
> {
>     ValueContainer values = new ValueContainer();
> 
>     values["int"] = 1;
>     values.insertKeyAndValue!(int)("key", 10);
> 
>     writef("int: %s\n", values["int"]);
>     writef("key: %s\n", values["key"]);
>     writef("key: %s\n", values.getValue!(int)("key"));
> 
>     return 0;
> }
> ---------------
> 
> $ dmd -g test.d                                                 
> test.d(46): no [] operator overload for type test.ValueContainer
> test.d(47): no [] operator overload for type test.ValueContainer
> test.d(48): function expected before (), not 0 of type int
> 
> Any thoughts on how to make this work well?

When I did this sort of thing I had to chnage the template name to be
different to the member inside the template *and* I had to instantiate the
member inside the class. The instantiation didn't surprise me but the
naming conflict one did. Anyhow, replace ...

     template opIndex(T)
     {   
         T opIndex(char[] key)
         {   
             return unbox!(T)(_objects[key]);
         }
     }

with ...

    template opIndex_T(T)
    {
        T opIndex(char[] key)
        {
            return unbox!(T)(_objects[key]);
        }
    }
    alias opIndex_T!(int).opIndex opIndex;

and it should work now.

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocracy!"
3/03/2006 3:14:03 PM