Templates - Numeric Types only

[Andrei Alexandrescu]

Jarrett Billingsley wrote:
> On Thu, Aug 6, 2009 at 9:53 PM, Andrei
> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>> Jarrett Billingsley wrote:
>>> On Thu, Aug 6, 2009 at 9:19 PM, Andrei
>>> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>>>> Kevin wrote:
>>>>> Hi,
>>>>> simple Question:
>>>>> Is it possible to allow just numeric types in templates?
>>>>> For example to create a Vector Class like this:
>>>>>
>>>>> class Vector(T:"numeric" = float, int size = 4)
>>>>>
>>>>> Thanks,
>>>>> Kevin
>>>> import std.traits;
>>>>
>>>> class Vector(T, int size = 4) if (isNumeric!T)
>>>> {
>>>>   ...
>>>> }
>>> Unfortunately,
>>>
>>> struct Vec(T) if(isNumeric!T) {}
>>> struct Vec(T) {}
>>> Vec!(int) x; // error
>>>
>>> foo.d(14): Error: template instance Vec!(int) matches more than one
>>> template declaration, Vec(T) if (isNumeric!(T)) and Vec(T)
>>>
>>> *sigh*
>>>
>>> Wouldn't it be nice.
>> struct Vec(T) if(isNumeric!T) {}
>> struct Vec(T) if(!isNumeric!T) {}
>>
>> is actually superior because it's more modular; the semantics of the latter
>> does not depend on the presence of the former.
> 
> But the condition of the latter now depends on the condition of the
> former.  And on the condition of any other 'specializations':
> 
> struct Vec(T) if(isNumeric!T) {}
> struct Vec(T) if(isSomeString!T) {}
> struct Vec(T) if(!isNumeric!T) {} // uhoh
> Vec!(string) x; // matches two
> 
> Now the fallback has to be declared as
> 
> struct Vec(T) if(!isNumeric!T && !isSomeString!T) {}
> 
> and it gets worse the more you add.

I know, and I explained why that's arguably better.

Andrei