Template argument deduction and default args

[Manu via Digitalmars-d]

On 24 July 2014 16:02, anonymous via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> On Thursday, 24 July 2014 at 04:53:41 UTC, Manu via Digitalmars-d
> wrote:
>
>  struct S(size_t len = 10)
>> {
>>   ubyte[len] data;
>> }
>>
>> S!100 x; // this works fine
>> S y; // this doesn't work (!)
>> S!() z; // this works
>>
>> The template arg has a default arg, why require !() ??
>>
>
> So that the type S!() is (easily) distinguishable from the
> template S.


Hmm.


 This causes problems in meta code, where you want to create an instance of
>> some T, and T may be a normal type with no template args, in which case
>> !()
>> is invalid, but a template type with default args should also be
>> acceptable, but it doesn't work because the meta code doesn't specify !().
>>
>
> Add !() at the instantiation site.


Then the code is broken for any type that isn't a template... it's a
mutually exclusive situation.


 struct S(T)
>> {
>>   this(T t)
>>   {
>>     m = t;
>>   }
>>
>>   T m;
>> }
>>
>> int myThing;
>> auto s = S(myThing); // error!
>> auto s = S!(typeof(myThing))(myThing); // works, but horrible, and breaks
>> down again when used in place of non-template counterparts in meta code.
>>
>
> Phobos goes with helper functions: S!T s(T)(T t) {return S!T(t);}
> I'm not a fan either.
>

Why? Surely this is easy to implement?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20140724/584c167a/attachment-0001.html>