template auto instantiation when parameters empty

[Erik Smith via Digitalmars-d-learn]

On Thursday, 5 May 2016 at 16:12:40 UTC, Steven Schveighoffer 
wrote:
> On 5/5/16 12:10 AM, Erik Smith wrote:
>> I want to have a struct template auto instantiate when the 
>> template
>> parameters are defaulted or missing.  Example:
>>
>> struct Resource(T=int) {
>>      static auto create() {return Resource(null);}
>>      this(string s) {}
>> }
>>
>> auto resource = Resource.create;
>>
>> As a plain struct it works, but not as a template:
>>
>> struct Resource {   // works
>> struct Resource() {  // fails
>> struct Resource(T=int) {  // fails
>>
>> At the call site, this works, but I'm hoping for a few less 
>> symbols:
>>
>> auto resource = Resource!().create;
>>
>> Any ideas?
>
> Instead of static method, use an external factory method:
>
> static auto createResource(T = int)()
> {
>    return Resource!T(null);
> }
>
> And I wouldn't bother with making Resource's T have a default, 
> as you'd still have to instantiate it with Resource!() to get 
> the default.
>
> -Steve

Alias works at the cost of adding a 2nd type name:

alias Res = Resource!();
auto res  = Res.create

The other problem is that the alias definition by itself 
instantiates, which I can't afford.

I have createResource() now, it just doesn't fit well with the 
rest of the calling styles.

There is also this approach, which might be slightly more 
idiomatic

struct Resource(T) {}
struct Policy {}

auto create(alias T)() {
     T!Policy r;
     return r;
}

auto r = create!Resource;