How to write a singleton template?

[Bruno Medeiros]

Kyle Furlong wrote:
> Bruno Medeiros wrote:
>> Kyle Furlong wrote:
>>>
>>> You might want to consider a mixin:
>>>
>>> template Singleton(T)
>>> {
>>>     static this()
>>>     {
>>>         _instance = new T();
>>>     }
>>>     public static T instance ()
>>>     {
>>>         return _instance;
>>>     }
>>>     private static T _instance;
>>> }
>>>
>>> class OnlyOne
>>> {
>>>     // Can't include this in the mixin,
>>>     // because D can only mixin static things to classes
>>>     private this() {}
>>>     mixin Singleton!(OnlyOne);
>>> }
>>>
>>
>> "because D can only mixin static things to classes"
>> -> That is incorrect, you can include instance methods and 
>> constructors just fine. See for yourself in a example such as this:
>>
>>   template Baz() {
>>     public void func() { writefln(x); }
>>     public this() { writefln("Construct!"); }
>>   }
>>
>>   class Foo {
>>     int x = 2;
>>     mixin Baz!();
>>   }
>> ...
>>   (new Foo).func();
>>
>>
>> The only problem is with private protection attributes and 
>> constructors, as reported in bug:
>> news://news.digitalmars.com:119/bug-49-3@http.d.puremagic.com/bugzilla/
>>
>>
> 
> The documentation leads one to believe that this is impossible. Look 
> under the limitations header. This needs to be clarified.

The limitations section is about instantiating templates that are 
defined inside a class, it is not about mixing in in a template inside a 
class, which is a different thing. The limitation exists only in the 
first case. This behaviour make sense.

-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D