How to write a singleton template?

[Kyle Furlong]

Bruno Medeiros wrote:
> 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.
> 

It is misleading because it asserts "Templates cannot be used to add non-static members or functions to classes." Even with the 
example, its still a blanket, general statement. It should specify that the example is the only case which has the limitation.