How to write a singleton template?

[Kyle Furlong]

Li Jie wrote:
> In article <dv8anc$2109$1 at digitaldaemon.com>, Kyle Furlong says...
>> 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);
>> }
>>
>> Or with inheritance:
>>
>> // Note here that there is some syntactic sugar for templated classes
>>
>> class Singleton(T)
>> {
>> 	static this()
>> 	{
>> 		_instance = new T();
>> 	}
>> 	public static T instance ()
>> 	{
>> 		return _instance;
>> 	}
>> 	protected static T _instance;
>> }
>>
>> class OnlyOne : Singleton!(OnlyOne)
>> {
>> 	private this() {}
>> }
> 
> Thanks.
> 
> In the version 2, must 'Singleton' template class and 'OnlyOne' class be in the
> same file?
> 

No, D templates can be in separate modules, just import the module you put the template in.

> I've never seen the usage of 'class XXX(T)', I read 'class.html' and
> 'template.html', but not found. Which document mention it?
> 
> 
> - Li Jie
> 
> 

Its in http://www.digitalmars.com/d/template.html, about 80% down the page, with the heading Class Templates. What is 
undocumented is the fact that struct templates are also legal. (a very nice feature IMHO)