I try to write a singleton template, but has not succeeded:

A:
template Singleton(T)
{
private T _instance;

static this()
{
_instance = new T();
}

public T instance ()
{
return _instance;
}
}

B:
template Singleton(T)
{
class Singleton
{
private static T _instance;

static this()
{
_instance = new T();
}

public static T instance ()
{
return _instance;
}
}
}

C:
template Singleton(T)
{
class Singleton
{
private static T _instance = null;

public static T instance ()
{
if (_instance == null)
_instance = new T();
return _instance;
}
}
}


// use it:

class AAA
{
public void hello ()
{
printf("hello\n");
}
}

int main(char[][] args)
{
alias Singleton!(AAA) aaa;
aaa.instance().hello();  // <== Segment fault !!!!!

return 0;
}

I want to know, how to write a singleton template?



Thanks.

- Li Jie

"Li Jie" <cpunion@gmail.com> wrote in message news:dv7s7e$1hee$1@digitaldaemon.com...

Versions A and B work fine, but version C doesn't work because of this line:

> if (_instance == null)

Change it to

if(_instance is null)

This is because the == operator calls opEquals on the first object, so _instance == null is the same as _instance.opEquals(null) - which is obviously going to cause a segfault if _instance is null.  Use the "is" (and the "!is") operator to determine what a class reference is pointing to (like null).

Li Jie wrote:
> I try to write a singleton template, but has not succeeded:
> 
> A:
> template Singleton(T)
> {
> private T _instance;
> 
> static this()
> {
> _instance = new T();
> }
> 
> public T instance ()
> {
> return _instance;
> }
> }
> 
> B:
> template Singleton(T)
> {
> class Singleton
> {
> private static T _instance;
> 
> static this()
> {
> _instance = new T();
> }
> 
> public static T instance ()
> {
> return _instance;
> }
> }
> }
> 
> C:
> template Singleton(T)
> {
> class Singleton
> {
> private static T _instance = null;
> 
> public static T instance ()
> {
> if (_instance == null)
> _instance = new T();
> return _instance;
> }
> }
> }
> 
> 
> // use it:
> 
> class AAA
> {
> public void hello ()
> {
> printf("hello\n");
> }
> }
> 
> int main(char[][] args)
> {
> alias Singleton!(AAA) aaa;
> aaa.instance().hello();  // <== Segment fault !!!!!
> 
> return 0;
> }
> 
> I want to know, how to write a singleton template?
> 
> 
> 
> Thanks.
> 
> - Li Jie
> 
> 

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() {}
}

In article <dv80jr$1m1g$1@digitaldaemon.com>, Jarrett Billingsley says...
>Versions A and B work fine, but version C doesn't work because of this line:
>
>> if (_instance == null)
>
>Change it to
>
>if(_instance is null)
>
>This is because the == operator calls opEquals on the first object, so _instance == null is the same as _instance.opEquals(null) - which is obviously going to cause a segfault if _instance is null.  Use the "is" (and the "!is") operator to determine what a class reference is pointing to (like null).

Thannnks.

It works find.
But, Versions A and B work can't work on my computer.
Segment fault at the same line, and I don't know why.

My system:
P4 3.0G, 1G RAM, Gentoo 2006.0
gcc 3.4.4, dmd 0.149


Thanks again.

- Li Jie

In article <dv8anc$2109$1@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?

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

Li Jie wrote:
> In article <dv8anc$2109$1@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)

> printf("hello\n");

string not null terminated, change to one of
printf("hello\n\x00");
writef("hello\n");
writefln("hello");

Frank

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/


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

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.

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