Singleton

[Ugo]

Hi guys,

how do you make a singleton access class?

Do you know a better way of this one:

var singletonClass = (function( )
{
// Private variable
var instance = null;

// Private Constructor
function myClass( )
{
//...
}

return new function( )
{
this.constructo r = null;

this.getInstanc e = function( )
{
if( ! instance )
{
instance = new myClass( );
instance.constr uctor = null;
}

return instance;
}
}
})( );

[Sam --]

Re: Singleton

On Wed, 7 May 2008 10:42:51 +0200, Ugo <privacy@nospam .itwrote:

>Hi guys,
>
>how do you make a singleton access class?
>
>Do you know a better way of this one:
>
>var singletonClass = (function( )
>{
// Private variable
var instance = null;
>
// Private Constructor
function myClass( )
{
//...
}
>
return new function( )
{
this.constructo r = null;
>
this.getInstanc e = function( )
{
if( ! instance )
{
instance = new myClass( );
instance.constr uctor = null;
}
>
return instance;
}
}
>})( );

Pure qui? :-))
se ti vede ZERO...


ciao

[RoLo]

Re: Singleton

Do you know a better way of this one:
Yes

change this:
   return new function( )

to this:
return function( )

;-)

[RoLo]

Re: Singleton

On May 7, 11:06 am, RoLo <roloswo...@gma il.comwrote:

Do you know a better way of this one:

>
Yes
>
change this:
    return new function( )
>
to this:
    return function( )
>
;-)

ok... sorry, I over read you code.. you can ignore my previous comment.

[Ugo]

Re: Singleton

>>Hi guys,

>>how do you make a singleton access class?
>>Do you know a better way of this one:

[cut]

Pure qui? :-))

ehh, già :)
Mi è tornato questo cruccio, e non so' fare nè trovare di meglio...
per tanto o provato a vedere cosa ne pensano qui...

se ti vede ZERO...

ssshhh, zitto, non dire niente che forse non se ne accorge
:P

ciao

Bye

[Richard Cornford]

Re: Singleton

Ugo wrote:

how do you make a singleton access class?

How do you define a "singleton access class"?

Do you know a better way of this one:

The best way of doing something depends at least in part on what it is
you are trying to do.

var singletonClass = (function( )
{
// Private variable
var instance = null;

There is little point in assigning null to this variable as its default
undefined value will be just as false as the null value when you test it
with - !instance -.

// Private Constructor
function myClass( )
{
//...
}
>
return new function( )

I can think of no circumstances under which it makes sense to use the -
new - operator with a function expression as its operand. In this case
it would be simpler (and more efficient) to have an object literal
returned at this point. That object could easily define - constructor -
and - getInstance - properties.

{
this.constructo r = null;
>
this.getInstanc e = function( )
{
if( ! instance )
{
instance = new myClass( );
instance.constr uctor = null;

As you are using a constructor to create this object instance it would
be possible to define the created object's - constructor property on the
prototype of the constructor. Though there may be no good reason for
using a contractor to create the object at all as you are only going to
be creating one instance of the object, and so again a literal could be
used instead.

}
>
return instance;
}
}
})( );

The result of those changes could look like:-

var singletonClass = (function( ){
// Private variable
var instance;
return ({
constructor:nul l,
getInstance:fun ction( ){
return (
instance||
(
instance = {
constructor:nul l
}
)
);
}
});
})();

- though that is almost certainly less than would be needed in any real
context, but you will always suffer that if you don't define what it is
exactly that you are trying to achieve.

Richard.

[Peter Michaux]

Re: Singleton

On May 7, 1:42 am, Ugo <priv...@nospam .itwrote:

Hi guys,
>
how do you make a singleton access class?
>
Do you know a better way of this one:
>
var singletonClass = (function( )
{
// Private variable
var instance = null;
>
// Private Constructor
function myClass( )
{
//...
}
>
return new function( )
{
this.constructo r = null;
>
this.getInstanc e = function( )
{
if( ! instance )
{
instance = new myClass( );
instance.constr uctor = null;
}
>
return instance;
}
}
>
})( );

This does not look like something a JavaScript programmer would write.
In JavaScript, assigning an object literal to a global variable would
likely achieve the purpose of a Singleton pattern in Java, for
example, for many situations. What is your real application?

Peter

[Ugo]

Re: Singleton

>how do you make a singleton access class?

How do you define a "singleton access class"?

i wanted a function/object which restricts the instantiation of one my
object one time and which it ables one global access

>Do you know a better way of this one:

>
The best way of doing something depends at least in part on what it is
you are trying to do.

A generic way to create a singleton object (and if it's possible I'd like
to adopt the GoF's "roles" - applyed to JS)

>var singletonClass = (function( )
>{
> // Private variable
> var instance = null;

>
There is little point in assigning null to this variable as its default
undefined value will be just as false as the null value when you test it
with - !instance -.

IMHO, (new myClass()) can not be null|false|unde fined..
so that initialization was good

> // Private Constructor
> function myClass( )
> {
> //...
> }
>>
> return new function( )

>
I can think of no circumstances under which it makes sense to use the -
new - operator with a function expression as its operand. In this case
it would be simpler (and more efficient) to have an object literal
returned at this point. That object could easily define - constructor -
and - getInstance - properties.

ahh, in effect :P

> {
> this.constructo r = null;
>>
> this.getInstanc e = function( )
> {
> if( ! instance )
> {
> instance = new myClass( );
> instance.constr uctor = null;

>
As you are using a constructor to create this object instance it would
be possible to define the created object's - constructor property on the
prototype of the constructor. Though there may be no good reason for
using a contractor to create the object at all as you are only going to
be creating one instance of the object, and so again a literal could be
used instead.

Mmm

> }
>>
> return instance;
> }
> }
>})( );

>
The result of those changes could look like:-

let's look at

var singletonClass = (function( ){
// Private variable
var instance;
return ({
constructor:nul l,
getInstance:fun ction( ){
return (
instance||
(
instance = {
constructor:nul l
}
)
);
}
});
})();

Ok, it's no much different of mine, you have converted my code with object
literal

- though that is almost certainly less than would be needed in any real
context, but you will always suffer that if you don't define what it is
exactly that you are trying to achieve.

see before

THKS

[Peter Michaux]

Re: Singleton

On May 23, 7:22 am, Ugo <priv...@nospam .itwrote:

how do you make a singleton access class?

How do you define a "singleton access class"?

>
i wanted a function/object which restricts the instantiation of one my
object one time and which it ables one global access
>

Do you know a better way of this one:

>

The best way of doing something depends at least in part on what it is
you are trying to do.

>
A generic way to create a singleton object

I doubt that is what Richard meant by "what it is you are trying to
do". You are more likely trying to "implement a tabbed pane widget" or
"send user data to the server". Unless this is just an academic
exercise.

A generic way to create a singleton object with global access is to
use a object literal.

var someSingleton = {
someProperty: function() {},
someOtherProper ty: 55
};

Peter

[Thomas 'PointedEars' Lahn]

Re: Singleton

Ugo wrote:

IMHO, (new myClass()) can not be null|false|unde fined..

Yes, it can ;-)

function myClass()
{
this.toString = function() {
var a = [null, false, a];
return a[Math.floor(Math .random() * a.length)];
};
}

window.alert(ne w myClass());


PointedEars
--
Use any version of Microsoft Frontpage to create your site.
(This won't prevent people from viewing your source, but no one
will want to steal it.)
-- from <http://www.vortex-webdesign.com/help/hidesource.htm>

[P. Prikryl]

Re: Singleton

On 23. Máj, 20:45 h., Thomas 'PointedEars' Lahn <PointedE...@we b.de>
wrote:

Ugo wrote:

IMHO, (new myClass()) can not be null|false|unde fined..

>
Yes, it can ;-)

You are just plain wrong.

function myClass()
{
this.toString = function() {
var a = [null, false, a];
return a[Math.floor(Math .random() * a.length)];
};
}
>
window.alert(ne w myClass());

Irrelevant code - what does it prove?

[Peter Michaux]

Re: Singleton

On May 23, 11:45 am, Thomas 'PointedEars' Lahn <PointedE...@we b.de>
wrote:

Ugo wrote:

IMHO, (new myClass()) can not be null|false|unde fined..

>
Yes, it can ;-)
>
function myClass()
{
this.toString = function() {
var a = [null, false, a];
return a[Math.floor(Math .random() * a.length)];
};
}
>
window.alert(ne w myClass());

That is "null"|"false"| "undefined"

Try this

function myClass() {
this.toString = function() {
var a = [null, false, a];
return a[Math.floor(Math .random() * a.length)];
};
}

var inst = (new myClass());
if (inst === null ||
inst === false ||
inst === undefined) {
alert('Thomas is right');
}
else {
alert('Thomas is wrong');
}

This is, of course, the expected behavior according to ECMAScript
spec. Please see section 13.2.2.

Peter

P.S. I do appreciate the humor. I was just playing Thomas.

Footnotes:
"P.S." is abbreviation for "post scriptum", a Latin expression meaning
"after writing"

[Thomas 'PointedEars' Lahn]

Re: Singleton

P. Prikryl wrote:

Thomas 'PointedEars' Lahn wrote:

>Ugo wrote:

>>IMHO, (new myClass()) can not be null|false|unde fined..

>Yes, it can ;-)

You are just plain wrong.

Or I could have made a joke.

> function myClass()
> {
> this.toString = function() {
> var a = [null, false, a];
> return a[Math.floor(Math .random() * a.length)];
> };
> }
>>
> window.alert(ne w myClass());

Irrelevant code - what does it prove?

It has proven that Google Groups and your irony detector are borken.


PointedEars
--
realism: HTML 4.01 Strict
evangelism: XHTML 1.0 Strict
madness: XHTML 1.1 as application/xhtml+xml
-- Bjoern Hoehrmann

[Ugo]

Re: Singleton

>>The best way of doing something depends at least in part on what it is

>>you are trying to do.

>A generic way to create a singleton object

I doubt that is what Richard meant by "what it is you are trying to
do". You are more likely trying to "implement a tabbed pane widget" or
"send user data to the server". Unless this is just an academic
exercise.

all this things :P

A generic way to create a singleton object with global access is to
use a object literal.
>
var someSingleton = {
someProperty: function() {},
someOtherProper ty: 55
};

But, in this way I can not to declare "private" variables/methods in an
elegant way:

I don't like it:

var singleton;

(function()
{
// "privare" prop
var a = 1;
// "private" method
function b( )
{
alert('hi');
}

// Obj
singleton = {
hi : b,
a: a
};
})();

[VK]

Re: Singleton

On May 24, 7:58 pm, Ugo <priv...@nospam .itwrote:

>The best way of doing something depends at least in part on what it is
>you are trying to do.
A generic way to create a singleton object

I doubt that is what Richard meant by "what it is you are trying to
do". You are more likely trying to "implement a tabbed pane widget" or
"send user data to the server". Unless this is just an academic
exercise.

>
all this things :P
>

A generic way to create a singleton object with global access is to
use a object literal.

>

var someSingleton = {
someProperty: function() {},
someOtherProper ty: 55
};

>
But, in this way I can not to declare "private" variables/methods in an
elegant way:
>
I don't like it:
>
var singleton;
>
(function()
{
// "privare" prop
var a = 1;
// "private" method
function b( )
{
alert('hi');
}
>
// Obj
singleton = {
hi : b,
a: a
};
>
})();

The way I'm using most often - without any claims that it is the best
out of anything:

function MySingleton() {
if (this == self) {
window.alert('C onctructor called as a function');
return MySingleton;
}
else if (
!!($isInstantia ted.flag) ||
(this instanceof MySingleton) ||
(MySingleton.is PrototypeOf(thi s))
) {
window.alert('O nly one instance is allowed');
return mySingleton;
}
else {
// instantiate your singleton
// with methods and properties
// ...
this.toString = $toString;
$isInstantiated .flag = true;
}
function $isInstantiated () {
return arguments.calle e.flag;
}
function $toString() {
return ''.concat(
'function mySingleton() {\n',
' [native code]\n',
'}'
);
}
}

The benefits as I see them:
1) It is much lesser convoluted as function expression ways.
Respectively it is easier to read, to maintain and to avoid parasite
closures.
2) It allows to distinct different "not allowed" situation so to
provide more informative errors - in a real case window.alert replaced
by throw new Error of course.
3) it prevents or at least makes much harder to use reliably runtime
cloning over source code dumping and reusing in eval or new Function.