digitalmars.D - how to implement singleton pattern for D?

clayasaurus (5/5) Jun 17 2004 hi, i tried converting the c++ singleton from

Mike Swieton (44/49) Jun 17 2004 Some of the specifics are different. There are odd details in C++ with w...

clayasaurus (42/91) Jun 18 2004 when i try to compile the code

Regan Heath (6/122) Jun 18 2004 rename the instance() method. getObj() works.

Chr. Grade (5/148) Jun 19 2004 To express what I think about singletons and some other naive design

Ant (4/10) Jun 18 2004 maybe a singleton is never necessary,

Mike Swieton (15/28) Jun 20 2004 Well, the biggest problem we've run into here is that they're difficult ...

Andy Friesen (3/8) Jun 18 2004 Here's one.

clayasaurus <clayasaurus_member pathlink.com> writes:

hi, i tried converting the c++ singleton from
http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D but i'm having
a lot of problems trying to convert it to D code. It is just a simple class but
I'm not quite sure how to achieve the same effect in D. Any help is appreciated.
Thanks.

Jun 17 2004

Mike Swieton <mike swieton.net> writes:

On Fri, 18 Jun 2004 02:23:17 +0000, clayasaurus wrote:

 hi, i tried converting the c++ singleton from
 http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D but i'm
having
 a lot of problems trying to convert it to D code. It is just a simple class but
 I'm not quite sure how to achieve the same effect in D. Any help is
appreciated.
 Thanks.

Some of the specifics are different. There are odd details in C++ with which
methods are written for you, which methods you need to use to prevent
inherited class from breaking singletonnes, etc. Try something along the lines
of this:

class mySingleton
{
private:
	static mySingleton _instance = null;

	static this()
	{
		_instance = new mySingleton;
	}

	static ~this()
	{
		// this is probably reduntant
		delete _instance;
	}

	this()
	{
		// init...
	}

	~this()
	{
		// deinit...
	}

public:
	static mySingleton instance()
	{
		return _instance;
	}

	...
}

I haven't tested it, but it ought to work, or at least be pretty close. You
also could put the instantiation in mySingleton.instance(), which is the C++
idiom for singletons, but I think the idea of a singleton fits better with the
static constructor.

The assignment operator overload and the copy constructor doesn't apply in D,
obviously.

If this doesn't work or isn't helpful, why not post some code?

Mike Swieton
__
We are all born originals - why is it so many if us die copies?
	- Edward Young

Jun 17 2004

clayasaurus <clayasaurus_member pathlink.com> writes:

In article <pan.2004.06.18.03.57.53.836604 swieton.net>, Mike Swieton says...
On Fri, 18 Jun 2004 02:23:17 +0000, clayasaurus wrote:

 hi, i tried converting the c++ singleton from
 http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D but i'm
having
 a lot of problems trying to convert it to D code. It is just a simple class but
 I'm not quite sure how to achieve the same effect in D. Any help is
appreciated.
 Thanks.

Some of the specifics are different. There are odd details in C++ with which
methods are written for you, which methods you need to use to prevent
inherited class from breaking singletonnes, etc. Try something along the lines
of this:

class mySingleton
{
private:
	static mySingleton _instance = null;

	static this()
	{
		_instance = new mySingleton;
	}

	static ~this()
	{
		// this is probably reduntant
		delete _instance;
	}

	this()
	{
		// init...
	}

	~this()
	{
		// deinit...
	}

public:
	static mySingleton instance()
	{
		return _instance;
	}

	...
}

I haven't tested it, but it ought to work, or at least be pretty close. You
also could put the instantiation in mySingleton.instance(), which is the C++
idiom for singletons, but I think the idea of a singleton fits better with the
static constructor.

The assignment operator overload and the copy constructor doesn't apply in D,
obviously.

If this doesn't work or isn't helpful, why not post some code?

Mike Swieton
__
We are all born originals - why is it so many if us die copies?
	- Edward Young


when i try to compile the code


class mySingleton
{
private:
static mySingleton _instance = null;

static this()
{
_instance = new mySingleton;
}

static ~this()
{
// this is probably reduntant
delete _instance;
}

this()
{
// init...
}

~this()
{
// deinit...
}
public:
static mySingleton instance()
{
return _instance;
}
//	...
}

int main(char[][] args)
{


return 0;
}

i get the errors

singleton.d(28): no identifier for declarator
singleton.d(28): semicolon expected, not 'instance'
singleton.d(28): TemplateIdentifier expected following instance
singleton.d(28): ';' expected after template instance
singleton.d(28): Declaration expected, not '('

singleton.d(33): unrecognized declaration

and i'm not quite sure how to fix it. can you help me? thanks.

Jun 18 2004

Regan Heath <regan netwin.co.nz> writes:

On Fri, 18 Jun 2004 19:22:21 +0000 (UTC), clayasaurus 
<clayasaurus_member pathlink.com> wrote:

 In article <pan.2004.06.18.03.57.53.836604 swieton.net>, Mike Swieton 
 says...
 On Fri, 18 Jun 2004 02:23:17 +0000, clayasaurus wrote:

 hi, i tried converting the c++ singleton from
 http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D but 
 i'm having
 a lot of problems trying to convert it to D code. It is just a simple 
 class but
 I'm not quite sure how to achieve the same effect in D. Any help is 
 appreciated.
 Thanks.

 Some of the specifics are different. There are odd details in C++ with 
 which
 methods are written for you, which methods you need to use to prevent
 inherited class from breaking singletonnes, etc. Try something along 
 the lines
 of this:

 class mySingleton
 {
 private:
 	static mySingleton _instance = null;

 	static this()
 	{
 		_instance = new mySingleton;
 	}

 	static ~this()
 	{
 		// this is probably reduntant
 		delete _instance;
 	}

 	this()
 	{
 		// init...
 	}

 	~this()
 	{
 		// deinit...
 	}

 public:
 	static mySingleton instance()
 	{
 		return _instance;
 	}

 	...
 }

 I haven't tested it, but it ought to work, or at least be pretty close. 
 You
 also could put the instantiation in mySingleton.instance(), which is 
 the C++
 idiom for singletons, but I think the idea of a singleton fits better 
 with the
 static constructor.

 The assignment operator overload and the copy constructor doesn't apply 
 in D,
 obviously.

 If this doesn't work or isn't helpful, why not post some code?

 Mike Swieton
 __
 We are all born originals - why is it so many if us die copies?
 	- Edward Young


 when i try to compile the code


 class mySingleton
 {
 private:
 static mySingleton _instance = null;

 static this()
 {
 _instance = new mySingleton;
 }

 static ~this()
 {
 // this is probably reduntant
 delete _instance;
 }

 this()
 {
 // init...
 }

 ~this()
 {
 // deinit...
 }
 public:
 static mySingleton instance()
 {
 return _instance;
 }
 //	...
 }

 int main(char[][] args)
 {


 return 0;
 }

 i get the errors

 singleton.d(28): no identifier for declarator
 singleton.d(28): semicolon expected, not 'instance'
 singleton.d(28): TemplateIdentifier expected following instance
 singleton.d(28): ';' expected after template instance
 singleton.d(28): Declaration expected, not '('

 singleton.d(33): unrecognized declaration

 and i'm not quite sure how to fix it. can you help me? thanks.

rename the instance() method. getObj() works.

Regan


-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

Jun 18 2004

"Chr. Grade" <tickle everymail.net> writes:

To express what I think about singletons and some other naive design 
patterns, Dickens needs to be quoted:
"How Mr. Winkle, instead of shooting at the pigeon and killing the crow, 
shot at the crow and wounded the pigeon."

Regan Heath wrote:
 On Fri, 18 Jun 2004 19:22:21 +0000 (UTC), clayasaurus 
 <clayasaurus_member pathlink.com> wrote:
 
 In article <pan.2004.06.18.03.57.53.836604 swieton.net>, Mike Swieton 
 says...

 On Fri, 18 Jun 2004 02:23:17 +0000, clayasaurus wrote:

 hi, i tried converting the c++ singleton from
 http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D 
 but i'm having
 a lot of problems trying to convert it to D code. It is just a 
 simple class but
 I'm not quite sure how to achieve the same effect in D. Any help is 
 appreciated.
 Thanks.


 Some of the specifics are different. There are odd details in C++ 
 with which
 methods are written for you, which methods you need to use to prevent
 inherited class from breaking singletonnes, etc. Try something along 
 the lines
 of this:

 class mySingleton
 {
 private:
     static mySingleton _instance = null;

     static this()
     {
         _instance = new mySingleton;
     }

     static ~this()
     {
         // this is probably reduntant
         delete _instance;
     }

     this()
     {
         // init...
     }

     ~this()
     {
         // deinit...
     }

 public:
     static mySingleton instance()
     {
         return _instance;
     }

     ...
 }

 I haven't tested it, but it ought to work, or at least be pretty 
 close. You
 also could put the instantiation in mySingleton.instance(), which is 
 the C++
 idiom for singletons, but I think the idea of a singleton fits better 
 with the
 static constructor.

 The assignment operator overload and the copy constructor doesn't 
 apply in D,
 obviously.

 If this doesn't work or isn't helpful, why not post some code?

 Mike Swieton
 __
 We are all born originals - why is it so many if us die copies?
     - Edward Young


 when i try to compile the code


 class mySingleton
 {
 private:
 static mySingleton _instance = null;

 static this()
 {
 _instance = new mySingleton;
 }

 static ~this()
 {
 // this is probably reduntant
 delete _instance;
 }

 this()
 {
 // init...
 }

 ~this()
 {
 // deinit...
 }
 public:
 static mySingleton instance()
 {
 return _instance;
 }
 //    ...
 }

 int main(char[][] args)
 {


 return 0;
 }

 i get the errors

 singleton.d(28): no identifier for declarator
 singleton.d(28): semicolon expected, not 'instance'
 singleton.d(28): TemplateIdentifier expected following instance
 singleton.d(28): ';' expected after template instance
 singleton.d(28): Declaration expected, not '('

 singleton.d(33): unrecognized declaration

 and i'm not quite sure how to fix it. can you help me? thanks.

 
 
 rename the instance() method. getObj() works.
 
 Regan

Jun 19 2004

Ant <duitoolkit yahoo.ca> writes:

On Sat, 19 Jun 2004 04:54:23 -0700, Chr. Grade wrote:

 
 To express what I think about singletons and some other naive design 
 patterns, Dickens needs to be quoted:
 "How Mr. Winkle, instead of shooting at the pigeon and killing the crow, 
 shot at the crow and wounded the pigeon."
 

maybe a singleton is never necessary,
but what are the dangers of using one?

Ant

Jun 18 2004

Mike Swieton <mike swieton.net> writes:

On Fri, 18 Jun 2004 23:35:18 -0400, Ant wrote:

 On Sat, 19 Jun 2004 04:54:23 -0700, Chr. Grade wrote:
 
 
 To express what I think about singletons and some other naive design 
 patterns, Dickens needs to be quoted:
 "How Mr. Winkle, instead of shooting at the pigeon and killing the crow, 
 shot at the crow and wounded the pigeon."
 

 
 maybe a singleton is never necessary,
 but what are the dangers of using one?
 
 Ant

Well, the biggest problem we've run into here is that they're difficult to
test. The most significant issue is that it's more difficult to ensure that
the program is in a clean state before each unit test. Of course, D has no
setUp/tearDown facility in it's unit testing anyway (D's unit testing has many
such oversights. Please, Walter, let us fix it!).

Generally, singleton's are not a good replacement for a global variable:
that's not their purpose. My general rule is that they should only be used
to represent something that there really is only one of, hardware, for
instance being the canonical example.

Mike Swieton
__
I figure that if God actually does exist, He's big enough to understand an
honest difference of opinion.
	- Isaac Asimov

Jun 20 2004

Andy Friesen <andy ikagames.com> writes:

clayasaurus wrote:
 hi, i tried converting the c++ singleton from
 http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp to D but i'm
having
 a lot of problems trying to convert it to D code. It is just a simple class but
 I'm not quite sure how to achieve the same effect in D. Any help is
appreciated.
 Thanks. 

Here's one.

  -- andy

Jun 18 2004

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digitalmars.D - how to implement singleton pattern for D?