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.

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

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.

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

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/

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
> 
> 

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

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