What is the best way in D to declare a singleton? Currently I use:

// the singleton type
class Bar
{
    void func();
}

// the singleton instance
struct Foo
{
   static Bar bar;

   static this()
   {
   bar = new Bar();
   }
}

// usage
void test()
{
    Foo.bar.func();
}

However, a user can happily instantiate additional instances of the singleton simply by

void test()
{
    Foo myFoo;
}

which is not cool. Also, a user can {Foo.bar = null;} since bar is not const. I tried using the 'final' keyword, but that doesn't appear to have an effect.

Ideas? Comments?

- Kris

Just make the ctor private.

But I tend to agree with other commentators on this NG on this subject, which is that D's module functionality - which, btw, is deterministic, unlike that of C++ - renders the representation of singletons as objects rather jejune. :-)


"Kris" <someidiot@earthlink.net> wrote in message news:c420o8$65e$1@digitaldaemon.com...
> What is the best way in D to declare a singleton? Currently I use:
>
> // the singleton type
> class Bar
> {
>     void func();
> }
>
> // the singleton instance
> struct Foo
> {
>    static Bar bar;
>
>    static this()
>    {
>    bar = new Bar();
>    }
> }
>
> // usage
> void test()
> {
>     Foo.bar.func();
> }
>
> However, a user can happily instantiate additional instances of the singleton simply by
>
> void test()
> {
>     Foo myFoo;
> }
>
> which is not cool. Also, a user can {Foo.bar = null;} since bar is not const. I tried using the 'final' keyword, but that doesn't appear to have
an
> effect.
>
> Ideas? Comments?
>
> - Kris
>
>

Kris wrote:
> 
> Ideas? Comments?
> 
> - Kris
> 
> 

Use a module:

//Your singleton
module bar;

void func()
{

}

usage:

module xyz;

void another_func()
{
	bar.func();
}

Stephan

"Matthew" <matthew@stlsoft.org> wrote in message news:c422t8$9ji$1@digitaldaemon.com...
> Just make the ctor private.
>

Did you mean this Matthew?

class Foo
{
    private this(){}
}

the private attribute is thoroughly ignored by the compiler ...

BTW, I searched (via Outlook) for other threads on Singleton, but didn't
find anything. Is there someplace to search the "older" threads?
(digitalmars/d/index does not support search).

Thanks;

Cool. Thanks Stephan.

In this particular case, I'd really like to keep the extended namespace intact, as in Foo.bar.func().  Is that possible using the module-technique?

- Kris


"Stephan Wienczny" <wienczny@web.de> wrote in message news:c42345$9uo$1@digitaldaemon.com...
> Kris wrote:
> >
> > Ideas? Comments?
> >
> > - Kris
> >
> >
>
> Use a module:
>
> //Your singleton
> module bar;
>
> void func()
> {
>
> }
>
> usage:
>
> module xyz;
>
> void another_func()
> {
> bar.func();
> }
>
> Stephan
>

On Fri, 26 Mar 2004 12:49:56 -0800, Kris wrote:

[...]
> BTW, I searched (via Outlook) for other threads on Singleton, but didn't
> find anything.

http://www.digitalmars.com/drn-bin/wwwnews?D/21642
==
<bu0da8$1qic$1@digitaldaemon.com>

So long!

Kris wrote:

>Cool. Thanks Stephan.
>
>In this particular case, I'd really like to keep the extended namespace
>intact, as in Foo.bar.func().  Is that possible using the module-technique?
>
>- Kris
>  
>
Surround it in structs and make everything static.

-- 
-Anderson: http://badmama.com.au/~anderson/

Cheers Manfred!


"Manfred Nowak" <svv1999@hotmail.com> wrote in message news:c42omb$1bhu$1@digitaldaemon.com...
> On Fri, 26 Mar 2004 12:49:56 -0800, Kris wrote:
>
> [...]
> > BTW, I searched (via Outlook) for other threads on Singleton, but didn't
> > find anything.
>
> http://www.digitalmars.com/drn-bin/wwwnews?D/21642
> ==
> <bu0da8$1qic$1@digitaldaemon.com>
>
> So long!

"Kris" <someidiot@earthlink.net> wrote in message news:c424kb$c4f$1@digitaldaemon.com...
> "Matthew" <matthew@stlsoft.org> wrote in message news:c422t8$9ji$1@digitaldaemon.com...
> > Just make the ctor private.
> >
>
> Did you mean this Matthew?
>
> class Foo
> {
>     private this(){}
> }

Yep.

> the private attribute is thoroughly ignored by the compiler ...

It sounds flippant, but I wouldn't worry about it. You shouldn't get confused between a language feature and a compiler bug, which will likely be fixed in the next release (as most do).

Thanks.

"Matthew" <matthew@stlsoft.org> wrote in message news:c4dkp1$8hh$3@digitaldaemon.com...
>
> "Kris" <someidiot@earthlink.net> wrote in message news:c424kb$c4f$1@digitaldaemon.com...
> > "Matthew" <matthew@stlsoft.org> wrote in message news:c422t8$9ji$1@digitaldaemon.com...
> > > Just make the ctor private.
> > >
> >
> > Did you mean this Matthew?
> >
> > class Foo
> > {
> >     private this(){}
> > }
>
> Yep.
>
> > the private attribute is thoroughly ignored by the compiler ...
>
> It sounds flippant, but I wouldn't worry about it. You shouldn't get confused between a language feature and a compiler bug, which will likely
be
> fixed in the next release (as most do).
>
>
>
>