immutable singleton pattern with static opCall

[Steven Schveighoffer]

On Sat, 26 Jun 2010 20:19:44 -0400, Michal Minich  
<michal.minich at gmail.com> wrote:

> On Sun, 27 Jun 2010 09:36:04 +0930, Justin Johansson wrote:
>
>> immutable class Foo
>> {
>>     static private Foo instance;
>>
>>     static this() {			// line 9
>>        instance = new Foo;
>>     }
>>
>>     static Foo opCall() {		// line 13
>>        return instance;
>>     }
>> }
>>
>> test.d(9): Error: function test.Foo._staticCtor2 without 'this' cannot
>> be const/immutable
>> test.d(13): Error: function test.Foo.opCall without 'this' cannot be
>> const/immutable
>
> there is bug report on this subject
>
> http://d.puremagic.com/issues/show_bug.cgi?id=3598
>
> Your example also uses static variable, which was not considered in the
> bug report. This makes things more complex to design properly, because it
> seems now that one does not want static functions to be affected by
> immutable attribute of class, but it should affect static data...

static variables inside an immutable class should be immutable.  That was  
considered in the bug report and purposefully left out.

The issue is that the compiler incorrectly labels static *functions* as  
immutable which makes no sense, static functions cannot be immutable  
ever.  Static data can be.

BTW, you can work around this problem like this:

class Foo
{
    static immutable instance; // no need to make private, it can never  
change

    static this() {
       instance = new Foo;  // not sure if this works, you may have to cast.
    }

    static immutable(Foo) opCall() {
       return instance;
    }

    immutable:

    // member functions
}

-Steve