Singleton Pattern with struct

[Maxim Fomin]

On Thursday, 24 January 2013 at 15:50:34 UTC, ParticlePeter wrote:
> Got it, thanks, I changed the instance method to:
> [code]
> 	static Singleton * instance()  {
> 		if ( s is null )
> 			s = new Singleton( 0 ) ;
> 		return s ;
> 	}
> [\code]
>
> and everything works as expected.
>
> Cheers, PP !

Yes, but this can be broken by:

import core.stdc.stdio : printf;

struct Singleton  {

private :
         this( int a = 0 ) {} ;
         static Singleton * s ;

public :
         @disable this() ;
         static Singleton* instance()  {
                 if ( s is null )
                         s = new Singleton(0) ;
                 return s ;
         }

         int val = 0 ;
}

void main()
{
         Singleton s = * Singleton.instance;
         printf( "%d\n", s.val ) ; //
         Singleton.instance.val = 2 ;
         printf( "%d\n", s.val ) ; //0
}

Here s is explicitly defined to be a struct object, not pointer 
(reference), so main.s is independent of further modification of 
Singleton.instance.