Re: Ada Singleton Pattern

[Luca Stasio <stasio2000@tin.it>]

Florian Weimer ha scritto:
> * Dmitry A. Kazakov:
> 
> 
>>On Mon, 13 Sep 2004 15:04:27 GMT, Luca Stasio wrote:
>>
>>
>>>Hi, there is a way to implement the Singleton Pattern in Ada?
>>>There are some examples out there?
>>>Thanx.
>>
>>package Foo is
>>   type Singleton (<>) is limited private;
>>   ... -- Public interface subroutines
>>
>>   The_Only_One : constant Singleton; -- The value
>>private
>>   type Singleton is new Integer; -- Any implementation you want
>>   The_Only_One : constant Singleton := 5;
>>
>>The public view of Singleton is unconstrained and limited, which prevents
>>it from either being copied or declaring new objects of this type.
> 
> 
> This is only a partial solution, unfortunately.
> 
> Ada *requires* that the package initialization code runs before
> The_Only_One is accessed.  This is different from many other languages
> (where the initialization order of global objects is mostly
> unspecified and not related to actual usage patterns) and solves a big
> chunk of the singleton problem.
> 
> However, you still have to deal with termination on your own: you must
> ensure that no client is still using the singleton object before you
> destroy it.  There is no direct support in the language for this part
> of the problem, and you have to use one of the traditional approaches
> if this matters to your application (or assignment 8-).
> 
> (I tried to avoid Ada-specific terminology above because Luca probably
> isn't familiar with the Ada language.)

Hi, is the one from Martin Dowie and Steve a right solution?
Thanx a lot.

Luca Stasio