Re: Ada Singleton Pattern

[Luca Stasio <stasio2000@tin.it>]

Matthew Heaney ha scritto:

> Luca Stasio <stasio2000@tin.it> writes:
> 
> 
>>Hi, there is a way to implement the Singleton Pattern in Ada?  There
>>are some examples out there?  Thanx.
> 
> 
> I prefer to do it this way:
> 
> package P is
>    pragma Elaborate_Body;
> 
>    type T (<>) is limited private;
>    type T_Access is access all T;
> 
>    function Object return T_Access;
> 
>    procedure Op (O : access T);
>    ...
> private
>    type T is limited record ... end record;
> end;
> 
> package body P is
>    State : aliased T;
> 
>    function Object return T_Access is
>    begin
>       return State'Access;
>    end;
>    ...
> end P;
> 
> Technically, since there's only one object, then you don't really need
> to declare the state as components of type T.  T can just be a dummy
> type (type T is limited null record;), and operations (like Op) can just
> ignore the object passed as a parameter, and manipulate package state
> directly.
> 
> But all of this is just syntactic overhead.  You can always get rid of
> the type declaration, and just let the operations manipulate the package
> state.  (Booch calls this an "abstract state machine.")
> 
> Of course, if this is a type hierarchy, then you probably do need a type
> (since packages aren't "first class citizens," to use Wegner's phrase).
> 

Hi... thank you too !!!