Re: Ada-Singleton-Why does it work like this?

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

patrick.gunia@googlemail.com schrieb:

> Still I don�t get how the unconstrained type limits the
> number of instances to 1.

It does, but outside package Singleton only.

The (<>) forces initialization.
The "limited" part prevents assignment (thus copying).
The type is private.

Any object to be created in whichever way must be
initialized because of (<>). Initialization is not
possible outside the package because Singleton_Type
has discriminants unknown outside the package.

Also, any object of the public Singleton_Type cannot be
assigned a new value (because it is limited).

The only place where objects of type Singleton_Type
can be created is where the full definition
of type Singleton_Type is visible.

The only way to get a reference to the object is
through function return_Single. This function
always returns the same access value, hence clients
of package Singleton always use the same (invisible)
object.