Re: Parameterless generic instance in place of opaque type instance

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Sun, 28 Jul 2013 08:36:36 +0200, Yannick Duchêne (Hibou57)  
<yannick_duchene@yahoo.fr> a écrit:
> The Ada 95 Q&S Guide mention this pattern, and seems to have no more to  
> say about it, not more than what is obvious:
> http://www.adaic.org/resources/add_content/docs/95style/html/sec_8/8-3-5.html
> They call this pattern, “Generic Abstract Data Object”.

An idea, and there may have missed another interesting variation,  
something with mixed properties from a GADO and an ADT.

Say a GADO still defines an ADT, with an unknown discriminant, so that it  
cannot be instantiated without initialization. What the private declares,  
is just an access type to a deferred type (defined in the body). The body  
defines an instance of the type it fully defines. The package offers a  
single function returning an (opaque) access to this unique and  
per‑paquage instance, instance. The function could be named  
`Instance_Reference`, to clearly show it's always the same behind, however  
a new distinct one per generic instance (not strictly a singleton, a  
per‑package singleton).

This would provide some of the benefit of an ADT and some of a GADO,  
together. The same as an ADT regarding the ability to pass it to a  
sub‑program, while avoiding the heap allocation (which I surely don't  
want) otherwise required with an access to a deferred type (or else an  
allocation in a static pool, … not so nice for many reasons) and avoiding  
to pollute the private part of the package (I don't enjoy when the private  
part monopolizes 85% of a package's text).

This could be named, say “Generic Abstract Singleton Data Type”, or  
something like that :-D

What's you feeling on that variation?

-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University