From: "Randy Brukardt" <randy@rrsoftware.com>
Subject: Re: Problems using Generic_Dispatching_Constructor
Date: Wed, 1 Jun 2022 16:25:16 -0500 [thread overview]
Message-ID: <t78lfu$rve$1@dont-email.me> (raw)
In-Reply-To: t77mrv$1e20$1@gioia.aioe.org
"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:t77mrv$1e20$1@gioia.aioe.org...
> On 2022-06-01 13:36, Mark Lorenzen wrote:
>
>> Why are access types used at all?
We needed this usable to implement dispatching stream attributes (the
generic dispatching constructor was intended to be a user-definable
generalization of the mechanism of the class-wide stream attribute). The
stream attributes probably used access types because "in out" parameters
were not allowed for functions when they were invented. (So mistakes piled
on mistakes. :-)
> Parameters are kind of factory object, you want to have the factory
> mutable.
Right. For instance, consider a factory where each object gets a unique ids
while being constructed. You would want to update the Next_Id component at
the end of each construction.
>> Is there another workaround?
>
> In my practice I never had a case when I could obtain the tag needed for
> generic dispatching constructor. All my designs ended up with a mapping
>
> key -> constructing function
>
> with an explicit registering the type in the mapping.
Right. Generally, one uses a mapping of some sort of key or menu choice or
whatever to tags. If you aren't adverse to a giant case statement, then you
might as well call the constructor directly. (And if you are willing to use
access-to-functions, you don't need OOP at all.) So this "factory" is mostly
a bone for OOP purists.
The one exception is the case where you have an external tag as the key,
since you can get the tag from that directly. But even that is really a
mapping (one built by the implementation).
Randy.
prev parent reply other threads:[~2022-06-01 21:25 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-01 11:36 Problems using Generic_Dispatching_Constructor Mark Lorenzen
2022-06-01 12:42 ` Dmitry A. Kazakov
2022-06-01 21:25 ` Randy Brukardt [this message]
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