[PD] test if object-class available

Mathieu Bouchard matju at artengine.ca
Mon Aug 10 03:36:25 CEST 2009

On Fri, 3 Apr 2009, Enrique Erne wrote:

> My workaround is an abstraction with the name dyn~, this sends a message 
> "dyn~ not available" if it gets loaded. Then I found out that pd-0.42 
> can do object aliasing (not sure if that is the correct term)

I didn't see any reply at all to this, so, I'm gonna have a try at it, 
even though it was 4 months ago.

if I try to guess what you mean, you mean how a class being loaded can 
override a previously-loaded class. well, this only happens if a class is 
being loaded, and pd doesn't look for it if it already knows the name. so, 
normally, it depends on a -lib option or anything else that acts like a 
-lib option.

Now, there is something else called aliasing, which can occur in 
combination with the above or not, and exists since the beginning of Pd. 
When people don't claim something like "[t] is an object", they'd claim 
the more correct "[t] is a class", which is how I'd say it, even though 
it's not exact. The exact thing is that [t] is a name of a class, and it's 
not the main name, as [trigger] is the main name, but from a user's 
perspective, both names are equal. they correspond to the exact same 

There's also an opposite to that, which is that a single name can 
correspond to several classes. This is the case of [sel], for example.

The only way I can think of having your [dyn~] thing work is by making the 
relevant -path directive the last folder in the search-path and making 
sure it stays that way. But I wouldn't like to do that. GridFlow 0.9.5 now 
adds a method called "else" in Pd's builtins (yes, an external can do 
that!) which is used for exactly what you want, but only during dynamic 
patching. Well, no, actually it could also work in hand-modified .pd 
files) :

   #X obj 666 42 dyn~ blah blah;
   #X else obj 666 42 dyn~-was-not-found blah blah;

and you define [dyn~-was-not-found] as something that apologises for the 
hopefully temporary inconvenience.

What "else" does is create an object ONLY IF the last created object 
became a dashed object, and in doing so, it deletes the dashed object so 
that it can pretend that the replacement is really what was intended. 
(saving this patch will save whichever actually loaded object as a normal 
object and the "else" will disappear).

> which i find a very confusing feature. I was worried that my workaround 
> wouldn't work anymore if the abstraction would "overwrite" the external, 
> but it seems all fine. So the question is are there plans to make 
> abstractions alias object classes ? (i hope not :) )

an abstraction is an object class.

but abstractions can't do the new 0.42 trick because they can't do any 
class_new or class_addcreator themselves, so they can't register any name 
on their own, so each file can only contain one abstraction whose name is 
the same as the file.

with the 0.42 trick, the name is overridden and not overwritten, but you 
can't tell the difference. with the same thing before 0.42, the new name 
seems to be disappearing, but it's really there and overridden by the old 
one. You can also use "evil" trickery to add, replace, reorder or remove 
any name, with any version of Pd so far, but then, it's risky.

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal, Québec

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