Re: [Xotcl] forwarding

From: Gustaf Neumann <neumann_at_wu-wien.ac.at>
Date: Tue, 24 Jan 2012 09:01:16 +0100

Dear Victor,

The best approach for forwarding to a child object is either
an alias, or a direct invocation of a subobject through the
method interface. Consider the following code, where we have
an object "obj" and an embedded child object "obj::child".

    nx::Object create obj {

       nx::Object create [self]::child {
       :public method foo {} {return [self]}
     }

     :public alias a [self]::child
     :public forward f [self]::child
    }


One could certainly use as well a forward, but this is
necessary. Technically, objects are Tcl commands, and Tcl
commands and procs reside in a namespace. Therefore, by
creating a child object of obj, one has to create in its
namespace a cmd "child", which is in turn very similar to a
per-object method. Therefore, one cannot define a per-object
method "foo" and a child object "foo" at the same time.

The idiom to use an object as a container for subobject and
delegate to these is quite common in some systems (used
intensively in xotclIDE). In our previous releases of nx,
the semantics of calling of the subobject have changed
relative to XOTcl 1.6 due to the requirements of ensemble
objects. The recent changes reintroduced backwards
compatibility in this respect as well, leading to a probably
more expected behavior in the case above (concerning "self"
in the methods of the subobject). As you can see below the
invocation of the child is the fastest approach.

   ? {obj a foo} {::obj::child}
   ? {obj f foo} {::obj::child}
   ? {obj child foo} {::obj::child}

v/child-obj-delegation.001: 0.71 mms, obj a foo
(overhead 0.62)
v/child-obj-delegation.002: 0.75 mms, obj f foo
(overhead 0.65)
v/child-obj-delegation.003: 0.65 mms, obj child foo
(overhead 0.68)

Note, that this requires the head version of nx (which works
now as well with Tcl version from fossil head); we will tag
a new beta release soon, documentation updates are pending.

In case, one does not want to allow calls of the subobject
via the method interface (3rd case above), one can use
access control ("private" or "protected"). Maybe we add for
nx some more syntactic sugar by allowing something along the
lines of

    nx::Object create obj {

     :public child nx::Object bar {
        :public method foo {} {return [self]}
     }

     :public alias a [self]::child
     :public forward f [self]::child
    }


for creating a child object "obj::bar" with public access.

-gustaf neumann

On 11.01.12 04:06, Victor Mayevski wrote:
> Is it possible to do a forward in the following manner:
>
> obj public forward link %self::child-obj
>
> The best I could do was:
>
> obj public forward link :child-obj
>
> which I have to call in the following way:
> [obj link] mymethod
>
> Also, I don't even know why it works that way, because I think it
> should work this way:
> obj public forward link -objframe :child-obj
> but that does not work.
>
> Thanks
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