Re: [TCLCORE] TclOO and "protected" methods

[Donal K. F. <don...@ma...>]

On 08/11/2010 18:19, Tom Jackson wrote:
> If there are three classes (http://rmadilo.com/files/tcloo/protected/):

It's taken me a bit to work through this example.  The notion of 
"protected" that we are using isn't one of guarding of the methods (for 
that, people should use code in C or multiple interpreters) so much as 
of restricting what methods can be called through the official entry 
point.  (Since it's pretty easy to work out what [my] is called for a 
particular object, any method on that object can be invoked without too 
much difficulty if you really want; [info object namespace] is the core 
of that particular trick.)  You *can* do the sort of protection by 
overriding the unknown method:

   method unknown {name args} {
       if {
           [catch {self caller} context] == 0
           && [canAccess [self] $name $context]
       } then {
           tailcall my $name {*}$args
       }
       return [next $name {*}$args]
   }

(I leave out the details of [canAccess] here.)

The problem with this is that method calls where the method name is 
unknown are not cached, so the above mechanism is rather slow.  I don't 
cache the mappings of unknown method names to implementations as there's 
an infinite number of such names, and adding caching of them would also 
make cache spoilage costs a bigger problem (right now, most code doesn't 
pay the cache spoilage costs at all; that's good because they're quite 
large).

(Other variations on the theme of protection are possible through the 
use of filters - the [self] command provides most of the tools required, 
and any that are missing are things I should be told about so I can fix 
the lack - though I don't feel like going into depth now.)

----

A quick refresher on [self caller]:

   % oo::class create Foo {
       method a args {my {*}$args}
       method b {} {return [self caller]}
   }
   ::Foo
   % Foo create bar
   ::bar
   % bar a b
   ::Foo ::bar a
   % bar b
   caller is not an object

As you can see, it reports information about how a method was called 
that is (in theory at least) enough to define access control rules.

Donal.