"Chris Tavares" <christophertavares@[...].net>  writes:
>  At the start of this thread, your post implied that 'data hiding' meant
>  something similar to C++ or Java's access specifiers: private, public, etc.
>  But the stuff you mention above has nothing to do with that! The applet
>  sandbox is a specific mode provided by the runtime - and rexec/bastion is
>  NOT a hack, it's just another way to implement a sandbox.

A private variable in a java class instance is intended to really be 
private, that is, it's supposed to be secure against access from code
outside the class implementation, even to code not running in a sandbox.
The sandbox is a separate mechanism and rexec/bastion does provide
something like it, but that's not always appropriate.

>  As far as security - well, I don't know how stuff like that is
>  supposed to work, but in my experience that's an OS issue rather
>  than a language issue.  Python compiles and runs on almost every
>  platform under the sun, and lots of them don't have ANY kind of
>  security, so what should it do? Implement some gigantic code-access
>  security features that most people don't need?

Obviously Python (or a JVM) can't be responsible for what happens
outside the interpreter.  If the application can access raw hardware
memory through the file system (/dev/mem) then it can get to anything
in the program.  But assuming there's no such external interfaces
then the security can be provided.

>  And in any event, C++ access specifiers don't do much but provide a
>  challenge to those "rogue programmers" you mentioned, and they don't
>  help at all in any of the above situations. So I'm curious what you
>  really mean, and what you really want.

C++ can't provide security for private variables and doesn't try to.
Because C++ lets you dereference naked pointers, if you know the
definiition of class x, you can figure out the offset of private
variable x.a, and cast &x to an int pointer and get the data from
that offset.  Java doesn't have pointers.  It really can and does
enforce the privacy.
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