Re: How to? Writeable from a readable?

["chris.danx" <spamoff.danx@ntlworld.com>]

John R. Strohm wrote:
> Maybe I'm way out in left field here.
> 
> In capability-based operating system terms, it sounds as though you are
> trying to take a read-only capability and amplify it into a read-write
> capability.  Every capability system I ever saw was specifically designed to
> prevent you from doing precisely that: if the owner of the object wanted you
> to have read-write access to the, he would have granted you a read-write
> capability for the object in the first place.
> 
> What I think you want to do is go back to the original system, structure it
> as a read-write object, and then make a read-only view of it that you give
> out to the world.  Only the ones who really have a legitimate need for
> read-write access get the read-write capability.

That was my first idea, but since it will be OO the read only view of 
the data would inherit the operations of the read-write view and so the 
client would be able to modify the read-only raster - unless the 
operations where overridden to throw an exception, but I'd think that'd 
be a bad design choice in this instance.

What I'm considering is

Raster is abstract tagged ...;

Read_Only_Raster is new Raster with tagged ...;

Write_Read_Raster is new Raster with tagged ...;

That would enable the client to use a read_only_raster or 
write_read_raster wherever they could use a raster.


Chris
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