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From: willett@cbnewsl.att.com (david.c.willett)
Newsgroups: comp.lang.ada
Subject: Re: Types defining h/w r/o or w/o registers
Summary: Limited private types (read only)
Message-ID: <1991May31.170353.21919@cbnewsl.att.com>
Date: 31 May 91 17:03:53 GMT
References: <3949@titan.tsd.arlut.utexas.edu>
Distribution: usa
Organization: AT&T Bell Laboratories
List-Id: <comp.lang.ada>

In article <3949@titan.tsd.arlut.utexas.edu>, gardner@titan.tsd.arlut.utexas.edu (Don Gardner) writes:
> Is there some way to define a type such that all variables of that
> type can be only read or only written?  For instance, I have a type that
> defines the status register of some peripheral which can only be 
> read.  It would be best if the compiler could flag any assignments 
> to variables of such a type as errors.
> 
> Is there some other means for commonly handling this problem?  The 
> compiler being used is VADSWorks, if an implementation-defined solution
> is required.

Don,
	The Ada LRM defines a "limited private" type as being one for which
the assignment operators are not valid outside the package in which the 
type is declared (see Booch, "Programming in Ada" pg. 125).  I think that
will take care of the read only case.  Only suggestion I have on write 
only is to hide the write operation in a package and provide no 
corresponding read function.

			Good Luck,
				Dave Willett