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From: jduarte@liege.ICS.UCI.EDU (J o s e D u a r t e)
Newsgroups: comp.lang.ada
Subject: Re: Types defining h/w r/o or w/o registers
Message-ID: <9105311025.aa04299@PARIS.ICS.UCI.EDU>
Date: 31 May 91 17:26:39 GMT
Reply-To: Jose Aleman Duarte <jduarte@liege.ICS.UCI.EDU>
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List-Id: <comp.lang.ada>

> 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.


Well...Ada is "the" language which explicitly allows you
to name variables which can ONLY be read/written/read-write using
the procedure syntax:


with REGISTERS; use REGISTERS;
procedure XYZ(Reg1: in REGISTER; Reg2: out REGISTER; Reg3: in out REGISTER) is
begin
  -- Reg1 can only be used effectively as a constant in this scope
  -- Reg2 can only be assigned to in this scope
  -- Reg3 can be assigned to or read from in this scope
  null;
end XYZ;

.
.
.

Alternatively, you can define a "type" and then define a constant variable
of that type for read only variables...

subtype XYZ is INTEGER;
NY_REG: constant XYZ := 0; -- define a  variable that can only be read

.
.
.

to define a type in a package that does not have ":="
implicitly defined...use a "limited private" syntax:
package REGS is

type XYZ is limited private;

-- insert your procedures/functions here

private
type XYZ is NEW WHATEVER;
end REGS;

--- This would not allow other programmers that use your package to assign
--- to a variable of type "XYZ" unless you also specified an "Assign" procedure.
--- Look up "limited private" types for more details. I believe that "limited
--- private" means that "=" (and thus "/=") and  ":=" are not implicitly
--- allowed for that type unless the  package also explicitly defines
--- "Assign" and "Read-From" procedures

Jose' D.