collberg@dna.lth.se (Christian S. Collberg) (06/05/90)
Is it possible to declare (the equivalent of) an exported readonly
variable in Ada? I'm referring to something similar to the READ_ONLY
attribute present in Mesa. I'm having a hard time reading the
appropriate sections in the LRM, and I don't have a compiler at hand
to try it out, so maybe someone out there could enlighten me...
Maybe something along these lines might work (excuse the syntax, I'm not an
Ada programmer):
---------------------------
package P;
type T is limited private;
v : T; -- We want a client of P to be able to
-- perform any operation on v, except
-- assignment.
private
type T is BOOLEAN;
end P;
---------------------------
---------------------------
with P;
declare
x : BOOLEAN;
begin
P.v := false; -- Obviously illegal, as it should be
-- for a read only variable.
if P.v = true then -- Doesn't seem like this would be
... -- legal since equality is undefined
-- for limited private types.
if P.v then -- legal ?
...
if P.v OR x then -- legal ?
...
end;
---------------------------
Would one, in order to achieve the desired effect, have to provide,
in the interface of P, overloaded operators for the type T (=, OR, AND,
etc for this example)?
Thanks in advance for any help you may offer in clearing this up for me.
Chris Collberg
collberg@dna.lth.seprogers@ajpo.sei.cmu.edu (Pat Rogers) (06/05/90)
In article <1990Jun5.085654.920@lth.se>, collberg@dna.lth.se (Christian S. Collberg) writes: > Is it possible to declare (the equivalent of) an exported readonly > variable in Ada? The best way I know is to use a discriminated record that is exported as a private/limited type. The discriminates are visible and thus readable, but since the type is actually private (or limited), they cannot be written to from outside the package's exported subprograms. You may also have internal variables (components of the exported record type) that cannot be seen at all, if you choose, or simply make the record empty via "null". For example: (This hasn't "experienced the joys of compilation" as a friend of mine says, but I have done it before, so to speak, and this is the general approach.) package Hardware is type Register( B0,B1,B2,B3,B4,B5,B6,B7 : Integer := 0 ) is private; procedure Set( R : in out Register; ...... ); -- other procedures and functions which operate on registers private type Register( <as before> ) is record null; -- or internal, hidden components end record; end Hardware; with Hardware; package Machine is Status : Hardware.Register; ... end Machine; From here, Machine.Status.B0 etc can be read, but not written, since the type is private. Is that close to what you need? Regards, P Rogers Software Arts and Sciences
collard@software.org (David Collard) (06/05/90)
In article <1990Jun5.085654.920@lth.se> collberg@DNA.LTH.Se (Christian S. Collberg) writes: > Is it possible to declare (the equivalent of) an exported readonly > variable in Ada? I'm referring to something similar to the READ_ONLY > attribute present in Mesa. I'm having a hard time reading the > package P; > type T is limited private; > v : T; -- We want a client of P to be able to > -- perform any operation on v, except > -- assignment. > private > type T is BOOLEAN; > end P; I think, for a read only VARIABLE, there is an easier answer -- the type can be visible, but declare the variable in the body of the package and only provide an interface to read it. package P is function T return Boolean; end P; package body P is Static_Package_Variable : Boolean; function T return Boolean is begin return Static_Package_Variable; end T; end P; with P; procedure P_User is begin if P.T then --Legal null; end if; P.Static_Package_Variable := False; -- Illegal because the variable is not visible. end P_User; A end P_User; -- ----------------------------------------------------------------------- D. Thor Collard Internet: collard@software.org Software Productivity Consortium UUNET: ...!uunet!software!collard 2214 Rock Hill Rd, Herndon VA 22070
eachus@linus.mitre.org (Robert I. Eachus) (06/06/90)
In article <1990Jun5.085654.920@lth.se> collberg@dna.lth.se (Christian S. Collberg) writes: > Is it possible to declare (the equivalent of) an exported readonly > variable in Ada? If what you want is a value which can is not writable except within the defining package, the usual Ada style is to export a function. For this example it would be easy to make V of type Boolean and have all the functionality you seem to want, but the private type example is also interesting: package P; type T is private; function V return T; -- We want a client of P to be able to -- perform any operation on v, except -- assignment. function True return T; function False return T; -- See if you can figure out how to write the body of these -- functions! pragma INLINE (V, True, False); private type T is new BOOLEAN; end P; with P; ... declare X : T; begin P.V := false; -- Obviously illegal, as it should be -- for a read only variable. if P.V = true then -- Legal if P.V then -- Illegal outside of P (V is not of a boolean type.) ... if P.v OR x then -- Legal only if OR has been overloaded... ... end; For completeness the body of package P is: package P; -- type T is private; Hidden_V:T := T'FIRST; function True return T is begin return T'LAST; end; function False return T is begin return T'FIRST; end; -- That was easy wasn't it... function V return T is return Hidden_V; end; ... end P; -- Robert I. Eachus Amiga 3000 - The hardware makes it great, the software makes it awesome, and the price will make it ubiquitous.