Puzzling question

Puzzling question

[Chris Sparks (Mr. Ada)]

Hi all,

I have the following files:

----------------------------------------------------------------------
package T is

  type Rec_Type is record
    A : Integer;
    B : Integer;
  end record;

  procedure S (U : out Rec_Type);

end T;
----------------------------------------------------------------------
with System;

package body T is

  procedure R (V : in System.Address) is begin

    null;

  end R;

  procedure S (U : out Rec_Type) is begin

    U.A := 1;
    U.B := 2;

    R (V => U.A'Address);  -- Why is this valid????????????????

  end S;

end T;
----------------------------------------------------------------------
with T; use T;

procedure T1 is

  M : Rec_Type;

begin
  T.S (U => M);
end;
----------------------------------------------------------------------

At work we are using Ada 83 and I thought that if a parameter is "out"
in a procedure then only assignments could be made to the variable.
In the call to R above, U.A would effectively be considered an input
which I thought would be illegal since U is an "out" type.  I know that
in Ada 95 this is ok, however, I don't know why this works under Ada 83.

Any help will be appreciated.

Chris Sparks

Re: Puzzling question

[Adam Beneschan]

"Chris Sparks (Mr. Ada)" <sparks@AISF.COM> writes:
 
 >with System;
 >
 >package body T is
 >
 >  procedure R (V : in System.Address) is begin
 >
 >    null;
 >
 >  end R;
 >
 >  procedure S (U : out Rec_Type) is begin
 >
 >    U.A := 1;
 >    U.B := 2;
 >
 >    R (V => U.A'Address);  -- Why is this valid????????????????
 >
 >  end S;
 >
 >end T;

6.2(5) says "Reading the bounds and discriminants of the formal
parameter and of its subcomponents is allowed, but no other reading."

I don't know whether "reading" is defined explicitly in the Ada83 RM,
but I would suspect that taking the 'ADDRESS of a parameter is not
considered "reading".

                                -- Adam

Re: Puzzling question

[Robert Dewar]

Chris asks

    R (V => U.A'Address);  -- Why is this valid????????????????

why not? I can't see anything in the Ada 83 that suggests that this 
would be illegal. Address can be applied to objects, and U.A is an
object. Now note that often applying address to parameters can give
totally meaningless results, or at the best implementation dependent
results, but address is ID anyway, so this is not really an issue.

<<At work we are using Ada 83 and I thought that if a parameter is "out"
  in a procedure then only assignments could be made to the variable.
  In the call to R above, U.A would effectively be considered an input
  which I thought would be illegal since U is an "out" type.  I know that
  in Ada 95 this is ok, however, I don't know why this works under Ada 83.>>

Why is U.A'Address "effectively" considered as an input, nothing is
being read! Ada 83 and Ada 95 are identical here, the fact that in one
you could write U.A is totally irrelevant.