paul@cernvax.UUCP (paul) (02/08/88)
<<< DISK$SYSTEM:[NOTES$LIBRARY]ADA.NOTE;1 >>>
-< Ada >-
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Note 12.6 A bug in Vax Ada compiler... 6 of 6
20514::LOUBOUTIN "Sylvain R.Y. (021/47 27 37)" 68 lines 19-JAN-1988 10:00
-< I don't think so >-
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Spencer Peterson says:
> The "exemple" procedure/program is ERRONEOUS. I quote, with meaningful
> substitutions, from the Ada Reference Manual( Chapter 13.10.1, Para. 6).
>
> "If A and B designate the same object, then accessing that object through
> B is erroneous if this access is performed (or attempted) after the call
> DISPOSE(A); the effect of each such access is not defined by the language."
>
> The program behaves as correctly as it can, given that it is 'legal' for
> a compiler to optimize use of storage by NOT allocating a fresh copy of
> the string contents, although it would be prefered that a second copy be
> made and thus, no problem would occur when the original is DISPOSEd. The
> effect of making a copy of the contents of an access type, in this case,
> is the same as if B were a constant of type ACCESS_STRING, i.e.
> B : constant ACCESS_STRING := A;
I don't agree with Spencer Peterson because in my example, B
is not an access to a string, but actually a string. So it doesn't
designate the same object than A.
My procedure would have been erroneous if I had written it like
that:
...
type ACCESS_STRING is access STRING;
procedure DISPOSE is new UNCHECKED_DEALLOCATION (STRING, ACCESS_STRING);
A: ACCESS_STRING := new STRING'("Ceci est un exemple...");
B: constant ACCESS_STRING := A;
----------------------
...
but the problem is different when the procedure is written like this:
...
type ACCESS_STRING is access STRING;
procedure DISPOSE is new UNCHECKED_DEALLOCATION (STRING, ACCESS_STRING);
A: ACCESS_STRING := new STRING'("Ceci est un exemple...");
B: constant STRING := A.all;
---------------
...
So I still pretend that there is a bug in the Vax Ada compiler.
Another point, the program behaviour is correct if I use a variable
instead of a constant:
...
type ACCESS_STRING is access STRING;
procedure DISPOSE is new UNCHECKED_DEALLOCATION (STRING, ACCESS_STRING);
A: ACCESS_STRING := new STRING'("Ceci est un exemple...");
B: STRING(A.all'RANGE) := A.all;
---------------
...
Sylvain Louboutin
D.I.
Swiss Federal
Institute of Technology
1015 Lausanne,
--------------
SWITZERLAND
Phone: (+41 21) 47 27 37
Email: LOUBOUTIN@CLSEPF51 (from BITNET/EARN)
LOUBOUTIN%CLSEPF51.BITNET@WISCVM.WISC.EDU (from ARPA Internet)