From: dewar@schonberg.cs.nyu.edu (Robert Dewar)
Subject: Re: Valid Attribute and Unchecked Conversion
Date: 1996/10/05
Date: 1996-10-05T00:00:00+00:00 [thread overview]
Message-ID: <dewar.844516953@schonberg> (raw)
In-Reply-To: 53151i$ddd@newsbf02.news.aol.com
John Herror said
"I could be wrong, but I thought that Unchecked_Conversion is erroneous in
_any_ case. I heard that the first validated Ada compiler, Ada/Ed, took
advantage of that fact and generated code to raise Program_Error wherever
the program tried to make use of Unchecked_Conversion."
This is completely wrong. In both Ada 83 and Ada 95, there is nothing
erroneous about unchecked conversion unless an invalid result is
produced. The wording from the Ada 83 RM for section 13.10.2 mentions
erroneousness only in one place:
"Whenever unchecked conversions are used, it is the programmers
responsibility to ensure that these conversions maintain the properties
that are guaranteed by the language for objects of the target type.
Programs that violate these properties by means of unchecked conversions
are erroneous."
The ACVC 1.11 suite contains a number of uses of unchecked conversion
that are required to work on all compilers and are not only NOT erroneous,
but are not even implementation dependent.
Now it is certainly the case that UC is implementation dependent, and an
implementation is allowed to place restrictions. In the case of Ada/Ed,
which is what you referred to, unchecked conversion made no sense in the
virtual machine used for the implementation, so all UC's were rejected.
Later on, Ada/Ed did implement many UC's, including those required by
the ACVC suite.
Never use the word erroneous casually, it is not a blanket term meaning
{undefined, implementation defined, wrong, illegal, inadvisable etc ..}
Only a few things in Ada 83 or Ada 95 are erroneous (fewer in Ada 95), and
the RM is very careful to document exactly what is and what is not
erroneous, so don't go by "I thought" here, read the RM carefully!
Note that in Ada 95, more uses of unchecked conversion are non-erroneous,
since a program can generate an abnormal value using UC, and then stay
non-erroneous as long as only 'Valid is used to query the result.
Indeed the Ada 95 section on unchecked conversion (section 13.9) does not
mention the word erroneous at all, so an unchecked conversoin BY ITS SELF
never makes a program execution erroneous in Ada 95.
next prev parent reply other threads:[~1996-10-05 0:00 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
1996-09-22 0:00 Rules for Representation of Subtypes Matthew Heaney
1996-09-23 0:00 ` David C. Hoos, Sr.
1996-09-23 0:00 ` Samuel T. Harris
1996-09-26 0:00 ` David C. Hoos, Sr.
1996-09-23 0:00 ` Robert A Duff
1996-09-24 0:00 ` Robert Dewar
1996-09-24 0:00 ` Robert Dewar
1996-09-26 0:00 ` Keith Thompson
1996-09-26 0:00 ` Matthew Heaney
1996-09-27 0:00 ` Robert Dewar
1996-09-27 0:00 ` Robert A Duff
1996-09-27 0:00 ` Robert Dewar
1996-09-27 0:00 ` Robert A Duff
1996-09-23 0:00 ` Robert A Duff
1996-09-24 0:00 ` Matthew Heaney
1996-09-26 0:00 ` Robert A Duff
1996-09-26 0:00 ` Larry Kilgallen
1996-09-27 0:00 ` Robert A Duff
1996-09-27 0:00 ` Mark A Biggar
1996-09-30 0:00 ` Robert A Duff
1996-10-01 0:00 ` Larry Kilgallen
1996-10-01 0:00 ` Robert A Duff
1996-10-01 0:00 ` Samuel Tardieu
1996-10-01 0:00 ` Valid Attribute and Unchecked Conversion Matthew Heaney
1996-10-02 0:00 ` Robert A Duff
1996-10-04 0:00 ` Keith Thompson
1996-10-04 0:00 ` Robert A Duff
1996-10-04 0:00 ` Robert Dewar
1996-10-11 0:00 ` Norman H. Cohen
1996-10-12 0:00 ` Robert Dewar
1996-10-06 0:00 ` Keith Thompson
1996-10-07 0:00 ` Robert Dewar
1996-10-09 0:00 ` Keith Thompson
1996-10-07 0:00 ` Ken Garlington
1996-10-08 0:00 ` Alan Brain
1996-10-04 0:00 ` Matthew Heaney
1996-10-07 0:00 ` Robert Dewar
1996-10-09 0:00 ` Keith Thompson
1996-10-07 0:00 ` Robert Dewar
1996-10-10 0:00 ` Ken Garlington
1996-10-11 0:00 ` Robert Dewar
1996-10-14 0:00 ` Keith Thompson
1996-10-14 0:00 ` Ken Garlington
1996-10-07 0:00 ` Kenneth Almquist
1996-10-01 0:00 ` Rules for Representation of Subtypes Ken Garlington
1996-09-28 0:00 ` Larry Kilgallen
1996-09-29 0:00 ` Robert A Duff
1996-09-29 0:00 ` Larry Kilgallen
1996-09-29 0:00 ` Matthew Heaney
1996-09-30 0:00 ` Robert A Duff
1996-10-01 0:00 ` Ken Garlington
1996-10-02 0:00 ` Robert A Duff
1996-10-02 0:00 ` Ken Garlington
1996-10-06 0:00 ` Robert Dewar
1996-09-29 0:00 ` Matthew Heaney
1996-09-30 0:00 ` Robert Dewar
1996-09-30 0:00 ` Matthew Heaney
1996-09-30 0:00 ` Robert A Duff
1996-10-03 0:00 ` Robert Dewar
1996-10-02 0:00 ` Valid Attribute and Unchecked Conversion Robert I. Eachus
1996-10-02 0:00 ` Matthew Heaney
1996-10-02 0:00 ` George Haddad
1996-10-03 0:00 ` John Herro
1996-10-04 0:00 ` Karl Cooper {46901}
1996-10-07 0:00 ` Mark A Biggar
1996-10-08 0:00 ` Robert Dewar
1996-10-05 0:00 ` Robert Dewar [this message]
1996-10-06 0:00 ` Keith Thompson
1996-10-14 0:00 ` Robert A Duff
1996-09-27 0:00 ` Rules for Representation of Subtypes Matthew Heaney
1996-09-27 0:00 ` Robert A Duff
-- strict thread matches above, loose matches on Subject: below --
1996-10-03 0:00 Valid Attribute and Unchecked Conversion Franco Mazzanti
1996-10-04 0:00 Franco Mazzanti
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