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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Ada and Automotive Industry
Date: 1996/12/17
Message-ID: <EACHUS.96Dec17091753@spectre.mitre.org>#1/1
X-Deja-AN: 204554478
references: <mazzanti-0512961053010001@131.114.200.115>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada
Date: 1996-12-17T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <32B197A6.2781E494@escmail.orl.lmco.com> Ted Dennison <dennison@escmail.orl.lmco.com> writes:

 > Hmmm. Can I take this to mean that it is not a good idea to raise
 > predefined exceptions manually? Are predefined exceptions somehow
 > handled differently than user-defined ones?


   I'll answer this in reverse order.  The Ada 95 11.6 allows the
optimization of language defined checks, not explicit raise
statements.  However, you can have situations where the failure of a
languagge defined check will cause an explicit raise statement to be
invoked:

   begin
     Foo := new Bar(A*B); --allocate A*B bytes.
   exception
     when others => raise Storage_Error;
     -- if A*B overflows, raise Storage_Error anyway.
   end;

   If Foo is never used, it is legal to optimize away the allocation,
whether the handler raises Storage_Error or Fubar.  For this reason I
prefer to explicitly raise a predefined exception when the raise can
get optimized away like this.

   So my rule on explicitly raising predefined exceptions is that I
will raise them either if the meaning is the predefined meaning of
that exception, or if suppression of a language defined check, either
explicitly or through 11.6, will potentially suppress the exception.

--

					Robert I. Eachus

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