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From: "Alex Mentis" <foo@invalid.invalid>
Newsgroups: comp.lang.ada
Subject: Unconstrained base subtype questions
Date: Thu, 31 Mar 2011 20:25:13 +0000 (UTC)
Organization: A noiseless patient Spider
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The following does not cause a constraint error in my version of GNAT
on my system:

...

Integer_Result := (Integer'Last + Integer'Last) / 2;

...


If I understand correctly, this is because the Integer operators are
defined for operands of type Integer'Base, which is an unconstrained
subtype and allows the operands to be stored in extended-length
registers so that intermediate values in calculations do not overflow.

My questions are:

1) Do I understand correctly what's going on?

2) Does the language make any guarantees about preventing spurious
overflow, or am I just getting lucky with my compiler/architecture? If
guarantees are made by the language, what are they?

Thanks,
Alex