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From: "David C. Hoos" <david.c.hoos.sr@ada95.com>
Newsgroups: comp.lang.ada
References: <bnlmg1$kqv$04$1@news.t-online.com>
Subject: Re: Modular integers
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"Lars" <lars-remove_me-m@gmx.net> wrote in message
news:bnlmg1$kqv$04$1@news.t-online.com...
> Hi,
>
> I was able to create an unsigned 64-bit integer using
>
>     type unsigned_64_bit_integer is mod 2**64;
>
> Then I defined a subtype:
>
>     subtype safe_unsigned_64_bit_integer is unsigned_64_bit_integer range
0
> .. 2**64-1;
>
> When a variable of type unsigned_64_bit_integer contains 2**64-1 and you
add
> 1, it is "reset" to 0, as expected.
> When a variable of type safe_unsigned_64_bit_integer contains 2**64-1 and
> you add 1, no exception is raised and it is "reset" to 0.
>
> Why is that? Shouldn't the compiler be able to insert a check that throws
an
> exception if an overflow occurs?

No.  By definition modular numbers cannot "overflow."  In fact, to say the
value
is "reset" to 0 is incorrect.  0 simply follows 2**64 - 1.

You can declare a _signed_ 64-bit integer type which will overflow when
adding
1 to 2**63-1, or underflow when subtracting 1 from -(2**63).

>
> -Lars
>
>
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