Re: [Dev-C++] (no subject)

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----- Original Message -----
From: David McKen
To: Dev-Cpp Mailing List
Sent: Wednesday, October 08, 2003 4:19 PM
Subject: [Dev-C++] (no subject)


Ok how are overflows handled in C++ for integer types?
I know about the resulting (number % upper_limit) effect which is
actually what I want. What I am interested in is determining that an
overflow has occurred.
So far my searches into the void ... whoops meant the C++ standard, seem
to suggest a exception is thrown when a overflow occurs, however I am
under the impression exceptions are not exactly completely portable just
yet, or am I wrong in this belief? The code makes use of classes so I am
fairly confident that exceptions are portably supported the question is
whether the exception is thrown for this exception. The following chunk
of the standard should explain why I am not too sure about this.
-- Begin Quote --
[Note: operators can be regrouped according to the usual mathematical
rules only where the operators really
are associative or commutative.9) For example, in the following fragment
int a, b;
/*...*/
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the
result of the sum (a + 32760) is
next added to b, and that result is then added to 5 which results in the
value assigned to a. On a machine in
which overflows produce an exception and in which the range of values
representable by an int is
[- 32768,+32767], the implementation cannot rewrite this expression as
a = ((a + b) + 32765);
since if the values for a and b were, respectively, - 32754 and - 15,
the sum a + b would produce an
exception while the original expression would not; nor can the
expression be rewritten either as
a = ((a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been, respectively, 4 and - 8
or - 17 and 12. However on a
machine in which overflows do not produce an exception and in which the
results of overflows are
reversible, the above expression statement can be rewritten by the
implementation in any of the above ways
because the same result will occur. ]

-- End Quote --
This seems to indicate that the overflow is only thrown on machines that
produce some sort of exception when an overflow occurs. Also a similar
exception class exist for underflows the structure seems to be extremely
similar to the overflow, I assume if one works then so does the other.
Also is there some mathematical way to detect such an overflow? So far I
am not finding any way, as the result can potentially be anywhere within
the full range of numbers represent by the data type. I would rather not
try to determine if an overflow would occur by performing the addition
myself in binary as I am exploring this method to try to increase the
efficiency of my algorithm and doing the addition "by hand" would
severely increase the complexity of my algorithm.
Thanks in Advance
David McKen
P.S. In case you are wondering this is not homework but a BigInt class I
am trying to create.


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