Raymond wrote:
> Source:
> http://moryton.blogspot.com/2007/08/detecting-overflowunderf low-when.html
>
> Example from source:
>
> char unsigned augend (255);
> char unsigned const addend (255);
> char unsigned const sum (augend + addend);
>
> if (sum < augend)
> {
> std::puts ("Overflowed!");
> }
>
> sum = augend + addend
> sum = 255 + 255
> sum = 510 modulo 256 // Behind the scenes.
> sum = 254
>
> Does it touch any implementation defined or undefined behaviour, or
> was that specific to signed integers (on some platforms)?

No, the behaviour is well-defined. All arithmetic operations with
unsigned values work modulo 2^N, where N is the number of bits in
the representation of the number.

>
> What other methods are there for detecting unsigned integer overflow
> and/or underflow in C++?

If you look in the archives (use Google Groups interface, e.g.), you
should find that unsigned does not overflow. There is no such thing
as "underflow" AFA integers are concerned, IIUIC.

There is no way to "detect" it in the current language. There is only
a way to "predict" it:

unsigned a = UINT_MAX - 2, b = UINT_MAX - 3; // or whatever
if (UINT_MAX - a < b)
std::cout << "Adding " << a << " to " << b
<< " would \"overflow\"\n";

V
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