Re: surprised by signed arithmetic...
* MiB:
On Feb 13, 7:22 pm, Ulrich Eckhardt <eckha...@satorlaser.com> wrote:
[...]
I just stumbled across a piece of weird behaviour: I took the difference of
two unsigned, 8-bit integers and it seems the result was converted to a
signed char and then a signed int and thus sign-extended. I'm actually
surprised that any unsigned operation will ever yield a signed value at
all, after all signed values always present the danger of undefined
behaviour when they overflow.
Can anyone help me to understand the background of this behaviour?
The 'unsigned char' values are promoted to 'int' values before evaluation of the
operator.
I am not sure what The Standard says, but if you evaluate 3u - 5u, you
should expect -2 even with both arguments unsigned.
Sorry, this is incorrect.
With both arguments as 'unsigned' you do not get any promotion.
The result is guaranteed by the standard, namely the 'unsigned' value 2^n - 2
where n is the number of value representation bits (in general you get whatever
value the expression evaluates to, modulo 2^n).
If you'd insist on
getting an unsigned result, the best you can hope for is a domain
error
Sorry, this is incorrect.
The standard guarantees that all unsigned arithmetic results are well defined.
There are no domain errors or UB with unsigned arithmetic such as 3u-5u.
, or an unsigned interpretation of the two's complement signed
bit pattern for -2.
This is correct, but it's a very roundabout and impractical way of viewing it.
This would give different results depending on the
bit length of int in your compiler,
This is correct.
also not very pleasant and confusing at best.
?
(e.g. with 16-bit int you get 0xFFFEu, with 32-bit
int 0xFFFFFFFEu). With the behavior you describe, you can still
achieve this by an explicit cast.
Note: AFAIR, while its the most common implementation in concurrent
compilers, int is not fixed to 32 bit length - the standard merely
demands length( short ) <= length( int ) <= length( long ). Maybe I'm
mixing up C and C++ here, I'm too lazy to look it up :-).
By reference to the C standard the C++ standard also restricts the minimum
number of bits, e.g. char is minimum 8 bits -- see the FAQ.
Cheers & hth.,
- Alf
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