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From: Olaf Klein on 5 Mar 2010 20:58
Dear group,

is it possible to initialize an unsigned variable with its maximum
value with the following lines?

unsigned u = -1;
std::size_t s = -1;

I am asking because the standard does not guarantee overflow behavior
of signed types while it does for unsigned types. What applies here?

Olaf Klein

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From: Daniel Krügler on 6 Mar 2010 08:15
On 6 Mrz., 14:58, Olaf Klein <olaf.klei...(a)googlemail.com> wrote:
> is it possible to initialize an unsigned variable with its maximum
> value with the following lines?
>
> unsigned u = -1;
> std::size_t s = -1;
>
> I am asking because the standard does not guarantee overflow behavior
> of signed types while it does for unsigned types. What applies here?

This is clearly expressed in [basic.fundamental]/4

"Unsigned integers, declared unsigned, shall obey the laws
of arithmetic modulo 2n where n is the number of bits in
the value representation of that particular size of integer.45"

with footnote 45:

"This implies that unsigned arithmetic does not overflow because
a result that cannot be represented by the resulting unsigned
integer type is reduced modulo the number that is one greater
than the largest value that can be represented by the resulting
unsigned integer type."

This are just different words for what you are trying to
realize.

HTH & Greetings from Bremen,

Daniel Kr�gler


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From: Bo Persson on 6 Mar 2010 08:15
Olaf Klein wrote:
> Dear group,
>
> is it possible to initialize an unsigned variable with its maximum
> value with the following lines?
>
> unsigned u = -1;
> std::size_t s = -1;
>
> I am asking because the standard does not guarantee overflow
> behavior of signed types while it does for unsigned types. What
> applies here?
>

Well, these are unsigned, right? :-)

The standard guarantees that u and s are both initialized to the
largest value of their respective type.


Bo Persson



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From: Thomas Richter on 7 Mar 2010 05:01
Daniel Kr�gler wrote:
> On 6 Mrz., 14:58, Olaf Klein <olaf.klei...(a)googlemail.com> wrote:
>> is it possible to initialize an unsigned variable with its maximum
>> value with the following lines?
>>
>> unsigned u = -1;
>> std::size_t s = -1;
>>
>> I am asking because the standard does not guarantee overflow behavior
>> of signed types while it does for unsigned types. What applies here?
>
> This is clearly expressed in [basic.fundamental]/4
>
> "Unsigned integers, declared unsigned, shall obey the laws
> of arithmetic modulo 2n where n is the number of bits in
> the value representation of that particular size of integer.45"

Is this really the right argument? After all, this is an implicit
conversion from an int to an unsigned integral type, thus the arithmetic
of unsigned types should not really matter here. After all, two types
are involved here, not just one.

So long,
Thomas

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From: Johannes Schaub (litb) on 7 Mar 2010 08:33
Thomas Richter wrote:

> Daniel Krügler wrote:
>> On 6 Mrz., 14:58, Olaf Klein <olaf.klei...(a)googlemail.com> wrote:
>>> is it possible to initialize an unsigned variable with its maximum
>>> value with the following lines?
>>>
>>> unsigned u = -1;
>>> std::size_t s = -1;
>>>
>>> I am asking because the standard does not guarantee overflow behavior
>>> of signed types while it does for unsigned types. What applies here?
>>
>> This is clearly expressed in [basic.fundamental]/4
>>
>> "Unsigned integers, declared unsigned, shall obey the laws
>> of arithmetic modulo 2n where n is the number of bits in
>> the value representation of that particular size of integer.45"
>
> Is this really the right argument? After all, this is an implicit
> conversion from an int to an unsigned integral type, thus the arithmetic
> of unsigned types should not really matter here. After all, two types
> are involved here, not just one.
>

I think i agree with you. For example, i believe this code is UB:

unsigned int i = -1.0f;

=> "The behavior is undefined if the truncated value cannot be represented
in the destination type."

However, the following is defined:

unsigned int i = -1;

=> "If the destination type is unsigned, the resulting value is the least
unsigned integer congruent to the source integer (modulo 2n where n is the
number of bits used to represent the unsigned type)."

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