«

On 1/23/2013 7:24 AM, Bint wrote:
> I haven't seen this syntax before. What does "!!" do in C++?

"Syntax"? There's no notable "syntax" of any kind involved here.

`!!` is just an unary `!` followed by another unary `!`.

Now, if you know what `!` operator does (consult your favorite C or C++
book if you don't), then you should realize that the `!!` combination
implements "is non-zero" functionality: it evaluates to `true` for
non-zero argument, and to `false` otherwise.

--
Best regards,
Andrey Tarasevich

On Jan 24, 7:23*pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
wrote:
> On 1/23/2013 7:24 AM, Bint wrote:
>
> > I haven't seen this syntax before. *What does "!!" do in C++?
>
> "Syntax"? There's no notable "syntax" of any kind involved here.
>
> `!!` is just an unary `!` followed by another unary `!`.
>
> Now, if you know what `!` operator does (consult your favorite C or C++
> book if you don't), then you should realize that the `!!` combination
> implements "is non-zero" functionality: it evaluates to `true` for
> non-zero argument, and to `false` otherwise.

Andrey - what you say is true if you *know* there is no !! operator,
but the OP presumably didn't know (or at least wasn't sure) of this.

After all, there is a unary - operator, but it doesn't follow that --x
will change the sign of x twice, returning the original value, does
it?

On 1/24/2013 4:49 PM, Paul N wrote:
> On Jan 24, 7:23 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
> wrote:
>> On 1/23/2013 7:24 AM, Bint wrote:
>>
>>> I haven't seen this syntax before. What does "!!" do in C++?
>>
>> "Syntax"? There's no notable "syntax" of any kind involved here.
>>
>> `!!` is just an unary `!` followed by another unary `!`.
>>
>> Now, if you know what `!` operator does (consult your favorite C or C++
>> book if you don't), then you should realize that the `!!` combination
>> implements "is non-zero" functionality: it evaluates to `true` for
>> non-zero argument, and to `false` otherwise.
>
> Andrey - what you say is true if you *know* there is no !! operator,

This should really be easy to "know" upon opening any C++ book worth the
paper it's printed on, on the page where all the operators are put in a
table or a list.

> but the OP presumably didn't know (or at least wasn't sure) of this.
>
> After all, there is a unary - operator, but it doesn't follow that --x
> will change the sign of x twice, returning the original value, does
> it?

Yes, but the same list of operators in the C++ book the OP is supposed
to have undoubtedly includes -- as a single operator, and it does NOT
include !! (like it does not include ^^ or **, for instance).

As an exercise, it's recommended to try to understand what the
expression a-----b "does" (syntactic meaning), and whether it should
compile (advanced topic for those who try to understand lvalues) for,
say, integral types of 'a' and 'b'.

V
--
I do not respond to top-posted replies, please don't ask

On Thu, 24 Jan 2013 19:27:45 -0500, Victor Bazarov wrote:

> As an exercise, it's recommended to try to understand what the
> expression a-----b "does" (syntactic meaning), and whether it should
> compile (advanced topic for those who try to understand lvalues) for,
> say, integral types of 'a' and 'b'.

Rot13: Vg fubhyq pbzcvyr ohg vgf orunivbhe vf haqrsvarq. Right?



--
[ T H I S S P A C E I S F O R R E N T ]
Troppo poca cultura ci rende ignoranti, troppa ci rende folli.
-- fathermckenzie di it.cultura.linguistica.italiano
<http://xkcd.com/397/>

On Tue, 29 Jan 2013 10:17:33 +0000, army1987 wrote:

> On Thu, 24 Jan 2013 19:27:45 -0500, Victor Bazarov wrote:
>
>> As an exercise, it's recommended to try to understand what the
>> expression a-----b "does" (syntactic meaning), and whether it should
>> compile (advanced topic for those who try to understand lvalues) for,
>> say, integral types of 'a' and 'b'.
>
> Rot13: Vg fubhyq pbzcvyr ohg vgf orunivbhe vf haqrsvarq. Right?

(Wrong. Brain fart.)



--
[ T H I S S P A C E I S F O R R E N T ]
Troppo poca cultura ci rende ignoranti, troppa ci rende folli.
-- fathermckenzie di it.cultura.linguistica.italiano
<http://xkcd.com/397/>

On 25 jan, 01:27, Victor Bazarov <v.baza...@comcast.invalid> wrote:
> On 1/24/2013 4:49 PM, Paul N wrote:
>
>
>
>
>
>
>
>
>
> > On Jan 24, 7:23 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
> > wrote:
> >> On 1/23/2013 7:24 AM, Bint wrote:
>
> >>> I haven't seen this syntax before. *What does "!!" do in C++?
>
> >> "Syntax"? There's no notable "syntax" of any kind involved here.
>
> >> `!!` is just an unary `!` followed by another unary `!`.
>
> >> Now, if you know what `!` operator does (consult your favorite C or C++
> >> book if you don't), then you should realize that the `!!` combination
> >> implements "is non-zero" functionality: it evaluates to `true` for
> >> non-zero argument, and to `false` otherwise.
>
> > Andrey - what you say is true if you *know* there is no !! operator,
>
> This should really be easy to "know" upon opening any C++ book worth the
> paper it's printed on, on the page where all the operators are put in a
> table or a list.
>
> > but the OP presumably didn't know (or at least wasn't sure) of this.
>
> > After all, there is a unary - operator, but it doesn't follow that --x
> > will change the sign of x twice, returning the original value, does
> > it?
>
> Yes, but the same list of operators in the C++ book the OP is supposed
> to have undoubtedly includes -- as a single operator, and it does NOT
> include !! (like it does not include ^^ or **, for instance).
>
> As an exercise, it's recommended to try to understand what the
> expression a-----b "does" (syntactic meaning), and whether it should
> compile (advanced topic for those who try to understand lvalues) for,
> say, integral types of 'a' and 'b'.
>
> V
> --
> I do not respond to top-posted replies, please don't ask

Interesting...

ROT13: Vg snvyf jvgu "reebe P2105: '--' arrqf y-inyhr". Nccneragyl vg
gevrf gb qrperzrag "-o". "n--- --o" jbexf. V jbhyq unir gubhtug gur
bevtvany rkcerffvba jbhyq unir orra vagrecergrq nf "(n--) - (--o)".

Cheers.

On 1/31/2013 5:39 AM, Valentin Bernard wrote:
> On 25 jan, 01:27, Victor Bazarov <v.baza...@comcast.invalid> wrote:
>> On 1/24/2013 4:49 PM, Paul N wrote:
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>> On Jan 24, 7:23 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
>>> wrote:
>>>> On 1/23/2013 7:24 AM, Bint wrote:
>>
>>>>> I haven't seen this syntax before. What does "!!" do in C++?
>>
>>>> "Syntax"? There's no notable "syntax" of any kind involved here.
>>
>>>> `!!` is just an unary `!` followed by another unary `!`.
>>
>>>> Now, if you know what `!` operator does (consult your favorite C or C++
>>>> book if you don't), then you should realize that the `!!` combination
>>>> implements "is non-zero" functionality: it evaluates to `true` for
>>>> non-zero argument, and to `false` otherwise.
>>
>>> Andrey - what you say is true if you *know* there is no !! operator,
>>
>> This should really be easy to "know" upon opening any C++ book worth the
>> paper it's printed on, on the page where all the operators are put in a
>> table or a list.
>>
>>> but the OP presumably didn't know (or at least wasn't sure) of this.
>>
>>> After all, there is a unary - operator, but it doesn't follow that --x
>>> will change the sign of x twice, returning the original value, does
>>> it?
>>
>> Yes, but the same list of operators in the C++ book the OP is supposed
>> to have undoubtedly includes -- as a single operator, and it does NOT
>> include !! (like it does not include ^^ or **, for instance).
>>
>> As an exercise, it's recommended to try to understand what the
>> expression a-----b "does" (syntactic meaning), and whether it should
>> compile (advanced topic for those who try to understand lvalues) for,
>> say, integral types of 'a' and 'b'.
>>
>> V
>> --
>> I do not respond to top-posted replies, please don't ask
>
> Interesting...
>
> ROT13: Vg snvyf jvgu "reebe P2105: '--' arrqf y-inyhr". Nccneragyl vg
> gevrf gb qrperzrag "-o". "n--- --o" jbexf. V jbhyq unir gubhtug gur
> bevtvany rkcerffvba jbhyq unir orra vagrecergrq nf "(n--) - (--o)".

The expression is parsed as ((a--) --) - b. The second decrement is
what doesn't work because the compiler is trying to apply it to the
result of 'a--', which is not an lvalue.

I thought that one had appeared (in a slightly different form) as one of
Herb Sutter's "gotcha of the week" some time ago, but upon a brief look
at gotw.ca/gotw, I couldn't find it. Perhaps it was somewhere else.
The question was something like "how many different operator symbols in
a row are legal in C++?"

V
--
I do not respond to top-posted replies, please don't ask

On Thu, 31 Jan 2013 09:10:36 -0500, Victor Bazarov wrote:

> I thought that one had appeared (in a slightly different form) as one of
> Herb Sutter's "gotcha of the week" some time ago, but upon a brief look
> at gotw.ca/gotw, I couldn't find it. Perhaps it was somewhere else. The
> question was something like "how many different operator symbols in a
> row are legal in C++?"

Without intervening identifiers and without repeating yourself, this
would be among the largest:
a++,+--*&b;

(With repetitions, the longest sequence is infinitely long)

>
> V

Bart v Ingen Schenau

On 2/2/2013 2:10 AM, Bart van Ingen Schenau wrote:

> Without intervening identifiers and without repeating yourself, this
> would be among the largest:
> a++,+--*&b;
>

What about
a++,!+--*&b;

You could also possibly throw in a - before the !,
and also a ! between the + and the --.

red floyd <> writes:

> On 2/2/2013 2:10 AM, Bart van Ingen Schenau wrote:
>
> > Without intervening identifiers and without repeating yourself, this
> > would be among the largest:
> > a++,+--*&b;
> >
>
> What about
> a++,!+--*&b;
>
> You could also possibly throw in a - before the !,
> and also a ! between the + and the --.

Still not the largest:

int &(**p)(), a;
p[0]()++>=~!-+--*&a;

BTW, sizeof is also an operator, so you can add that, too.


urs