Variables as array size declarators.

**Jakob Bieling** · Jul 22 '05, 06:59 AM

Re: Variables as array size declarators.

"Ayaz Ahmed Khan" <resilient@myre albox.com> wrote in message
news:pan.2004.0 2.26.15.51.50.7 45504.1849@myre albox.com...[color=blue]
> Can a variable containing, say, an integer value be used as size
> declarator for an array of integers? g++ does not complain, but other
> compilers, such as Visual C++/6.0, Borland Builder C++, Turbo C++, and
> Dev-C++/4.0, that I tested it on decline to use variables as size
> declarators.[/color]

Not sure, but I think C allows this and g++ allows it as an extension.
However, it is not part of Standard C++. If you want to use this kind of
stuff, use the std::vector container class. It will do exactly what you
want.

hth
--
jb

(replace y with x if you want to reply by e-mail)

**Rolf Magnus** · Jul 22 '05, 06:59 AM

Re: Variables as array size declarators.

Ayaz Ahmed Khan wrote:
[color=blue]
> Can a variable containing, say, an integer value be used as size
> declarator for an array of integers?[/color]

No.
[color=blue]
> g++ does not complain, but other compilers, such as Visual C++/6.0,
> Borland Builder C++, Turbo C++, and Dev-C++/4.0, that I tested it on
> decline to use variables as size declarators.[/color]

You probably didn't use g++ in ansi/iso mode. Try to use at least the
following command line switches:

-ansi -pedantic -W -Wall

**Default User** · Jul 22 '05, 06:59 AM

Re: Variables as array size declarators.

Jakob Bieling wrote:[color=blue]
>
> "Ayaz Ahmed Khan" <resilient@myre albox.com> wrote in message
> news:pan.2004.0 2.26.15.51.50.7 45504.1849@myre albox.com...[color=green]
> > Can a variable containing, say, an integer value be used as size
> > declarator for an array of integers?[/color][/color]

[color=blue]
> Not sure, but I think C allows this and g++ allows it as an extension.
> However, it is not part of Standard C++. If you want to use this kind of
> stuff, use the std::vector container class. It will do exactly what you
> want.[/color]

These (variable-length arrays) were added to C in the latest standard.
As there are few compilers actually implementing that standard yet,
actually having that capability in a standard-compliant way is spotty.

Brian Rodenborn

**Ayaz Ahmed Khan** · Jul 22 '05, 07:00 AM

Re: Variables as array size declarators.

"Jakob Bieling" typed:
[color=blue]
> "Ayaz Ahmed Khan" <resilient@myre albox.com> wrote in message
> news:pan.2004.0 2.26.15.51.50.7 45504.1849@myre albox.com...[color=green]
>> Can a variable containing, say, an integer value be used as size
>> declarator for an array of integers? g++ does not complain, but
>> other compilers, such as Visual C++/6.0, Borland Builder C++, Turbo
>> C++, and Dev-C++/4.0, that I tested it on decline to use variables
>> as size declarators.[/color]
>
>
> Not sure, but I think C allows this and g++ allows it as an
> extension.
> However, it is not part of Standard C++. If you want to use this
> kind of stuff, use the std::vector container class. It will do
> exactly what you want.[/color]

I had been under the impression that it was a feature incorporated
into standard C++. I found a reference to VLAs[1], but am not sure what
it, actually, is worth.

Note:

Service Unavailable

http://www.cray.com/craydoc/manuals/004-2179-003/html-004-2179-003/z893434830malz.html

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________ _______________ __
I ditched Windows for Linux over two years ago.
Life's never been better since.

**Ayaz Ahmed Khan** · Jul 22 '05, 07:00 AM

Re: Variables as array size declarators.

"Rolf Magnus" typed:
[color=blue]
> Ayaz Ahmed Khan wrote:
>[color=green]
>> Can a variable containing, say, an integer value be used as size
>> declarator for an array of integers?[/color]
>
> No.
>[color=green]
>> g++ does not complain, but other compilers, such as Visual C++/6.0,
>> Borland Builder C++, Turbo C++, and Dev-C++/4.0, that I tested it
>> on decline to use variables as size declarators.[/color]
>
> You probably didn't use g++ in ansi/iso mode. Try to use at least
> the following command line switches:
>
> -ansi -pedantic -W -Wall[/color]

Yes. It now complains when I use the above switches. But if I wasn't
using g++ in ANSI/ISO mode before, why did it flag with an "ISO for
scope" message, or something to that effect, an identifier declared
in the for header that I tried to use elsewhere in the listing?

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________ _______________ __
I ditched Windows for Linux over two years ago.
Life's never been better since.

**Rob Williscroft** · Jul 22 '05, 07:00 AM

Re: Variables as array size declarators.

Ayaz Ahmed Khan wrote in
news:pan.2004.0 2.27.04.40.24.3 76057.7652@myre albox.com:
[color=blue]
> "Jakob Bieling" typed:
>[color=green]
>> "Ayaz Ahmed Khan" <resilient@myre albox.com> wrote in message
>> news:pan.2004.0 2.26.15.51.50.7 45504.1849@myre albox.com...[color=darkred]
>>> Can a variable containing, say, an integer value be used as size
>>> declarator for an array of integers? g++ does not complain, but
>>> other compilers, such as Visual C++/6.0, Borland Builder C++, Turbo
>>> C++, and Dev-C++/4.0, that I tested it on decline to use variables
>>> as size declarators.[/color]
>>
>>
>> Not sure, but I think C allows this and g++ allows it as an
>> extension.
>> However, it is not part of Standard C++. If you want to use this
>> kind of stuff, use the std::vector container class. It will do
>> exactly what you want.[/color]
>
>
> I had been under the impression that it was a feature incorporated
> into standard C++. I found a reference to VLAs[1], but am not sure
> what it, actually, is worth.
>[/color]

VLA's are a feature of Standard C, C++ compilers that support them
(eg: g++) do so as an extension to Standard C++. IOW using VLA's in
C++ is *not* portable.
[color=blue]
> Note:
>
> http://www.cray.com/craydoc/manuals/...-2179-003/z893
> 434830malz.html
>[/color]

The page title says "Chapter 3. Cray C and C++ Extensions".

Rob.
--

http://www.victim-prime.dsl.pipex.com/

**André Pönitz** · Jul 22 '05, 07:01 AM

Re: Variables as array size declarators.

In gnu.g++.help Ayaz Ahmed Khan <resilient@myre albox.com> wrote:[color=blue]
> I had been under the impression that it was a feature incorporated
> into standard C++.[/color]

It's not in the current C++ Standard (1998) but in C (1999).
I wouldn't use it in my code.

Andre'

**Rolf Magnus** · Jul 22 '05, 07:01 AM

Re: Variables as array size declarators.

Ayaz Ahmed Khan wrote:
[color=blue][color=green][color=darkred]
>>> g++ does not complain, but other compilers, such as Visual C++/6.0,
>>> Borland Builder C++, Turbo C++, and Dev-C++/4.0, that I tested it
>>> on decline to use variables as size declarators.[/color]
>>
>> You probably didn't use g++ in ansi/iso mode. Try to use at least
>> the following command line switches:
>>
>> -ansi -pedantic -W -Wall[/color]
>
>
> Yes. It now complains when I use the above switches. But if I wasn't
> using g++ in ANSI/ISO mode before, why did it flag with an "ISO for
> scope" message, or something to that effect, an identifier declared
> in the for header that I tried to use elsewhere in the listing?[/color]

I think that g++ gives that error message when you try to use a variable
that was defined in a for loop after that loop. So e.g. the following
erroneous code would produce that:

#include <iostream>

int main()
{
for (int i = 0; i < 10; ++i)
std::cout << i << '\n';

i = 3; // error, i is not defined after the for loop
}

In some old C++ dialects before ISO, that was allowed, and i was defined
up until the end of the enclosing block. VC++ is known to still handle
this incorrectly.

**Ayaz Ahmed Khan** · Jul 22 '05, 07:03 AM

Re: Variables as array size declarators.

"Rolf Magnus" typed:
[color=blue]
> Ayaz Ahmed Khan wrote:
>[color=green][color=darkred]
>>>> g++ does not complain, but other compilers, such as Visual
>>>> C++/6.0, Borland Builder C++, Turbo C++, and Dev-C++/4.0, that I
>>>> tested it on decline to use variables as size declarators.
>>>
>>> You probably didn't use g++ in ansi/iso mode. Try to use at least
>>> the following command line switches:
>>>
>>> -ansi -pedantic -W -Wall[/color]
>>
>>
>> Yes. It now complains when I use the above switches. But if I
>> wasn't using g++ in ANSI/ISO mode before, why did it flag with an
>> "ISO for scope" message, or something to that effect, an identifier
>> declared in the for header that I tried to use elsewhere in the
>> listing?[/color]
>
> I think that g++ gives that error message when you try to use a
> variable that was defined in a for loop after that loop. So e.g. the
> following erroneous code would produce that:
>
> #include <iostream>
>
> int main()
> {
> for (int i = 0; i < 10; ++i)
> std::cout << i << '\n';
>
> i = 3; // error, i is not defined after the for loop
> }
> }
> In some old C++ dialects before ISO, that was allowed, and i was
> defined up until the end of the enclosing block. VC++ is known to
> still handle this incorrectly.[/color]

Yes. Exactly that error. But the first time I came across it using
g++, I thought that g++ was, by default, running in ANSI/ISO
standards-complaint mode.

Anyway, thanks for your time.

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________ _______________ __
I ditched Windows for Linux over two years ago.
Life's never been better since.

**Ayaz Ahmed Khan** · Jul 22 '05, 07:03 AM

Re: Variables as array size declarators.

"André Pönitz" typed:
[color=blue]
> In gnu.g++.help Ayaz Ahmed Khan <resilient@myre albox.com> wrote:[color=green]
>> I had been under the impression that it was a feature incorporated
>> into standard C++.[/color]
>
> It's not in the current C++ Standard (1998) but in C (1999). I
> wouldn't use it in my code.[/color]

Thanks, Andre.

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________ _______________ __
I ditched Windows for Linux over two years ago.
Life's never been better since.

**Ayaz Ahmed Khan** · Jul 22 '05, 07:03 AM

Re: Variables as array size declarators.

"Rob Williscroft" typed:
[color=blue]
> Ayaz Ahmed Khan wrote in
> news:pan.2004.0 2.27.04.40.24.3 76057.7652@myre albox.com:
>[color=green]
>> "Jakob Bieling" typed:
>>[color=darkred]
>>> "Ayaz Ahmed Khan" <resilient@myre albox.com> wrote in message
>>> news:pan.2004.0 2.26.15.51.50.7 45504.1849@myre albox.com...
>>>> Can a variable containing, say, an integer value be used as size
>>>> declarator for an array of integers? g++ does not complain, but
>>>> other compilers, such as Visual C++/6.0, Borland Builder C++,
>>>> Turbo C++, and Dev-C++/4.0, that I tested it on decline to use
>>>> variables as size declarators.
>>>
>>>
>>> Not sure, but I think C allows this and g++ allows it as an
>>> extension.
>>> However, it is not part of Standard C++. If you want to use this
>>> kind of stuff, use the std::vector container class. It will do
>>> exactly what you want.[/color]
>>
>>
>> I had been under the impression that it was a feature incorporated
>> into standard C++. I found a reference to VLAs[1], but am not sure
>> what it, actually, is worth.
>>
>>[/color]
> VLA's are a feature of Standard C, C++ compilers that support them
> (eg: g++) do so as an extension to Standard C++. IOW using VLA's in
> C++ is *not* portable.[/color]

So I see. But aren't there advantages in using VLAs that are defined a
certain size, such as provided by the input variables, at run-time,
over other arrays for the same purposes that have to be specified a
size large enough to accomodate the input data, which can so much
as only be guessed anytime before run-time?

[color=blue][color=green]
>> Note:
>>
>> http://www.cray.com/craydoc/manuals/...-2179-003/z893
>> 434830malz.html
>>
>>[/color]
> The page title says "Chapter 3. Cray C and C++ Extensions".[/color]

Extensions. Perhaps, I didn't read it so thoroughly as I should have
had.

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________ _______________ __
I ditched Windows for Linux over two years ago.
Life's never been better since.

**Rob Williscroft** · Jul 22 '05, 07:03 AM

Re: Variables as array size declarators.

Ayaz Ahmed Khan wrote in news:pan.2004.0 2.28.17.55.17.5 10698.1784
@myrealbox.com:
[color=blue][color=green]
>> VLA's are a feature of Standard C, C++ compilers that support them
>> (eg: g++) do so as an extension to Standard C++. IOW using VLA's in
>> C++ is *not* portable.[/color]
>
>
> So I see. But aren't there advantages in using VLAs that are defined a
> certain size, such as provided by the input variables, at run-time,
> over other arrays for the same purposes that have to be specified a
> size large enough to accomodate the input data, which can so much
> as only be guessed anytime before run-time?
>
>[/color]

Yes, but with C++ we have std::vector<> to solve that problem.

#include <vector>

extern void takes_a_char_bu ffer( char *buf, std::size_t sz );

int main()
{
std::size_t sz = /* some runtime value */;

std::vector< char > buf( sz );

takes_a_char_bu ffer( &buf[0], sz );
}

Rob.
--

http://www.victim-prime.dsl.pipex.com/

**Ayaz Ahmed Khan** · Jul 22 '05, 07:04 AM

Re: Variables as array size declarators.

"Rob Williscroft" typed:
[color=blue]
> Ayaz Ahmed Khan wrote in news:pan.2004.0 2.28.17.55.17.5 10698.1784
> @myrealbox.com:
>[color=green][color=darkred]
>>> VLA's are a feature of Standard C, C++ compilers that support them
>>> (eg: g++) do so as an extension to Standard C++. IOW using VLA's
>>> in C++ is *not* portable.[/color]
>>
>>
>> So I see. But aren't there advantages in using VLAs that are
>> defined a certain size, such as provided by the input variables, at
>> run-time, over other arrays for the same purposes that have to be
>> specified a size large enough to accomodate the input data, which
>> can so much as only be guessed anytime before run-time?
>>
>>
>>[/color]
> Yes, but with C++ we have std::vector<> to solve that problem.
>
> #include <vector>
>
> extern void takes_a_char_bu ffer( char *buf, std::size_t sz );
>
> int main()
> {
> std::size_t sz = /* some runtime value */;
>
> std::vector< char > buf( sz );
>
> takes_a_char_bu ffer( &buf[0], sz );
> }
> }
> Rob.[/color]

Thanks, Rob. I'm still far behind in studying containers and STL in
C++, having recently started learning "Classes and Objects".

--
Ayaz Ahmed Khan

Yours Forever in, | Webmaster,
Cyberspace. | http://fast-ce.org/
_______________ _______________
"To death, I shall hate thee."

Variables as array size declarators.

Variables as array size declarators.

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