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Ayaz Ahmed Khan
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 Posted: Thu Feb 26, 2004 4:28 pm Post subject: Variables as array size declarators. |
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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.
--
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.
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Jakob Bieling
Guest
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| Posted: Thu Feb 26, 2004 4:30 pm Post subject: Re: Variables as array size declarators. |
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"Ayaz Ahmed Khan" <resilient (AT) myrealbox (DOT) com> wrote
| Quote: | 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.
hth
--
jb
(replace y with x if you want to reply by e-mail)
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Rolf Magnus
Guest
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| Posted: Thu Feb 26, 2004 4:35 pm Post subject: Re: Variables as array size declarators. |
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Ayaz Ahmed Khan wrote:
| Quote: | Can a variable containing, say, an integer value be used as size
declarator for an array of integers?
|
No.
| Quote: | 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.
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You probably didn't use g++ in ansi/iso mode. Try to use at least the
following command line switches:
-ansi -pedantic -W -Wall
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Default User
Guest
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| Posted: Thu Feb 26, 2004 7:22 pm Post subject: Re: Variables as array size declarators. |
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Jakob Bieling wrote:
| Quote: |
"Ayaz Ahmed Khan" <resilient (AT) myrealbox (DOT) com> wrote in message
news:pan.2004.02.26.15.51.50.745504.1849 (AT) myrealbox (DOT) com...
Can a variable containing, say, an integer value be used as size
declarator for an array of integers?
|
| Quote: | 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.
|
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
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Ayaz Ahmed Khan
Guest
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| Posted: Fri Feb 27, 2004 4:54 am Post subject: Re: Variables as array size declarators. |
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"Jakob Bieling" typed:
| Quote: | "Ayaz Ahmed Khan" <resilient (AT) myrealbox (DOT) com> wrote in message
news:pan.2004.02.26.15.51.50.745504.1849 (AT) myrealbox (DOT) 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.
|
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:
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.
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Ayaz Ahmed Khan
Guest
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| Posted: Fri Feb 27, 2004 4:54 am Post subject: Re: Variables as array size declarators. |
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"Rolf Magnus" typed:
| Quote: | Ayaz Ahmed Khan wrote:
Can a variable containing, say, an integer value be used as size
declarator for an array of integers?
No.
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
|
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.
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Rob Williscroft
Guest
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| Posted: Fri Feb 27, 2004 5:16 am Post subject: Re: Variables as array size declarators. |
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Ayaz Ahmed Khan wrote in
news:pan.2004.02.27.04.40.24.376057.7652 (AT) myrealbox (DOT) com:
| Quote: | "Jakob Bieling" typed:
"Ayaz Ahmed Khan" <resilient (AT) myrealbox (DOT) com> wrote in message
news:pan.2004.02.26.15.51.50.745504.1849 (AT) myrealbox (DOT) 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.
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.
|
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.
The page title says "Chapter 3. Cray C and C++ Extensions".
Rob.
--
http://www.victim-prime.dsl.pipex.com/
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André Pönitz
Guest
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| Posted: Fri Feb 27, 2004 9:36 am Post subject: Re: Variables as array size declarators. |
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In gnu.g++.help Ayaz Ahmed Khan <resilient (AT) myrealbox (DOT) com> wrote:
| Quote: | I had been under the impression that it was a feature incorporated
into standard C++.
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It's not in the current C++ Standard (1998) but in C (1999).
I wouldn't use it in my code.
Andre'
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Rolf Magnus
Guest
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| Posted: Fri Feb 27, 2004 10:04 am Post subject: Re: Variables as array size declarators. |
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Ayaz Ahmed Khan wrote:
| Quote: | 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
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?
|
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.
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Ayaz Ahmed Khan
Guest
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| Posted: Sat Feb 28, 2004 6:23 pm Post subject: Re: Variables as array size declarators. |
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"André Pönitz" typed:
| Quote: | In gnu.g++.help Ayaz Ahmed Khan <resilient (AT) myrealbox (DOT) com> wrote:
I had been under the impression that it was a feature incorporated
into standard C++.
It's not in the current C++ Standard (1998) but in C (1999). I
wouldn't use it in my code.
|
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.
|
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| Back to top |
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Ayaz Ahmed Khan
Guest
|
| Posted: Sat Feb 28, 2004 6:23 pm Post subject: Re: Variables as array size declarators. |
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"Rolf Magnus" typed:
| Quote: | Ayaz Ahmed Khan wrote:
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
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?
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
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.
|
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.
|
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| Back to top |
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Ayaz Ahmed Khan
Guest
|
| Posted: Sat Feb 28, 2004 6:23 pm Post subject: Re: Variables as array size declarators. |
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"Rob Williscroft" typed:
| Quote: | Ayaz Ahmed Khan wrote in
news:pan.2004.02.27.04.40.24.376057.7652 (AT) myrealbox (DOT) com:
"Jakob Bieling" typed:
"Ayaz Ahmed Khan" <resilient (AT) myrealbox (DOT) com> wrote in message
news:pan.2004.02.26.15.51.50.745504.1849 (AT) myrealbox (DOT) 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.
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.
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.
|
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?
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.
|
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| Back to top |
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Rob Williscroft
Guest
|
| Posted: Sat Feb 28, 2004 6:50 pm Post subject: Re: Variables as array size declarators. |
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Ayaz Ahmed Khan wrote in news:pan.2004.02.28.17.55.17.510698.1784
@myrealbox.com:
| Quote: | 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.
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?
|
Yes, but with C++ we have std::vector<> to solve that problem.
#include <vector>
extern void takes_a_char_buffer( char *buf, std::size_t sz );
int main()
{
std::size_t sz = /* some runtime value */;
std::vector< char > buf( sz );
takes_a_char_buffer( &buf[0], sz );
}
Rob.
--
http://www.victim-prime.dsl.pipex.com/
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Ayaz Ahmed Khan
Guest
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| Posted: Sun Feb 29, 2004 7:11 pm Post subject: Re: Variables as array size declarators. |
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"Rob Williscroft" typed:
| Quote: | Ayaz Ahmed Khan wrote in news:pan.2004.02.28.17.55.17.510698.1784
@myrealbox.com:
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.
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?
Yes, but with C++ we have std::vector<> to solve that problem.
#include <vector
extern void takes_a_char_buffer( char *buf, std::size_t sz );
int main()
{
std::size_t sz = /* some runtime value */;
std::vector< char > buf( sz );
takes_a_char_buffer( &buf[0], sz );
}
}
Rob.
|
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."
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