Variables as array size declarators.

[Ayaz Ahmed Khan]

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.

[Jakob Bieling]

"Ayaz Ahmed Khan" <re*******@myrealbox.com> wrote in message
news:pa*********************************@myrealbox .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.

hth
--
jb

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

[Rolf Magnus]

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

[Default User]

Jakob Bieling wrote:

"Ayaz Ahmed Khan" <re*******@myrealbox.com> wrote in message
news:pa*********************************@myrealbox .com...

Can a variable containing, say, an integer value be used as size
declarator for an array of integers?

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

[Ayaz Ahmed Khan]

"Jakob Bieling" typed:

"Ayaz Ahmed Khan" <re*******@myrealbox.com> wrote in message
news:pa*********************************@myrealbox .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/...34830malz.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]

"Rolf Magnus" typed:

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.

[Rob Williscroft]

Ayaz Ahmed Khan wrote in
news:pa*********************************@myrealbox .com:

"Jakob Bieling" typed:

"Ayaz Ahmed Khan" <re*******@myrealbox.com> wrote in message
news:pa*********************************@myrealbox .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.
Note:

http://www.cray.com/craydoc/manuals/...-2179-003/z893
434830malz.html

The page title says "Chapter 3. Cray C and C++ Extensions".
Rob.
--
http://www.victim-prime.dsl.pipex.com/

[André Pönitz]

In gnu.g++.help Ayaz Ahmed Khan <re*******@myrealbox.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.

Andre'

[Rolf Magnus]

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 <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]

"Rolf Magnus" typed:

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 <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.

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]

"André Pönitz" typed:

In gnu.g++.help Ayaz Ahmed Khan <re*******@myrealbox.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.

[Ayaz Ahmed Khan]

"Rob Williscroft" typed:

Ayaz Ahmed Khan wrote in
news:pa*********************************@myrealbox .com:

"Jakob Bieling" typed:

"Ayaz Ahmed Khan" <re*******@myrealbox.com> wrote in message
news:pa*********************************@myrealbox .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?

Note:

http://www.cray.com/craydoc/manuals/...-2179-003/z893
434830malz.html

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

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]

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

[Ayaz Ahmed Khan]

"Rob Williscroft" typed:

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."