Arrays in C++

[Sunny]

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
}

This code declares an array Arr of size len which is not fixed at
compile time. This program compiles using g++ (gcc version 3.2.3
20030502) and works properly. I am surprised to see this because I
thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

[Ian Collins]

Sunny wrote:

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
}

This code declares an array Arr of size len which is not fixed at
compile time. This program compiles using g++ (gcc version 3.2.3
20030502) and works properly. I am surprised to see this because I
thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

--
Ian Collins.

[Ian Collins]

Ian Collins wrote:

Sunny wrote:

>>#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
}

This code declares an array Arr of size len which is not fixed at
compile time. This program compiles using g++ (gcc version 3.2.3
20030502) and works properly. I am surprised to see this because I
thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

I should have added "as an extension to ", not deviating from the standard.

--
Ian Collins.

[Sunny]

On Mar 22, 3:05 pm, Ian Collins <[e-mail] mwrote:

Ian Collins wrote:

Sunny wrote:

>#include <iostream>

>int main()
{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
}

>This code declares an array Arr of size len which is not fixed at
compile time. This program compiles using g++ (gcc version 3.2.3
20030502) and works properly. I am surprised to see this because I
thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

I should have added "as an extension to ", not deviating from the standard.

--
Ian Collins.- Hide quoted text -

- Show quoted text -

Thanks for the reply, Ian.

[Sunny]

On Mar 22, 3:09 pm, "Sunny" <shiladitya.bis [e-mail] ote:

On Mar 22, 3:05 pm, Ian Collins <[e-mail] mwrote:

Ian Collins wrote:

Sunny wrote:

>>#include <iostream>

>>int main()
>>{
> int len;
> std::cin >len;
> int Arr[len];
> int *p = new int[len];
> Arr[len-1]=5;
> std::cout << &len << " " << &Arr << " " << p << std::endl;
> return 0;
>>}

>>This code declares an array Arr of size len which is not fixed at
>>compile time. This program compiles using g++ (gcc version 3.2.3
>>20030502) and works properly. I am surprised to see this because I
>>thought arrays in C++ should have size fixed at compile time.
>Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

I should have added "as an extension to ", not deviating from the standard.

--
Ian Collins.- Hide quoted text -

- Show quoted text -

Thanks for the reply, Ian.- Hide quoted text -

- Show quoted text -

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int Arr1[2*len];
int width;
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << &Arr1<<"" <<&width <<
std::endl;
return 0;

}

This code declares two variable size arrays. I am entering len as 20.
The program produces the following output:

len : 0xbfff99a0
Arr : 0xbfff9940
Arr1 : 0xbfff98a0
width :0xbfff999c

^
8a0 |
|
940 |
|
9a0 |
|
99c |

So width is allocated 4 bytes, len 0x60 bytes, Arr 0xA0 bytes. I am
unable to understand how compiler generates size of Arr as 0xA0 bytes
when it is of length 0x14*4 = 0x50 bytes. And why len gets 0x60
bytes.

Thanks

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

On 22 Mar, 13:04, "Sunny" <shiladitya.bis [e-mail] ote:

On Mar 22, 3:09 pm, "Sunny" <shiladitya.bis [e-mail] ote:

On Mar 22, 3:05 pm, Ian Collins <[e-mail] mwrote:

Ian Collins wrote:
Sunny wrote:

>#include <iostream>

>int main()
>{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
>}

>This code declares an array Arr of size len which is not fixed at
>compile time. This program compiles using g++ (gcc version 3.2.3
>20030502) and works properly. I am surprised to see this because I
>thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

I should have added "as an extension to ", not deviating from the standard.

--
Ian Collins.- Hide quoted text -

- Show quoted text -

Thanks for the reply, Ian.- Hide quoted text -

- Show quoted text -

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int Arr1[2*len];
int width;
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << &Arr1<<"" <<&width <<
std::endl;
return 0;

}

This code declares two variable size arrays. I am entering len as 20.
The program produces the following output:

len : 0xbfff99a0
Arr : 0xbfff9940
Arr1 : 0xbfff98a0
width :0xbfff999c

^
8a0 |
|
940 |
|
9a0 |
|
99c |

So width is allocated 4 bytes, len 0x60 bytes, Arr 0xA0 bytes. I am
unable to understand how compiler generates size of Arr as 0xA0 bytes
when it is of length 0x14*4 = 0x50 bytes. And why len gets 0x60
bytes.

It's a pointer, it does not matter how large the array is since it's
not allocated on the stack, which the other variables (len, width).
Arr must be a pointer since the compiler doesn't know at compile-time
how much space to reserve on the stack for the array, so it makes room
for an array and handles the allocation and deallocation for you. So
basically it's just a compiler generated wrapper around

int* Arr;
arr = new int[len];

// At end of scope
delete[] Arr;

--
Erik Wikström

[Adrian Hawryluk]

#include <iostream>

>
int main()
{
int len;
std::cin >len;
int Arr[len];
int Arr1[2*len];
int width;
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << &Arr1<<"" <<&width <<
std::endl;
return 0;

}

This code declares two variable size arrays. I am entering len as 20.
The program produces the following output:

len : 0xbfff99a0
Arr : 0xbfff9940
Arr1 : 0xbfff98a0
width :0xbfff999c

^
8a0 |
|
940 |
|
9a0 |
|
99c |

So width is allocated 4 bytes, len 0x60 bytes, Arr 0xA0 bytes. I am
unable to understand how compiler generates size of Arr as 0xA0 bytes
when it is of length 0x14*4 = 0x50 bytes. And why len gets 0x60
bytes.

How the compiler works internally is implementation dependent. Most
likely, the extra are housekeeping bytes. If exception handling is
turned on/off, you will probably notice other differences too.

If you would like more compiler specific answers, you will need to go to
a more compiler specific newsgroup. This is an ANSI C++ group AFAIK ;),
and is not worried so much about the internals.

Good luck.
Adrian
--
_______________ _______________ _______________ _______________ _________
\/Adrian_Hawryluk BSc. - Specialties: UML, OOPD, Real-Time Systems\/
\ My newsgroup writings are licensed under a Creative Commons /
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\_______[http://creativecommons.org/licenses/by-sa/3.0/]______/
\/_______[blog:_http://adrians-musings.blogspo t.com/]______\/

[Fei Liu]

Sunny wrote:

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int *p = new int[len];
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << p << std::endl;
return 0;
}

This code declares an array Arr of size len which is not fixed at
compile time. This program compiles using g++ (gcc version 3.2.3
20030502) and works properly. I am surprised to see this because I
thought arrays in C++ should have size fixed at compile time.
Is g++ deviating from standards here?

If you turn on -Wall -ansi -pendatic, then you will see a warning that
you are using variable length array.

Fei

[Sunny]

On Mar 22, 6:19 pm, "Erik Wikström" <eri...@student .chalmers.se>
wrote:

On 22 Mar, 13:04, "Sunny" <shiladitya.bis [e-mail] ote:

On Mar 22, 3:09 pm, "Sunny" <shiladitya.bis [e-mail] ote:

On Mar 22, 3:05 pm, Ian Collins <[e-mail] mwrote:

Ian Collins wrote:
Sunny wrote:

>>#include <iostream>

>>int main()
>>{
> int len;
> std::cin >len;
> int Arr[len];
> int *p = new int[len];
> Arr[len-1]=5;
> std::cout << &len << " " << &Arr << " " << p << std::endl;
> return 0;
>>}

>>This code declares an array Arr of size len which is not fixed at
>>compile time. This program compiles using g++ (gcc version 3.2.3
>>20030502) and works properly. I am surprised to see this because I
>>thought arrays in C++ should have size fixed at compile time.
>Is g++ deviating from standards here?

Yes, gcc supports C99 style variable length arrays.

I should have added "as an extension to ", not deviating from the standard.

--
Ian Collins.- Hide quoted text -

- Show quoted text -

Thanks for the reply, Ian.- Hide quoted text -

- Show quoted text -

#include <iostream>

int main()
{
int len;
std::cin >len;
int Arr[len];
int Arr1[2*len];
int width;
Arr[len-1]=5;
std::cout << &len << " " << &Arr << " " << &Arr1<<"" <<&width <<
std::endl;
return 0;

}

This code declares two variable size arrays. I am entering len as 20.
The program produces the following output:

len : 0xbfff99a0
Arr : 0xbfff9940
Arr1 : 0xbfff98a0
width :0xbfff999c

^
8a0 |
|
940 |
|
9a0 |
|
99c |

So width is allocated 4 bytes, len 0x60 bytes, Arr 0xA0 bytes. I am
unable to understand how compiler generates size of Arr as 0xA0 bytes
when it is of length 0x14*4 = 0x50 bytes. And why len gets 0x60
bytes.

It's a pointer, it does not matter how large the array is since it's
not allocated on the stack, which the other variables (len, width).
Arr must be a pointer since the compiler doesn't know at compile-time
how much space to reserve on the stack for the array, so it makes room
for an array and handles the allocation and deallocation for you. So
basically it's just a compiler generated wrapper around

int* Arr;
arr = new int[len];

// At end of scope
delete[] Arr;

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Hi Eric,
I investigated further and printed out &Arr[0], &Arr[1],.... These
addresses looked like stack addresses ad not heap addresses. So it is
unlikely that compiler is using new.

[Adrian Hawryluk]

Sunny wrote:

>
Hi Eric,
I investigated further and printed out &Arr[0], &Arr[1],.... These
addresses looked like stack addresses ad not heap addresses. So it is
unlikely that compiler is using new.

You are right. It is allocating it on the stack. The sizes and address
locations indicate such. However, this is not (currently) in the C++
spec, this is (as Ian stated) an extension. Do this at your own peril.
If you move to another compiler, it may not work.
Adrian
--
_______________ _______________ _______________ _______________ _________
\/Adrian_Hawryluk BSc. - Specialties: UML, OOPD, Real-Time Systems\/
\ _---_ Q. What are you doing here? _---_ /
\ / | A. Just surf'n the net, teaching and | \ /
\__/___\___ learning, learning and teaching. You?_____/___\__/
\/______[blog:__http://adrians-musings.blogspo t.com/]______\/