[Container] --begin() on empty deque crashes

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TONGARI

19 Jan 2013 19 Jan '13

12:06 p.m.

Hi there, The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ---------------------------------------- boost::container::deque<int> d; --d.begin(); ---------------------------------------- I took a look into the impl, seems that it dereferences from (this->m_node - 1) which is invalid at the time. I guess it's the problem. Thoughts?

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Ion Gaztañaga

19 Jan 19 Jan

6:18 p.m.

El 19/01/2013 13:06, TONGARI escribió:

...

Hi there,

The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ---------------------------------------- boost::container::deque<int> d; --d.begin(); ----------------------------------------

The problem is that decrementing a begin iterator is not a valid operator, since it would point out of the container. Ion

TONGARI

20 Jan 20 Jan

4:23 a.m.

2013/1/20 Ion Gaztañaga

...

El 19/01/2013 13:06, TONGARI escribió:

Hi there,

...
The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ------------------------------**---------- boost::container::deque<int> d; --d.begin(); ------------------------------**----------

The problem is that decrementing a begin iterator is not a valid operator, since it would point out of the container.

But wouldn't it be a valid operation as long as we don't dereference from it?

Marshall Clow

4:39 a.m.

On Jan 19, 2013, at 8:23 PM, TONGARI wrote:

...

2013/1/20 Ion Gaztañaga El 19/01/2013 13:06, TONGARI escribió:

Hi there,

The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ---------------------------------------- boost::container::deque<int> d; --d.begin(); ----------------------------------------

The problem is that decrementing a begin iterator is not a valid operator, since it would point out of the container.

But wouldn't it be a valid operation as long as we don't dereference from it?

I don't believe so. If it was a pointer, then yes. The closest I could find in the standard was section 24.2.6 (talking about bidirectional iterators), it says that a precondition for --r is: there exists s such that r == ++s -- Marshall Marshall Clow Idio Software mailto:mclow.lists@gmail.com A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait). -- Yu Suzuki

TONGARI

5:19 a.m.

2013/1/20 Marshall Clow

...

On Jan 19, 2013, at 8:23 PM, TONGARI wrote:

2013/1/20 Ion Gaztañaga

...
El 19/01/2013 13:06, TONGARI escribió:

Hi there,

...
The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ------------------------------**---------- boost::container::deque<int> d; --d.begin(); ------------------------------**----------

The problem is that decrementing a begin iterator is not a valid operator, since it would point out of the container.

But wouldn't it be a valid operation as long as we don't dereference from it?

I don't believe so. If it was a pointer, then yes.

The closest I could find in the standard was section 24.2.6 (talking about bidirectional iterators), it says that a precondition for --r is: there exists s such that r == ++s

OK, thanks. According to the requirement it's invalid indeed, I thought that we have ++end() a valid op so it must apply to --begin() as well, but that's not true.

TONGARI

5:23 a.m.

2013/1/20 TONGARI

...

2013/1/20 Marshall Clow

...
On Jan 19, 2013, at 8:23 PM, TONGARI wrote:

2013/1/20 Ion Gaztañaga

...
El 19/01/2013 13:06, TONGARI escribió:

Hi there,

...
The code below would simply crash on g++ 4.7.1/MinGW with default optimization level, it doesn't shown with -O/1/2/3 ------------------------------**---------- boost::container::deque<int> d; --d.begin(); ------------------------------**----------

The problem is that decrementing a begin iterator is not a valid operator, since it would point out of the container.

But wouldn't it be a valid operation as long as we don't dereference from it?

I don't believe so. If it was a pointer, then yes.

The closest I could find in the standard was section 24.2.6 (talking about bidirectional iterators), it says that a precondition for --r is: there exists s such that r == ++s

OK, thanks. According to the requirement it's invalid indeed, I thought that we have ++end() a valid op so it must apply to --begin() as well, but that's not true.

Ah, wrong again, ++end() is invalid, sorry for the noise.

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Ion Gaztañaga
Marshall Clow
TONGARI