Incorrect behaviour with vector::erase( vector<T>::iterator ) ?

[Jeroen Dobbelaere]

Hi,

I think your code contains a bug :

Taking a look a note 5 on :

< http://www.sgi.com/Technology/STL/Vector.html >
--
[5] A vector's iterators are invalidated when its memory is reallocated. Additionally, inserting or deleting
an element in the middle of a vector invalidates all iterators that point to elements following the
insertion
or deletion point. It follows that you can prevent a vector's iterators from being invalidated if you use
reserve() to preallocate as much memory as the vector will ever use, and if all insertions and deletions
are at the vector's end. 
--

My guess is that the iterator doom->where is not valid any more after one of the other insertions.
(It can point to something that looks valid (the value of (*(doomed->where))->junk  is what we would
expect),
but isn't (the object to which it points doesn't belong any more to the current 'vec'))

I can't try it out right now, but you could try adding 'vec.reserve(20)' after the declaration of vec.

Greetings,

Jeroen Dobbelaere


Michael K Vance wrote:
> 
> Output on g++-2.8.1:
> 
> 13
> 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
> 
> Output on egcs-1.1.2:
> 
> 13
> 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
> 1 2 3 4 134527404 134527408 134527412 134528056 134528060 134528064 134528068
> 134528072 134528076 134528080 134528084 134528952 17 18 19
> 
> These both seem incorrect, though the g++-2.8.1 seems at least a little more
> correct. Am I missing something here?
> 
> Thanks,
> 
> m.
> 
> --
> 
> #include <vector>
> 
> class Bob
> {
> public:
>     int junk;
>     vector<Bob*>::iterator where;
> 
>     Bob( int i ) : junk( i ) { };
> };
> 
> int main( void )
> {
>     vector<Bob*> vec;
>     Bob* doomed;
> 
>     for( int i = 0; i < 20; i++ ) {
>         Bob* b = new Bob( i );
>         vector<Bob*>::iterator where = vec.insert( vec.end( ), b );
>         b->where = where;
> 
>         if( i == 13 ) {
>             doomed = b;
>         }
> 
>     }
> 
>     cerr << (*(doomed->where))->junk << endl;
> 
>     for( int i = 0; i < 20; i++ ) {
>         cerr << vec[i]->junk << " ";
>     }
> 
>     vec.erase( doomed->where );
>     cerr << endl;
> 
>     for( int i = 0; i < 19; i++ ) {
>         cerr << vec[i]->junk << " ";
>     }
> 
>     cerr << endl;
> 
>     return 0;
> }
> 
> --
> "We can deny everything, except that we have the possibility of being better.
>  Simply reflect on that."
>  -- His Holiness The Dalai Lama