Problem with Map Iterator

Problem with Map Iterator

[Sanjib Talukdar]

Hi,

I am getting a segmentation fault error while executing the following
code:

	if (!this->colDtlsByName.empty())
	{
	     std::map< std::string, CS_DATAFMT*, 
	     std::less<std::string> >::iterator iterMap; 
	    for (iterMap = this->colDtlsByName.begin();
		iterMap != this->colDtlsByName.end(); ++iterMap)
	   {
	       CS_DATAFMT* obj = iterMap->second;
	       if (obj != NULL)
	             delete obj;
	       this->colDtlsByName.erase(iterMap);
	   }
	}

The backtrace from gdb is as follows:

	#0  0xff0e29b4 in std::_Rb_tree_increment () from
/usr/sfw/lib/libstdc++.so.6
	#1  0x0009854c in std::_Rb_tree_iterator<std::pair<std::string
const, _cs_datafmt*> >::operator++ (this=0xfe77a0c4) at stl_tree.h:180
	#2  0x0005a84c in MACDBQueryHandler::clearColInfo
(this=0xfe77bd30, inType=ALL)
	    at MACDBQueryHandler.C:123

The above code is in the method MACDBQueryHandler::clearColInfo() which
is being invoked in the class destructor.
We are using gcc version 3.4.3 on Solaris 10. The library libstdc++.so.6
is pointing to libstdc++.so.6.0.3.

Can anyone please point out the reason for the error?
Thank You.
Sanjib.

Re: Problem with Map Iterator

[John Love-Jensen]

Hi Sanjib,

The erase(iterMap) invalidates all iterators into the map.

HTH,
--Eljay

Re: Problem with Map Iterator

[Burak Serdar]

As far as I know, map::erase invalidates the iterator, so incrementing
the iterator after erasing it is invalid.

On 5/15/06, Sanjib Talukdar <sanjibt@ascindia.com> wrote:
> Hi,
>
> I am getting a segmentation fault error while executing the following
> code:
>
>         if (!this->colDtlsByName.empty())
>         {
>              std::map< std::string, CS_DATAFMT*,
>              std::less<std::string> >::iterator iterMap;
>             for (iterMap = this->colDtlsByName.begin();
>                 iterMap != this->colDtlsByName.end(); ++iterMap)
>            {
>                CS_DATAFMT* obj = iterMap->second;
>                if (obj != NULL)
>                      delete obj;
>                this->colDtlsByName.erase(iterMap);
>            }
>         }
>
> The backtrace from gdb is as follows:
>
>         #0  0xff0e29b4 in std::_Rb_tree_increment () from
> /usr/sfw/lib/libstdc++.so.6
>         #1  0x0009854c in std::_Rb_tree_iterator<std::pair<std::string
> const, _cs_datafmt*> >::operator++ (this=0xfe77a0c4) at stl_tree.h:180
>         #2  0x0005a84c in MACDBQueryHandler::clearColInfo
> (this=0xfe77bd30, inType=ALL)
>             at MACDBQueryHandler.C:123
>
> The above code is in the method MACDBQueryHandler::clearColInfo() which
> is being invoked in the class destructor.
> We are using gcc version 3.4.3 on Solaris 10. The library libstdc++.so.6
> is pointing to libstdc++.so.6.0.3.
>
> Can anyone please point out the reason for the error?
> Thank You.
> Sanjib.
>

Re: Problem with Map Iterator

[Aseem Rastogi]

Not all. Only the one pointed to by the iterator.

John Love-Jensen wrote:

>Hi Sanjib,
>
>The erase(iterMap) invalidates all iterators into the map.
>
>HTH,
>--Eljay
>
>


-- 
Nothing will work if u don't.

Re: Problem with Map Iterator

[Aseem Rastogi]

could be because when u have erased the map node pointed to by iterMap, 
iterMap++ crashes as the memory location pointed to by iterMap becomes 
invalid.

so, instead of erasing in loop, do a clear () on map after the loop.

see inline.

hope this helps.

Sanjib Talukdar wrote:

>Hi,
>
>I am getting a segmentation fault error while executing the following
>code:
>
>	if (!this->colDtlsByName.empty())
>	{
>	     std::map< std::string, CS_DATAFMT*, 
>	     std::less<std::string> >::iterator iterMap; 
>	    for (iterMap = this->colDtlsByName.begin();
>		iterMap != this->colDtlsByName.end(); ++iterMap)
>	   {
>	       CS_DATAFMT* obj = iterMap->second;
>	       if (obj != NULL)
>	             delete obj;
>	       this->colDtlsByName.erase(iterMap);             /////////   Remove this line
>	   }
>

 >>> call this->colDtlsByName.clear ();

>
>	}
>
>The backtrace from gdb is as follows:
>
>	#0  0xff0e29b4 in std::_Rb_tree_increment () from
>/usr/sfw/lib/libstdc++.so.6
>	#1  0x0009854c in std::_Rb_tree_iterator<std::pair<std::string
>const, _cs_datafmt*> >::operator++ (this=0xfe77a0c4) at stl_tree.h:180
>	#2  0x0005a84c in MACDBQueryHandler::clearColInfo
>(this=0xfe77bd30, inType=ALL)
>	    at MACDBQueryHandler.C:123
>
>The above code is in the method MACDBQueryHandler::clearColInfo() which
>is being invoked in the class destructor.
>We are using gcc version 3.4.3 on Solaris 10. The library libstdc++.so.6
>is pointing to libstdc++.so.6.0.3.
>
>Can anyone please point out the reason for the error?
>Thank You.
>Sanjib.
>


-- 
Nothing will work if u don't.

Re: Problem with Map Iterator

[John Love-Jensen]

Hi Aseem,

> Not all. Only the one pointed to by the iterator.

I stand corrected.  Thanks.

Sincerely,
--Eljay

vector
all vector iterators are invalidated on any change

string & wstring
all string iterators are invalidated on any change

deque
all deque iterators are invalidated on any change, except for beginning and
end removals

list
no list iterators are invalidated, except for those referring to a deleted
element

set & multiset
no iterators are invalidated except ones pointing to a removed element

map & multimap
no iterators are invalidated except ones pointed to a removed element

Re: Problem with Map Iterator

[Peter Doerfler]

Aseem Rastogi wrote:
> could be because when u have erased the map node pointed to by iterMap, 
> iterMap++ crashes as the memory location pointed to by iterMap becomes 
> invalid.
> 
> so, instead of erasing in loop, do a clear () on map after the loop.
> 

Alternatively, if you can't do that you can increment first and do the 
erase() then. This creates a bit of overhead and a while loop is more 
appropriate -- along these lines:

while (iterMap != iterMapEnd) {
            CS_DATAFMT* obj = iterMap->second;
            if (obj != NULL)
                  delete obj;
            this->colDtlsByName.erase(iterMap++);
}

In your example, where you actually erase all nodes, the clear() method 
is more appropriate, but if you need to remove nodes conditionally, the 
above is an option.

I also seem to recall reading that std::map::erase will return an 
iterator pointing to the next element in the future which would simplify 
things.... Found it: It's on the v7 branch.

Best, Peter

> see inline.
> 
> hope this helps.
> 
> Sanjib Talukdar wrote:
> 
>> Hi,
>>
>> I am getting a segmentation fault error while executing the following
>> code:
>>
>>     if (!this->colDtlsByName.empty())
>>     {
>>          std::map< std::string, CS_DATAFMT*,          
>> std::less<std::string> >::iterator iterMap;         for (iterMap = 
>> this->colDtlsByName.begin();
>>         iterMap != this->colDtlsByName.end(); ++iterMap)
>>        {
>>            CS_DATAFMT* obj = iterMap->second;
>>            if (obj != NULL)
>>                  delete obj;
>>            this->colDtlsByName.erase(iterMap);             /////////   
>> Remove this line
>>        }
>>
> 
>  >>> call this->colDtlsByName.clear ();
> 
>>
>>     }
>>
>> The backtrace from gdb is as follows:
>>
>>     #0  0xff0e29b4 in std::_Rb_tree_increment () from
>> /usr/sfw/lib/libstdc++.so.6
>>     #1  0x0009854c in std::_Rb_tree_iterator<std::pair<std::string
>> const, _cs_datafmt*> >::operator++ (this=0xfe77a0c4) at stl_tree.h:180
>>     #2  0x0005a84c in MACDBQueryHandler::clearColInfo
>> (this=0xfe77bd30, inType=ALL)
>>         at MACDBQueryHandler.C:123
>>
>> The above code is in the method MACDBQueryHandler::clearColInfo() which
>> is being invoked in the class destructor.
>> We are using gcc version 3.4.3 on Solaris 10. The library libstdc++.so.6
>> is pointing to libstdc++.so.6.0.3.
>>
>> Can anyone please point out the reason for the error?
>> Thank You.
>> Sanjib.
>>
> 
> 

Re: Problem with Map Iterator

[Dima Sorkin]

Hi.
 There is an item exactly on this issue (deleting iterators) in
S.Meyers "Effective STL". I think there was something about incrementing
too.

Regards,
 Dima.

On 5/15/06, Peter Doerfler wrote:
>
>
> Aseem Rastogi wrote:
> > could be because when u have erased the map node pointed to by iterMap,
> > iterMap++ crashes as the memory location pointed to by iterMap becomes
> > invalid.
> >
> > so, instead of erasing in loop, do a clear () on map after the loop.
> >
>
> Alternatively, if you can't do that you can increment first and do the
> erase() then. This creates a bit of overhead and a while loop is more
> appropriate -- along these lines:
>
> while (iterMap != iterMapEnd) {
>            CS_DATAFMT* obj = iterMap->second;
>            if (obj != NULL)
>                  delete obj;
>            this->colDtlsByName.erase(iterMap++);
> }
>
> In your example, where you actually erase all nodes, the clear() method
> is more appropriate, but if you need to remove nodes conditionally, the
> above is an option.
>
> I also seem to recall reading that std::map::erase will return an
> iterator pointing to the next element in the future which would simplify
> things.... Found it: It's on the v7 branch.
>
> Best, Peter