Re: std::list safely pop out iterator

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 8 Jan 2008 08:28:23 -0800

Message-ID:

<bxNgj.38$mK7.13@newsfe06.lga>

TBass wrote:

Hi,

I've got a class that uses a std::list to store tags that need to be
written out to a device. I've had no problem with the lists in terms
of iterating and reading values. My problem is when I need to pop that
value off the list when I'm done with it.

In this case, I think the iterator is getting "lost" because I pop off
the value it is pointing to at the end of this function. But how do I
pop values off one at a time without the iterator getting lost? The
erase function will just have the same problem.

Instead of hacking at it, I was wondering if anyone could point me
towards the standard solution when dealing with std::list.

Thanks in advance,
T

unsigned int duty_count;
std::list<CFusionTag *> *mylist;
std::list<CFusionTag *>::iterator it;
FU_Tag fu_tag;
FusionPlugin *myplg = m_pDevice->Driver();
CFusionTag *mytag;

mylist = m_pDevice->GetWriteTagList();
for( it=mylist->begin(), duty_count = 0;
it != mylist->end() && duty_count < m_pDevice->DutyCycle();
++it, ++duty_count )
{

mytag = *it;

if ( mytag->IsRegistered() )
{

if ( mytag->ClientAccess() == mytag->AccessWriteOnly

mytag->ClientAccess() == mytag->AccessReadWrite )

{

memcpy( fu_tag.Address, mytag->Address(), strlen( mytag-

Address() ) );

fu_tag.DeviceID = m_pDevice->RKey();
fu_tag.TagID = mytag->RKey();
fu_tag.TagType = mytag->Type();
mytag->CreateFUTagValue( &fu_tag.Value );
mytag->GetValue( &fu_tag.Value );

int nret = myplg->m_PluginModules->SetTagValue( &fu_tag );
if ( nret <= 0 )
{
mytag->Quality( 0 );
}

/* DELETE THE VALUE POINTER WE CREATED */
delete fu_tag.Value;

/* POP THIS OFF THE LIST */
mylist->pop_front();
}

}

}

Okay, you are using pop_front() to delete an element. How do you know that
it is pointing to the first element though? You are incrementing it in the
for loop, it may not be pointing to the first element. Wouldn't it be
better to do mylist->erase( it )? And erase returns an element to the next
item in the list. The normal algorithm for this is:

for ( std::list<CFusionTag *>::iterator it = MyList.being(); it !=
MyList.end(); }
{
   if ( somecondition )
      it = MyList.erase( it );
   else
      ++it;
}

Notice that it is not being incremented in the for loop. It is only
incremented if the element is not erased.

Now, if you really are wanting to erase the front elements only (which seems
like a bug in your code, but I may be wrong) then simply have it point to
the first element again after erasing it.
  it = MyList->begin();

--
Jim Langston
tazmaster@rocketmail.com