List: Erase element at iterator during iterations

[Halsafar]

I em having trouble with a list iteration.

I have to tranverse a list rather frequently, if an element is flagged for deletion then it must be removed.

for (list<int> iter = l.begin(); iter != l.end(); ++iter)
{
iter = l.erase(iter);
cout << (*iter) << endl;
}

This deletes every even number... since the first element is deleted, the 2nd is returns, then it is incremented by the for loop and deleted.

[bilm_ks]

Increment "manually"
for (iter = l.begin(); iter!=l.end()
{
if (must erase)
iter = l.erase(iter);
else
++iter;
}

[dis1411]

go through the list backwards

for (list<int> iter = l.end(); iter != l.begin(); --iter)
{
l.erase(iter);
cout << (*iter) << endl;
}

[twanvl]

This will not work, when you call l.erase(iter) the iterator becomes invalid, any other action with that iterator is illegal.

The idiomatic C++ way to do what you want to do is using list::remove_if. An alternative is to increment the iterator before erasing it:

for (list<int>::iterator iter = l.begin(); iter != l.end(); )
{
    list<int>::iterator to_erase = iter;
    ++iter;
    if (i_want_to_erase_it) {
        iter = l.erase(to_erase);
    }
}

[Halsafar]

twanvl && bilm_ks methods worked fine with the expected results.

Now, twanvl your method works identically to bilm_ks, I cannot see why your method is more "correct".

If you go iter = l.erase(iter). The returned iterator is valid.
If you do not erase then ++iter.

So

while (iter != l.end())
{
if (erase) iter = l.erase(iter);
else ++iter;
}

[CornedBee]

Indeed, bilm_ks's method is also recommended by Scott Meyers in Effective STL.

But twanvl is right, remove_if would be the typical way to do this.