Re: Deleting items from std::list

From:

Markus Schoder <a3vr6dsg-usenet@yahoo.de>

Newsgroups:

comp.lang.c++

Date:

Wed, 28 Jun 2006 02:57:52 +0200

Message-ID:

<44a1d411$0$26272$9b4e6d93@newsread2.arcor-online.net>

P.J. Plauger wrote:

"Markus Schoder" <a3vr6dsg-usenet@yahoo.de> wrote in message
news:44a19d4b$0$29149$9b4e6d93@newsread4.arcor-online.net...

P.J. Plauger wrote:

"Markus Schoder" <a3vr6dsg-usenet@yahoo.de> wrote in message
news:44a02da0$0$29123$9b4e6d93@newsread4.arcor-online.net...

Howard Hinnant wrote:

In article <1151276851.123572.175280@m73g2000cwd.googlegroups.com>,
"Markus Svilans" <msvilans@gmail.com> wrote:

std::list<int> data;

// [ code here to load data with values ]

std::list<int>::reverse_iterator ri = data.rbegin();

while (ri != data.rend())
{
    // Get next iterator in case ri is erased.
    std::list<int>::reverse_iterator next = ri;
    ++next;

    // Check if ri needs to be erased
    if (*ri == VALUE_TO_ERASE)
    {
        // [ code here to process *ri before erasing ]
        data.erase(ri); // <-- Causes compiler error
    }

    ri = next;
}

Obviously an erase method is not defined for reverse iterators in
std::list.

The solution below is better but just for completeness sake

data.erase(ri.base());

should work.

for (std::list<int>::iterator ri = data.end(); ri != data.begin();)
{
    if (*--ri == VALUE_TO_ERASE)
    {
        // [ code here to process *ri before erasing ]
        ri = data.erase(ri);
    }
}

For completeness sake you have to specify how to make it past the
erased element with your reverse iterator. It's way messier than
for Hinnant's direct solution.

Maybe I wasn't clear enough but I meant to say his (Hinnant's)
solution is better. Just wanted to point out that you can erase
through a reverse iterator. The way I did it is however not correct
since &*(ri.base()-1) == &*ri.

[snip]

2) Your expression above is ill formed, since base() returns a
bidirectional iterator, which you can't subtract one from.

I expect your defect report on comp.std.c++ shortly since the standard uses
similar notation in the reverse_iterator section. Hint: It is not actual
code but just explanatory.

[snip]

So my point was that sketching a partial solution, which is
demonstrably hard to get right, doesn't add much to "completeness."
At least we're in violent agreement that Hinnant did it best.

Since the OP stated that it is not possible to erase through a
reverse_iterator I consider it helpful and adding to "completeness" to
point out that it is indeed possible with the base() function even if it is
difficult to handle (which I proved by getting it wrong) and should better
be avoided if possible.