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std::map and erase

Started by Paradigm Shifter Apr 26, 2004 at 12:38 PM 2 replies 750+ views

Original Post

145

Apr 26, 2004 at 12:38 PM

Is it safe to ++ a map iterator when its equal to the end()? Or -- when its equal to begin()? map::erase returns an iterator to the next valid element, or end() if the last element was erased, so if we have this code

for( itMap = myMap.begin() ; itMap != myMap.end() ; ++itMap )
{
    if( /* some condition */ )
    {
        itNext = myMap.erase( itMap ) ;
        // make sure next iteration is correct

        itMap = --itNext;
    }
}

is it safe, or do I have to put checks for itNext == end() and begin() in as well? "Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

Agony

177

Agony

Apr 26, 2004 at 12:49 PM

My loops that involve erasing typically go like this:

std::something<something>::iterator i = Container.begin();
while (i != Container.end())
{
  if (SomeCondition)
    i = Container.erase(i);
  else
    ++i;
}

Thus, I only increment if I didn't erase, since the erase in effect does the increment for me already. However, I'm guessing that doing a decrement when you erase, and then incrementing every time regardless of erasing or not, would work fine on at least some types of containers, such as vectors. I couldn't guarantee anything, though, especially for containers like lists and maps, since incrementing and decrementing iterators is not just simple pointer addition/subtraction. Just to be on the safe side, I wouldn't recommend it.

[edited by - Agony on April 26, 2004 4:02:53 PM]

TheSalmon

Apr 26, 2004 at 2:55 PM

quote:
Original post by Agony
Thus, I only increment if I didn''t erase

Actually, you always increment with that code no matter what since you have ++i in your for loop. Typo I guess?

Agony

177

Agony

Apr 26, 2004 at 3:02 PM

Wow. Typo indeed. Thanks. I''ll edit it. I meant to use a while loop... Silly me.

Most of the greatest evils that man has inflicted upon man have come through people feeling quite certain about something which, in fact, was false.

-Bertrand Russell, Unpopular Essays, "Ideas That Have Harmed Mankind"

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