Re: std::list remove element mid iteration

James Kanze <>
Thu, 20 Dec 2007 01:40:23 -0800 (PST)
On Dec 20, 2:10 am, red floyd <no.s...@here.dude> wrote:

Christopher wrote:

The situation is that a std::list<std::set<std::string> > is being
iterated through. Upon certain criteria some sets become empty. I need
to remove the empty sets from the list.

Is it safe to iterate through a list and call list::erase( iterator )
in mid iteration?

Well, you can use

struct set_is_empty
     bool operator()(const std::set& s) const { return s.empty(); }

std::erase(std::remove_if(l.begin(), l.end(), set_is_empty());

Which could be unnecessarily expensive. In the case of
std::list, the canonical form is:

    l.remove_if( set_is_empty() ) ;

However, the original poster said that sets "become" empty
during his iteration, so this can't be used.

or else, the canonical iteration for this list is:

for (it = l.begin(); it != l.end(); )
    if (it->empty())
       it = l.erase(it);

Adopted to his case, you'd add braces and put the if at the end
of the loop. (Also, I'd write this with a while, rather than a
for. Something like:

    std::list<...>::iterator iter = l.begin() ;
    while ( iter != l.end() ) {
        // processing...
        if ( iter->empty() ) {
            iter = l.erase( iter ) ;
        } else {
            ++ iter ;

I'd prefer even more if that if could be replaced with a ?: on
the right side of an assignment, since the most important aspect
here is the update of the iterator, and not how it's being
updated, but I can't think of a nice way of doing this off hand.

James Kanze (GABI Software)
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