In article
<c2d30ddb-efea-4c07-be42-b34304602329@[EMAIL PROTECTED]
>,
Greg Herlihy <greghe@[EMAIL PROTECTED]
> wrote:
> On Apr 26, 2:34 am, Carl Barron <cbarron...@[EMAIL PROTECTED]
> wrote:
> > In article
> > <6eaa63d2-cb20-432a-a500-dc610e9f8...@[EMAIL PROTECTED]
>,
> > Greg Herlihy <gre...@[EMAIL PROTECTED]
> wrote:
> > > Calling std::remove_if() on a std::list can invalidate the list's
> > > iterators (meaning that after the call to remove_if() - each
iterator
> > > in the list might no longer reference the same value bas it did
before
> > > the call)
> >
> > True the iterators of the 'removed' items are invalid, but the
> > iterators to the remaining ones are valid.
> >
> > bool is_even(int);
> > std::list<int> foo;
> > for(int i=0;i!=10;++i) foo.push_back(i); // list =
{0,1,2,3,4,5,..9{
> > std::list<int>::iterator last =
std::remove_if(foo.begin(),foo.end(),
> > is_even);
> >
> > list is now {1,3,5,7,9,j,k,k,k,k} still 10 entries but only first 5
> > [last points to j] are in the sequence. all iterators in
> > [foo.begin(),last) are valid.
>
> No, none of the list's original iterators - including all five
> iterators whose values remove_if() did not delete - is still valid.
> After the call to std::remove_if() in the example above, the iterators
> for the values 1, 3, 5, 7, 9 - either wind up referring to a different
> value or referring to no existing value in the list at all (and in
> either case - an iterator that no longer points to the same
> (undeleted) value in the list - is - as far as both the programmer
> and the C++ Standard is concerned - an invalidated iterator.
>
List provides remove/remove_if members because the operations are
'cheap' and only involve internal pointer manipuation to modify the
list. Further this can be done without invoking a copy ctor. Both
good reasons for implementation in std::list<T,A>.
For what its worth , this shows what I said is true and the iterators
in [ints,begin(),last) are all valid.
run this:
<code>
#include <list>
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
inline bool is_even(int x) { return x%2 == 0;}
int main()
{
std::list<int> ints;
std::vector<std::list<int>::iterator> its;
for(int i=0;i!=10;++i) ints.push_back(i);
for(std::list<int>::iterator i=ints.begin();i!=ints.end();++i)
its.push_back(i);
std::list<int>::iterator last = std::remove_if
(ints.begin(),ints.end(),is_even);
std::copy(ints.begin(),last,std::ostream_iterator<int>(std::cout,"\n"));
std::cout << "\nvalues of original iterators:\n";
for(std::vector<std::list<int>::iterator>::iterator it =
its.begin();it != its.end();++it)
{
std::cout << **it << '\n';
};
std::cout << "\nvalues of incrementing original iterators:\n";
for(std::vector<std::list<int>::iterator>::iterator it =
its.begin();it != its.end();++it)
{
std::list<int>::iterator p = *it;
if(p==ints.end()) continue;
++p;
std::cout << *p << '\n';
}
}
</code>
<ans>
1
3
5
7
9
values of original iterators:
1
3
5
7
9
5
6
7
8
9
values of incrementing original iterators:
3
5
7
9
5
6
7
8
9
</ans>
looks like iterators pointing into [ints.begin(),last) all are valid
and they increment properly. iterators pointing to the removed but
not erased point to the same values and increment correctly to me.
--
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