Re: changing vector while processing one of its elements?

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 10 Jun 2008 02:53:38 -0700 (PDT)
Message-ID:
<acc372cd-54a4-4345-ad63-13ba77d54f8c@s50g2000hsb.googlegroups.com>
On Jun 9, 11:43 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

Markus Dehmann wrote:

I observed a behavior that I didn't expect: I have a vector
of sets. I iterate over the first of these sets, and while I
iterate over it, I add another set at the end of the vector.
I thought that shouldn't affect the first set I am iterating
over, but it does, and I get a segmentation fault.


It does not affect the first set _as a value_. However, it
affects the first set _as an object (region of memory)_
because inserting an element into the vector can trigger a
re-allocation of the vector and then all current elements are
moved around.

See the example program below.

It works fine if I make set0 a *copy* of v[0], instead of a reference
or a pointer, but I would rather not copy it (this routine is called
very often in my program and I don't see why I should make an
expensive copy).

How can I fix this? Thanks!

int main(int argc, char** argv){
  std::vector<std::set<int> > v;
  std::set<int> s;
  s.insert(1);
  s.insert(2);
  v.push_back(s);
  std::set<int>& set0 = v[0]; // using reference because we don't want=

to copy to local var (too expensive)


You are using a reference. Insertions into a vector can
invalidate all references, pointers, and iterators into the
vector (for the reasons mentioned above).

  for(std::set<int>::const_iterator it = set0.begin(); it !=
set0.end(); ++it){
    std::cout << *it << std::endl;
    std::set<int> tmp;
    tmp.insert(10);
    v.push_back(tmp); // will be v[1], so it shouldn't change set0 or
its
iterator?
  }
  return 0;
}

As output I get:
1
10
1
10
1
10
...
Segmentation fault


Yup, that the undefined behavior from using an invalidated reference.

Your options include:

a) Use std::list instead of std::vector.
b) Instead of inserting the new sets right away, put them on hold.
c) Use std::vector< some_smart_pointer< std::set<int> > >.


d) Use v[0] each time you need it, rather than saving the
   reference.

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James Kanze (GABI Software) email:james.kanze@gmail.com
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