CODEWITHSUNDEEP
c++stliteratorcontainers
Luca
Luca

Reputation: 1776

container iterators and operations on containers

I'm studying C++ and I'm reading about STL containers,iterators and the operations that can be performed on them. I know that every container type (or better, the corresponding template of which each type is an instance) defines a companio type that acts like a pointer-like type and it's called iterator. What I understand is that once you get an iterator to a container,performing operations like adding an element may invalidate that iterator,so I tried to test this statement with an example:

#include <vector>
#include <iostream>

using namespace std;

int main()
{
  vector<int> ivec={1,2,3,4,5,6,7,8,9,0}; 
  auto beg=ivec.begin();
  auto mid=ivec.begin()+ivec.size()/2;
  while (beg != mid) {
    if (*beg==2) 
      ivec.insert(beg,0);
    ++beg;
  }  
  for (auto i:ivec)
    cout<<i<<" ";
}

here,I'm simply contructing a vector of ints, brace initialize it,and performing a condition based operation,inserting an element in the first half of the container. The code is flawed I think, because I'm initializing two iterator objects beg and end and then I use them in the while statement as a condition. BUT, if the code should change the contents of the container (and it sure does) what happens to the iterators? The code seems to run just fine,it add a 0 in the ivec[1] position and prints the result.

What I thought is that the beg iterator would point to the newly added element and that the mid iterator would have pointed to the element before the formerly pointed to by mid (it's like the iterators point to the same memory locations while the underlying array,"slides" under.. unless it's reallocated that is)

Can someone explain me this behaviour??

Upvotes: 0

Views: 133

Answers (1)

John Zwinck
John Zwinck

Reputation: 249582

When the standard says iterators are invalidated, this does not guarantee that they will be invalid in the sense of preventing your program from working. Using an invalid iterator is undefined behavior which is a huge and important topic in C++. It doesn't mean your program will crash, but it might. Your program might also do something else--the behavior is completely undefined.

Upvotes: 5

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