List iterator vs. vector iterator

I have two questions regarding iterators.

I thought the once you define an iterator to an STL container such as a vector or a list, if you add elements to the containers then these iterators won't be able to access them. But the following code defines a list of five elements and then adds another element in each loop iteration and results in an infinite loop:

#include <iostream>
#include <list>

using namespace std;

int main()
{
    list<int> ls;

    for(int i = 0; i < 5; i++)
    {
        ls.push_back(i);
    }

    int idx = 0;

    for(list<int>::iterator iter = ls.begin(); iter != ls.end(); iter++)
    {
        cout << "idx: " << idx << ", *iter: " << *iter << endl;
        ls.push_back(7);
        idx++;
    }
}

However, doing the same for a vector results in an error:

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> vec;

    for(int i = 0; i < 5; i++)
    {
        vec.push_back(i);
    }

    int idx = 0;

    for(vector<int>::iterator iter = vec.begin(); iter != vec.end(); iter++)
    {
        cout << "idx: " << idx << ", *iter: " << *iter << endl;
        vec.push_back(7);
        idx++;
    }
}

I thought that when the vector container must be resized, it does so at powers of 2 and is located to a new area of memory, which is why you shouldn't define an iterator to a vector if you adding elements to it (since the iterators don't get passed to the new memory location). For example, I thought a vector containing 16 elements, after calling the push_back function, will be allocated space for 32 elements and the entire vector will be relocated. However, the this didn't happen for the following code. Was I just mistaken?
```
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> vec;

    for(int i = 0; i < 4; i++)
    {
        vec.push_back(i);
        cout << "address of vec: " << &vec << ", capacity: " << vec.capacity() << endl;
    }



    for(int i = 0; i < 20; i++)
    {
        vec.push_back(i);
        cout << "address of vec: " << &vec << ", capacity: " << vec.capacity() << endl;
    }
}
```

Solution

Different container's iterators have different properties. Here are the iterator invalidation rules.

The list loop: When you push onto a list all previous iterators still valid. You will never hit the end if every time you iterator forward one you also add a new element, obviously.

The vector loop: For a vector, your iterators are invalid once a push_back results in the new size exceeding the old capacity. As soon as this happens, using iter is undefined behavior (you will likely crash).

I thought that when the vector container must be resized, it does so at powers of 2 and is located to a new area of memory

This is unspecified by the standard. Some implementations of the C++ standard library double the capacity of a vector when the size exceeds the old capacity, and others grow at different rates.