What is the behavior of the vector here? - stl

Question

I am unable to comprehend why in the test shown below, iterator p never reaches the end and therefore the loop breaks only when k = 20? What exactly is the push_back doing to cause undefined behavior? Is it because the vector dynamically allocated a bunch of additional storage for the new elements I want to use, and the amount is not necessarily the amount I will use?
#include <iostream>
#include <vector>
#include <list>
using namespace std;
const int MAGIC = 11223344;
void test()
{
bool allValid = true;
int k = 0;
vector<int> v2(5, MAGIC);
k = 0;
for (vector<int>::iterator p = v2.begin(); p != v2.end(); p++, k++)
{
if (k >= 20) // prevent infinite loop
break;
if (*p != MAGIC)
{
cout << "Item# " << k << " is " << *p << ", not " << MAGIC <<"!" << endl;
allValid = false;
}
if (k == 2)
{
for (int i = 0; i < 5; i++)
v2.push_back(MAGIC);
}
}
if (allValid && k == 10)
cout << "Passed test 3" << endl;
else
cout << "Failed test 3" << "\n" << k << endl;
}
int main()
{
test();
}

Answer 1

Insertion to a vector while iterating over it is really a bad idea. Data insertion may cause memory reallocation that invalidates iterators. In this case, the capacity was not enough to insert additional elements, which caused memory allocation with a different address. You can check it yourself:
void test()
{
bool allValid = true;
int k = 0;
vector<int> v2(5, MAGIC);
k = 0;
for (vector<int>::iterator p = v2.begin(); p != v2.end(); p++, k++)
{
cout << v2.capacity() << endl; // Print the vector capacity
if (k >= 20) // prevent infinite loop
break;
if (*p != MAGIC) {
//cout << "Item# " << k << " is " << *p << ", not " << MAGIC <<"!" << endl;
allValid = false;
}
if (k == 2) {
for (int i = 0; i < 5; i++)
v2.push_back(MAGIC);
}
}
if (allValid && k == 10)
cout << "Passed test 3" << endl;
else
cout << "Failed test 3" << "\n" << k << endl;
}
This code will output something like the following:
5
5
5
10 <-- the capacity has changed
10
... skipped ...
10
10
Failed test 3
20
We can see that where k is equal to 2 (third line), the capacity of the vector doubled (fourth line) because we are adding new elements. The memory is redistributed, and the vector elements are most likely now located elsewhere. You can also check it by printing vector base address with data member function instead of capacity:
Address: 0x136dc20 k: 0
Address: 0x136dc20 k: 1
Address: 0x136dc20 k: 2
Address: 0x136e050 k: 3 <-- the address has changed
Address: 0x136e050 k: 4
... skipped ...
Address: 0x136e050 k: 19
Address: 0x136e050 k: 20
Failed test 3
20
The code is poorly written, you can make it more robust by using indices instead of iterators.