Vector of pointers undefined behaviour

Question

I'm trying to make a vector of pointers whose elements are pointing to vector of int elements. (I'm solving a competitive programming-like problem, that's why it sounds kinda nonsense).

but here's the code:

#include <bits/stdc++.h>

using namespace std;

int ct = 0;

vector<int> vec;

vector<int*> rf;

void addRef(int n){
    vec.push_back(n);
    rf.push_back(&vec[ct]);
    ct++;
}


int main(){
    addRef(1);
    addRef(2);
    addRef(5);
    for(int i = 0; i < ct; i++){
        cout << *rf[i] << ' ';
    }
    cout << endl;
    for(int i = 0; i < ct; i++){
        cout << vec[i] << ' ';  
    }
}

When I execute the code, it's showing weird behaviour that I don't understand. The first element of rf (vector<int*>) seems not pointing to the vec's (vector<int>) element, where the rest of the elements are pointing to it.

here's the output when I run it on Dev-C++:

1579600 2 5
1 2 5

When I tried to run the code here, the output is even weirder:

1197743856 0 5 
1 2 5 

The code is intended to have same output between the first line and the second.

Can you guys explain why it happens? Is there any mistake in my implementation?

thanks

Answer 1

Adding elements to a std::vector with push_back or similar may invalidate all iterators and references to its elements. See https://en.cppreference.com/w/cpp/container/vector/push_back.

The idea is that in order to grow the vector, it may not have enough free memory to expand into, and thus may have to move the whole array to some other location in memory, freeing the old block. That means in particular that your pointers now point to memory that has been freed, or reused for something else.

If you want to keep this approach, you will need to resize() or reserve() a sufficient number of elements in vec before starting. Which of course defeats the whole purpose of a std::vector, and you might as well use an array instead.