Weird behaviour with class fields when adding to a std::vector

Question

I have found some very weird behaviour (on clang and GCC) in the following situation. I have a vector, nodes, with one element, an instance of class Node

Answer 1

nodes.push_back(Node());

will reallocate the vector, thus changing the address of nodes[0], but this is not updated.
try replacing the set method with this code:

    void set(){
        X = 3;
        cout << "Before, X = " << X << endl;
        cout << "Before, this = " << this << endl;
        cout << "Before, &nodes[0] = " << &nodes[0] << endl;
        nodes.push_back(Node());
        cout << "After, X = " << X << endl;
        cout << "After, this = " << this << endl;
        cout << "After, &nodes[0] = " << &nodes[0] << endl;
    }

note how &nodes[0] is different after calling push_back.

-fsanitize=address will catch this, and even tell you on which line the memory was freed if you also compile with -g.