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. I then call a function on nodes[0] that adds a new Node to the vector. When the new Node is added, the calling object's fields are reset! However, they seem to return to normal again once the function has finished.

I believe this is a minimal reproducible example:

#include <iostream> #include <vector>  using namespace std;  struct Node; vector<Node> nodes;  struct Node{     int X;     void set(){         X = 3;         cout << "Before, X = " << X << endl;         nodes.push_back(Node());         cout << "After, X = " << X << endl;     } };  int main() {     nodes = vector<Node>();     nodes.push_back(Node());      nodes[0].set();     cout << "Finally, X = " << nodes[0].X << endl; } 

Which outputs

Before, X = 3 After, X = 0 Finally, X = 3 

Though you would expect X to remain unchanged by the process.

Other things I have tried:

• If I remove the line that adds a Node inside set(), then it outputs X = 3 every time.
• If I create a new Node and call it on that (Node p = nodes[0]) then the output is 3, 3, 3
• If I create a reference Node and call it on that (Node &p = nodes[0]) then the output is 3, 0, 0 (perhaps this one is because the reference is lost when the vector resizes?)

Is this undefined behaviour for some reason? Why?

Answer 1

Your code has undefined behavior. In

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

The access to X is really this->X and this is a pointer to the member of the vector. When you do nodes.push_back(Node()); you add a new element to the vector and that process reallocates, which invalidates all iterators, pointers and references to elements in the vector. That means

cout << "After, X = " << X << endl; 

is using a this that is no longer valid.