Function changes const object

Question

I have function which accepts const reference as argument. It should not change argument, but it does (variable "_isVertex"). How can this be fixed? Here is the code:

#include <vector> #include <iostream>  using namespace std;  class Element { public:     bool isVertex() const     { return _isVertex; };  private:     bool _isVertex = true; };  class ElementContainer : public vector <Element> { public:     void push(const Element &t)     {         // here everything is fine         cerr << t.isVertex() << ' ';         push_back(t);         // and here _isVertex is false, should be true!         cerr << t.isVertex() << '\n';     } };  int main() {     ElementContainer vertex;      vertex.push({});     vertex.push(vertex[0]); } 

Answer 1

Consider carefully vertex.push(vertex[0]);. t in the function push is a constant reference to vertex[0].

But after the push_back, the contents of the vector have moved (due to a memory reallocation), and therefore vector[0] has moved elsewhere. t is now a dangling reference.

That's undefined behaviour. Boom.