C++11 member variable of reference type, different behaviour after vector push_back

I was using somebody else's class which was acting odd when I pushed it into a vector. It involves a member variable which is a reference to another member variable. Here is the smallest self-contained example:

#include <iostream>
#include <vector>

class Myclass {
public: 
  Myclass() : a(1.0) {}

  float a;
  float &a_ref = a;

  void addOne() {
    a = a + 1.0;
  }
};

int main() {
  Myclass instance1;
  instance1.addOne();

  //prints 2:
  std::cout << "instance.a_ref is " << instance1.a_ref << std::endl;

  std::vector<Myclass> vec;
  Myclass instance2;
  vec.push_back(instance2);

  vec.at(0).addOne();

  //prints 1;
  std::cout << "vec.at(0).a_ref is " << vec.at(0).a_ref << std::endl;
  return 0;
}

I was compiling with g++ and -std=c++11, so I didn't notice the problem for a while. I see now the issue is probably to do with the synthesised copy constructor and the reference member. But what I'm not sure about is:

1. Why is there different behaviour when the object is in a vector ?


2. Why does g++ not give any warnings about this, using c++11 standard ?

Bonus question because I'm curious:

3. What is initialized first, a or a_ref?