#include <iostream>
#include <vector>
 
class MyClass {
public: 
  double& at(size_t index) { return data_[index]; }
  double const& at(size_t index) const { return data_[index]; }
  MyClass(size_t size) : data_(size) {}
  ~MyClass() { std::cout << "MyClass destroyed" << std::endl; }
private:
  std::vector<double> data_;
};
 
const MyClass getMyClass() {
  MyClass mc(3);
  mc.at(0) = 3.0;
  mc.at(1) = 4.0;
  mc.at(2) = 5.0;
  return mc;
}
 
int main() {
  const double &value = getMyClass().at(0);
 
  // Undefined behaviour, reference to element of vector that doesn't exist!
  std::cout << value << std::endl; 
}

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