Why does this not compile:
#include <iostream>

std::string getString() {
  return std::string("Hi");
}
int main() {
  std::string& s = getString();
  std::cout << s << std::endl;
}

but this does:
#include <iostream>

std::string getString() {
  return std::string("Hi");
}
int main() {
  const std::string& s = getString();
  std::cout << s << std::endl;
}