C++ auto may cause unwanted copies with getters

auto_pitfall.cpp

#include <iostream>
#include <string>
#include <type_traits>

using namespace std;

struct Foo
{
  Foo() {}
  Foo(const Foo&)
  {
    cout << "Copied a Foo" << endl;
  }
};

struct Bar
{
  const Foo& GetFoo() const { return m_foo; }

private:
  Foo m_foo;
};


int main(void)
{
  Bar b;

  cout << "Before auto" << endl;
  auto f = b.GetFoo();
  static_assert(is_same<decltype(f), Foo>::value, "f is a Foo");
  cout << "After auto" << endl;

  cout << "Before const auto&" << endl;
  const auto& f_cr = b.GetFoo();
  static_assert(is_same<decltype(f_cr), const Foo&>::value, "f_cr is a const Foo&");
  cout << "After const auto&" << endl;
}