class A {
  .....
};

class B {
protected: 
  A & m_1;
  A & m_2;
  ...
public
  B(A& a1, A& a2);
  ~B();
  void foo();
};

B::B(A& a1, A& a2)
: m_1(a1), m_2(a2)
{
}
B::~B() {}


void B::foo()
{
  A& a(m_1);
  if (some condition)
    a = m_2;  // error here: class 'A' does not have 'operator ='

  ....
}