class A {

 public:

  A();

  A& operator=(const A& rhs)
  {
    b = rhs.b;
    return *this;
  }

 private:

  B b;

};

class A {

 public:

  A() : b(0)
  {
    b = new B;
  }

  A& operator=(const A& rhs)
  {
    if (this != &rhs) {
      B* b1 = 0;

      try {
        b1 = new B(*rhs.b);
      }
      catch {
        delete b1;
        throw;
      }

      delete b;
      b = b1;
    }
    return *this;
  }

 private:

  B* b;

};

class A {

 public:

  A() : b(0)
  {
    b = new B;
  }

  A& operator=(const A& rhs)
  {
    *b = *rhs.b;
    return *this;
  }

 private:

  B* b;

};

const B* A::GetB() const
{
  return b;
}

A a1, a2;

const B* my_b = a1.GetB();

a1 = a2; // this leaves my_b dangling!