My Most Important C++ Aha Moments (2006)(artima.com)

  (mapc visitor-object objlist)

  (mapc (lambda (list-obj) (accept visitor-object list-obj)) objlist)

  (defmethod accept ((visitor this-class) (obj that-class))
     ;; handle this-class + that-class combination
    )

  // Things.h
  namespace Foo { class A { C* c; }; }
  namespace Bar { class B { C* c; }; }

  // TU1.cpp
  class C {};

  // TU2.cpp
  namespace Foo { class C {}; }
  namespace Bar { class C {}; }

  // TU3.cpp
  class C {};
  namespace Foo { class C {}; }
  namespace Bar { class C {}; }

  void foo() { D * d; }

  void bar() { E < 42 > e; }

  void baz() { F < (42) > f; }

  void he_comes() { G < (42) > g(); }

  v̹̊̈́̇ͦ̌ͭ͂ͅo̅̉̀ͪ̚ī̵̠̘̋d̸͚̪̝̹͙͆͛̿ ͉̜̳ͪͬ́ͣ̌ẑ̸̹̹̩̩a̡̰ͣͭ̄͒ͫ̇̚l̛̹̫͓̣͖͈̐ͬg̢͕̘͙o̪͆̏ͥ͟(̖̭̟̱̰ͩ)̘̬͉̺̉̓ ̦͆́̚{ ̶̪͙̹͈ͮͧ̋ͪͤͅĜ͔̙̉͢ ̝̯ͭ̈́͘<̨͊̂͋̎ ̤͇̼̝͔͇ͯ̑̋ͫ̾͐̚(ͨͪͦ̈̈́ͥ4͍͗̆2̲̞̦̳̖͡)͖͇̟̥ͮ͘ ̺̖͙̘̳͛̐̊͊>̼͍̘̦͖̎ ̧̘̻̭͉̺g̟͓̑ͦ̌̃(̪ͬͪͤ̎̉͘ȉ̮̥͖́n̸̝̪̗̯͇̓̈́̀̔̄̍́t͓̟͍ͨͪ(̰͈̻͎̣͉̀̿̈͑)̝͚̆̃̇)̠̃;̺͎̙̰̈́̏̚͘ ̶̜͍̠̼ͬ}̡̤̰̮̳͈̌͛ͤ̈́

   // file: A.h
   class B;
   class A {
     B* _b; // or any of the other variants.
   };

  // file: A.h
  class B;

  class A {
    B* _b;
  };

  // file: B.h
  class B {
    A* _a;
  };

    class B;
    class A {
      B* _b;
    };