Began tutorial and examples.

[SVN r21584]

doc/iterator_facade_tutorial.rst

@@ -0,0 +1,73 @@
+.. Copyright David Abrahams 2004. Use, modification and distribution is
+.. subject to the Boost Software License, Version 1.0. (See accompanying
+.. file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+Introduction
+------------
+
+In this section we'll walk through the implementation of a few
+iterators using ``iterator_facade``, based around the simple
+example of a linked list of polymorphic objects.
+
+The Problem
+-----------
+
+Say we've written a polymorphic linked list node as follows::
+
+  # include <iostream>
+
+  struct node_base
+  {
+      node_base() : m_next(0) {}
+
+      virtual ~node_base() { delete m_next; }
+
+      node_base* next() const { return m_next; }
+
+      // print to the stream
+      virtual void print(std::ostream& s) const = 0;
+      
+      // double the value
+      virtual void twice() = 0;
+
+      void append(node_base* p)
+      {
+          if (m_next)
+              m_next->append(p);
+          else
+              m_next = p;
+      }
+
+   private:
+      node_base* m_next;
+  };
+
+Lists can hold objects of different types by linking together
+specializations of the following template::
+
+  template <class T>
+  struct node : node_base
+  {
+      node(T x)
+        : m_value(x)
+      {}
+
+      void print(std::ostream& s) const { s << this->m_value; }
+      void twice() { m_value += m_value; }
+
+   private:
+      T m_value;
+  };
+
+And we can print any node using the following streaming operator::
+
+  inline std::ostream& operator<<(std::ostream& s, node_base const& n)
+  {
+      n.print(s);
+      return s;
+  }
+
+
+.. sidebar:: Note
+
+   This is in progress; check the ../example directory for code.