Introduce all_objfiles and next_iterator

This introduces an iterable object which can be used to iterate over
objfiles.  It also introduces a generic "next_iterator", which can be
used to iterate over types that have a "next" field.

gdb/ChangeLog
2019-01-09  Tom Tromey  <tom@tromey.com>

	* common/next-iterator.h: New file.
	* objfiles.h (class all_objfiles): New.
	(struct objfile_iterator): New.

gdb/common/next-iterator.h

@@ -0,0 +1,102 @@
+/* A "next" iterator for GDB, the GNU debugger.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COMMON_NEXT_ITERATOR_H
+#define COMMON_NEXT_ITERATOR_H
+
+/* An iterator that uses the 'next' field of a type to iterate.  This
+   can be used with various GDB types that are stored as linked
+   lists.  */
+
+template<typename T>
+struct next_iterator
+{
+  typedef next_iterator self_type;
+  typedef T *value_type;
+  typedef T *&reference;
+  typedef T **pointer;
+  typedef std::forward_iterator_tag iterator_category;
+  typedef int difference_type;
+
+  explicit next_iterator (T *item)
+    : m_item (item)
+  {
+  }
+
+  /* Create a one-past-the-end iterator.  */
+  next_iterator ()
+    : m_item (nullptr)
+  {
+  }
+
+  value_type operator* () const
+  {
+    return m_item;
+  }
+
+  bool operator== (const self_type &other) const
+  {
+    return m_item == other.m_item;
+  }
+
+  bool operator!= (const self_type &other) const
+  {
+    return m_item != other.m_item;
+  }
+
+  self_type &operator++ ()
+  {
+    m_item = m_item->next;
+    return *this;
+  }
+
+private:
+
+  T *m_item;
+};
+
+/* A range adapter that allows iterating over a linked list.  */
+
+template<typename T>
+class next_adapter
+{
+public:
+
+  explicit next_adapter (T *item)
+    : m_item (item)
+  {
+  }
+
+  using iterator = next_iterator<T>;
+
+  iterator begin () const
+  {
+    return iterator (m_item);
+  }
+
+  iterator end () const
+  {
+    return iterator ();
+  }
+
+private:
+
+  T *m_item;
+};
+
+#endif /* COMMON_NEXT_ITERATOR_H */