MXS-2219 Add iterator adapter

Iterator adapter for making it possible to iterate over an
intrusive singly linked list in an STL fashion.

maxutils/maxbase/include/maxbase/iterator.hh

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2019 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+#pragma once
+
+#include <maxbase/ccdefs.hh>
+#include <algorithm>
+
+namespace maxbase
+{
+
+/**
+ * This template can be used for providing an STL compatible iterator
+ * for an intrusive singly linked list, that is, a list where the
+ * elements themselves contain a @c next pointer.
+ */
+template<class T>
+class intrusive_slist_iterator : public std::iterator<std::input_iterator_tag, T>
+{
+public:
+    explicit intrusive_slist_iterator(T& t)
+        : m_pT(&t)
+    {
+    }
+
+    explicit intrusive_slist_iterator()
+        : m_pT(nullptr)
+    {
+    }
+
+    intrusive_slist_iterator& operator++()
+    {
+        mxb_assert(m_pT);
+        m_pT = m_pT->next;
+        return *this;
+    }
+
+    intrusive_slist_iterator& operator++(int)
+    {
+        intrusive_slist_iterator prev(*this);
+        ++(*this);
+        return prev;
+    }
+
+    bool operator == (const intrusive_slist_iterator& rhs) const
+    {
+        return m_pT == rhs.m_pT;
+    }
+
+    bool operator != (const intrusive_slist_iterator& rhs) const
+    {
+        return !(m_pT == rhs.m_pT);
+    }
+
+    T& operator * () const
+    {
+        mxb_assert(m_pT);
+        return *m_pT;
+    }
+
+private:
+    T* m_pT;
+};
+
+}