CxxUtils: Add releasing_iterator.

Helper to convert an iterator over unique_ptr to one over T*.

Control/CxxUtils/CxxUtils/releasing_iterator.h

+// This file's extension implies that it's C, but it's really -*- C++ -*-.
+/*
+ * Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration.
+ */
+/**
+ * @file CxxUtils/releasing_iterator.h
+ * @author scott snyder <snyder@bnl.gov>
+ * @date Feb, 2023
+ * @brief Adapter to retrieve elements from a unique_ptr iterator via release().
+ */
+
+
+#ifndef CXXUTILS_RELEASING_ITERATOR_H
+#define CXXUTILS_RELEASING_ITERATOR_H
+
+
+#include <iterator>
+
+
+namespace CxxUtils {
+
+
+/**
+ * @brief Adapter to retrieve elements from a unique_ptr iterator via release().
+ *
+ * This allows, say,
+ *@code
+ *   std::vector<std::unique_ptr<Foo> > c1 = ...;
+ *   std::vector<Foo*> c2;
+ *   c2.assign (releasing_iterator(c1.begin()), releasing_iterator(c1.end()));
+ @endcode
+ *
+ * All elements in @c c1 will be release'd.
+ *
+ * Since this implies that a given iterator can only be dereferenced once,
+ * this iterator models @c input_iterator.
+ */
+template <class ITERATOR>
+class releasing_iterator
+{
+public:
+  using value_type = typename std::iterator_traits<ITERATOR>::value_type::pointer;
+  using reference = value_type&;
+  using pointer = value_type*;
+  using difference_type = typename std::iterator_traits<ITERATOR>::difference_type;
+  using iterator_category = std::input_iterator_tag;
+
+  releasing_iterator (ITERATOR it) : m_it (it) {}
+  value_type operator*() { return m_it->release(); }
+  releasing_iterator& operator++() { ++m_it; return *this; }
+  bool operator!= (const releasing_iterator& other) const { return m_it != other.m_it; }
+
+
+private:
+  ITERATOR m_it;
+};
+
+
+} // namespace CxxUtils
+
+
+#endif // not CXXUTILS_RELEASING_ITERATOR_H