AthContainers: Add JaggedVecConstAccessor.

Add a ConstAccessor class specialized for jagged vectors.

Control/AthContainers/AthContainers/tools/JaggedVecConversions.h

+// This file's extension implies that it's C, but it's really -*- C++ -*-.
+/*
+ * Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration.
+ */
+/**
+ * @file AthContainers/tools/JaggedVecConversions.h
+ * @author scott snyder <snyder@bnl.gov>
+ * @date Jul, 2024
+ * @brief Conversions for accessing jagged vector variables.
+ */
+
+
+#ifndef ATHCONTAINERS_JAGGEDVECCONVERSIONS_H
+#define ATHCONTAINERS_JAGGEDVECCONVERSIONS_H
+
+
+#include "AthContainers/JaggedVecImpl.h"
+#include "AthContainers/tools/AuxDataTraits.h"
+#include "AthContainersInterfaces/AuxDataSpan.h"
+#include "CxxUtils/range_with_conv.h"
+#include <cassert>
+#include <stdexcept>
+
+
+namespace SG { namespace detail {
+
+
+/**
+ * @brief Helper: Make a span from a jagged vector element.
+ *        An instance of this class is constructed from an @c AuxDataSpan
+ *        over the payload vector.  (This has a reference to the range
+ *        held by the @c AuxTypeVector, so if the payload vector changes,
+ *        we'll see it here.)  @c operator() then takes a @c JaggedVecElt
+ *        and produces a span representing that element.
+ */
+template <class PAYLOAD_T>
+class JaggedVecConstConverter
+{
+public:
+  /// The type of the jagged vector payload.
+  using Payload_t = PAYLOAD_T;
+
+  /// An @c AuxDataSpan representing the payload vector.
+  using const_Payload_span = detail::AuxDataSpan<Payload_t>;
+
+  /// The type of the span we produce.
+  using element_type = CxxUtils::range_with_conv<typename AuxDataTraits<Payload_t>::const_span>;
+
+
+  /**
+   * @brief Constructor.
+   * @param payload Span over the payload vector.
+   */
+  JaggedVecConstConverter (const_Payload_span payload);
+
+
+  /**
+   * @brief Convert to a span.
+   * @param elt The jagged vector element to transform.
+   */
+  const element_type operator() (const JaggedVecEltBase& elt) const;
+
+
+private:
+  /// The span over the payload vector.
+  const_Payload_span m_payload;
+};
+
+
+} } // namespace SG::detail
+
+
+#include "AthContainers/tools/JaggedVecConversions.icc"
+
+
+#endif // not ATHCONTAINERS_JAGGEDVECCONVERSIONS_H