RField.hxx

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/// \file ROOT/RField.hxx
/// \ingroup NTuple ROOT7
/// \author Jakob Blomer <jblomer@cern.ch>
/// \date 2018-10-09
/// \warning This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback
/// is welcome!
 
/*************************************************************************
 * Copyright (C) 1995-2019, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#ifndef ROOT7_RField
#define ROOT7_RField
 
#include <ROOT/RColumn.hxx>
#include <ROOT/RError.hxx>
#include <ROOT/RColumnElement.hxx>
#include <ROOT/RFieldValue.hxx>
#include <ROOT/RNTupleUtil.hxx>
#include <ROOT/RSpan.hxx>
#include <ROOT/RStringView.hxx>
#include <ROOT/RVec.hxx>
#include <ROOT/TypeTraits.hxx>
 
#include <TGenericClassInfo.h>
#include <TVirtualCollectionProxy.h>
 
#include <algorithm>
#include <array>
#include <cstddef>
#include <functional>
#include <iostream>
#include <iterator>
#include <memory>
#include <string>
#include <type_traits>
#include <typeinfo>
#include <variant>
#include <vector>
#include <utility>
 
class TClass;
 
namespace ROOT {
namespace Experimental {
 
class RCollectionField;
class RCollectionNTupleWriter;
class REntry;
class RNTupleModel;
 
namespace Internal {
struct RFieldCallbackInjector;
} // namespace Internal
 
namespace Detail {
 
class RFieldVisitor;
class RPageStorage;
 
// clang-format off
/**
\class ROOT::Experimental::RFieldBase
\ingroup NTuple
\brief A field translates read and write calls from/to underlying columns to/from tree values
 
A field is a serializable C++ type or a container for a collection of sub fields. The RFieldBase and its
type-safe descendants provide the object to column mapper. They map C++ objects to primitive columns.  The
mapping is trivial for simple types such as 'double'. Complex types resolve to multiple primitive columns.
The field knows based on its type and the field name the type(s) and name(s) of the columns.
*/
// clang-format on
class RFieldBase {
   friend class ROOT::Experimental::RCollectionField; // to move the fields from the collection model
   friend struct ROOT::Experimental::Internal::RFieldCallbackInjector; // used for unit tests
   using ReadCallback_t = std::function<void(RFieldValue &)>;
 
public:
   /// No constructor needs to be called, i.e. any bit pattern in the allocated memory represents a valid type
   /// A trivially constructible field has a no-op GenerateValue() implementation
   static constexpr int kTraitTriviallyConstructible = 0x01;
   /// The type is cleaned up just by freeing its memory. I.e. DestroyValue() is a no-op.
   static constexpr int kTraitTriviallyDestructible = 0x02;
   /// A field of a fundamental type that can be directly mapped via `RField<T>::Map()`, i.e. maps as-is to a single
   /// column
   static constexpr int kTraitMappable = 0x04;
   /// Shorthand for types that are both trivially constructible and destructible
   static constexpr int kTraitTrivialType = kTraitTriviallyConstructible | kTraitTriviallyDestructible;
 
private:
   /// The field name relative to its parent field
   std::string fName;
   /// The C++ type captured by this field
   std::string fType;
   /// The role of this field in the data model structure
   ENTupleStructure fStructure;
   /// For fixed sized arrays, the array length
   std::size_t fNRepetitions;
   /// A field qualifies as simple if it is both mappable and has no post-read callback
   bool fIsSimple;
   /// When the columns are connected to a page source or page sink, the field represents a field id in the
   /// corresponding RNTuple descriptor. This on-disk ID is set in RPageSink::Create() for writing and by
   /// RFieldDescriptor::CreateField() when recreating a field / model from the stored descriptor.
   DescriptorId_t fOnDiskId = kInvalidDescriptorId;
   /// Free text set by the user
   std::string fDescription;
 
protected:
   /// Collections and classes own sub fields
   std::vector<std::unique_ptr<RFieldBase>> fSubFields;
   /// Sub fields point to their mother field
   RFieldBase* fParent;
   /// Points into fColumns.  All fields that have columns have a distinct main column. For simple fields
   /// (float, int, ...), the principal column corresponds to the field type. For collection fields expect std::array,
   /// the main column is the offset field.  Class fields have no column of their own.
   RColumn* fPrincipalColumn;
   /// The columns are connected either to a sink or to a source (not to both); they are owned by the field.
   std::vector<std::unique_ptr<RColumn>> fColumns;
   /// Properties of the type that allow for optimizations of collections of that type
   int fTraits = 0;
   /// List of functions to be called after reading a value
   std::vector<ReadCallback_t> fReadCallbacks;
 
   /// Creates the backing columns corresponsing to the field type for writing
   virtual void GenerateColumnsImpl() = 0;
   /// Creates the backing columns corresponsing to the field type for reading.
   /// The method should to check, using the page source and fOnDiskId, if the column types match
   /// and throw if they don't.
   virtual void GenerateColumnsImpl(const RNTupleDescriptor &desc) = 0;
 
   /// Called by Clone(), which additionally copies the on-disk ID
   virtual std::unique_ptr<RFieldBase> CloneImpl(std::string_view newName) const = 0;
 
   /// Operations on values of complex types, e.g. ones that involve multiple columns or for which no direct
   /// column type exists.
   virtual std::size_t AppendImpl(const RFieldValue &value);
   virtual void ReadGlobalImpl(NTupleSize_t globalIndex, RFieldValue *value);
   virtual void ReadInClusterImpl(const RClusterIndex &clusterIndex, RFieldValue *value) {
      ReadGlobalImpl(fPrincipalColumn->GetGlobalIndex(clusterIndex), value);
   }
 
   /// Throws an exception if the column given by fOnDiskId and the columnIndex in the provided descriptor
   /// is not of one of the requested types.
   ROOT::Experimental::EColumnType EnsureColumnType(const std::vector<EColumnType> &requestedTypes,
                                                    unsigned int columnIndex, const RNTupleDescriptor &desc);
 
   /// Set a user-defined function to be called after reading a value, giving a chance to inspect and/or modify the
   /// value object.
   /// Returns an index that can be used to remove the callback.
   size_t AddReadCallback(ReadCallback_t func);
   void RemoveReadCallback(size_t idx);
 
private:
   void InvokeReadCallbacks(RFieldValue &value)
   {
      for (const auto &func : fReadCallbacks)
         func(value);
   }
 
public:
   /// Iterates over the sub tree of fields in depth-first search order
   class RSchemaIterator {
   private:
      struct Position {
         Position() : fFieldPtr(nullptr), fIdxInParent(-1) { }
         Position(RFieldBase *fieldPtr, int idxInParent) : fFieldPtr(fieldPtr), fIdxInParent(idxInParent) { }
         RFieldBase *fFieldPtr;
         int fIdxInParent;
      };
      /// The stack of nodes visited when walking down the tree of fields
      std::vector<Position> fStack;
   public:
      using iterator = RSchemaIterator;
      using iterator_category = std::forward_iterator_tag;
      using value_type = RFieldBase;
      using difference_type = std::ptrdiff_t;
      using pointer = RFieldBase*;
      using reference = RFieldBase&;
 
      RSchemaIterator() { fStack.emplace_back(Position()); }
      RSchemaIterator(pointer val, int idxInParent) { fStack.emplace_back(Position(val, idxInParent)); }
      ~RSchemaIterator() {}
      /// Given that the iterator points to a valid field which is not the end iterator, go to the next field
      /// in depth-first search order
      void Advance();
 
      iterator  operator++(int) /* postfix */        { auto r = *this; Advance(); return r; }
      iterator& operator++()    /* prefix */         { Advance(); return *this; }
      reference operator* () const                   { return *fStack.back().fFieldPtr; }
      pointer   operator->() const                   { return fStack.back().fFieldPtr; }
      bool      operator==(const iterator& rh) const { return fStack.back().fFieldPtr == rh.fStack.back().fFieldPtr; }
      bool      operator!=(const iterator& rh) const { return fStack.back().fFieldPtr != rh.fStack.back().fFieldPtr; }
   };
 
   /// The constructor creates the underlying column objects and connects them to either a sink or a source.
   /// If `isSimple` is `true`, the trait `kTraitMappable` is automatically set on construction. However, the
   /// field might be demoted to non-simple if a post-read callback is set.
   RFieldBase(std::string_view name, std::string_view type, ENTupleStructure structure, bool isSimple,
              std::size_t nRepetitions = 0);
   RFieldBase(const RFieldBase&) = delete;
   RFieldBase(RFieldBase&&) = default;
   RFieldBase& operator =(const RFieldBase&) = delete;
   RFieldBase& operator =(RFieldBase&&) = default;
   virtual ~RFieldBase();
 
   /// Copies the field and its sub fields using a possibly new name and a new, unconnected set of columns
   std::unique_ptr<RFieldBase> Clone(std::string_view newName) const;
 
   /// Factory method to resurrect a field from the stored on-disk type information
   static RResult<std::unique_ptr<RFieldBase>> Create(const std::string &fieldName, const std::string &typeName);
   /// Check whether a given string is a valid field name
   static RResult<void> EnsureValidFieldName(std::string_view fieldName);
 
   /// Generates an object of the field type and allocates new initialized memory according to the type.
   RFieldValue GenerateValue();
   /// Generates a tree value in a given location of size at least GetValueSize(). Assumes that where has been
   /// allocated by malloc().
   virtual RFieldValue GenerateValue(void *where) = 0;
   /// Releases the resources acquired during GenerateValue (memory and constructor)
   /// This implementation works for simple types but needs to be overwritten for complex ones
   virtual void DestroyValue(const RFieldValue &value, bool dtorOnly = false);
   /// Creates a value from a memory location with an already constructed object
   virtual RFieldValue CaptureValue(void *where) = 0;
   /// Creates the list of direct child values given a value for this field.  E.g. a single value for the
   /// correct variant or all the elements of a collection.  The default implementation assumes no sub values
   /// and returns an empty vector.
   virtual std::vector<RFieldValue> SplitValue(const RFieldValue &value) const;
   /// The number of bytes taken by a value of the appropriate type
   virtual size_t GetValueSize() const = 0;
   /// For many types, the alignment requirement is equal to the size; otherwise override.
   virtual size_t GetAlignment() const { return GetValueSize(); }
   int GetTraits() const { return fTraits; }
   bool HasReadCallbacks() const { return !fReadCallbacks.empty(); }
 
   /// Write the given value into columns. The value object has to be of the same type as the field.
   /// Returns the number of uncompressed bytes written.
   std::size_t Append(const RFieldValue& value) {
      if (~fTraits & kTraitMappable)
         return AppendImpl(value);
 
      fPrincipalColumn->Append(value.fMappedElement);
      return value.fMappedElement.GetSize();
   }
 
   /// Populate a single value with data from the tree, which needs to be of the fitting type.
   /// Reading copies data into the memory wrapped by the ntuple value.
   /// The fast path is conditioned by the field qualifying as simple, i.e. maps as-is to a single column and has no
   /// read callback.
   void Read(NTupleSize_t globalIndex, RFieldValue *value) {
      if (fIsSimple)
         return (void)fPrincipalColumn->Read(globalIndex, &value->fMappedElement);
 
      if (fTraits & kTraitMappable)
         fPrincipalColumn->Read(globalIndex, &value->fMappedElement);
      else
         ReadGlobalImpl(globalIndex, value);
      if (R__unlikely(!fReadCallbacks.empty()))
         InvokeReadCallbacks(*value);
   }
 
   void Read(const RClusterIndex &clusterIndex, RFieldValue *value) {
      if (fIsSimple)
         return (void)fPrincipalColumn->Read(clusterIndex, &value->fMappedElement);
 
      if (fTraits & kTraitMappable)
         fPrincipalColumn->Read(clusterIndex, &value->fMappedElement);
      else
         ReadInClusterImpl(clusterIndex, value);
      if (R__unlikely(!fReadCallbacks.empty()))
         InvokeReadCallbacks(*value);
   }
 
   /// Ensure that all received items are written from page buffers to the storage.
   void Flush() const;
   /// Perform housekeeping tasks for global to cluster-local index translation
   virtual void CommitCluster() {}
 
   /// Add a new subfield to the list of nested fields
   void Attach(std::unique_ptr<Detail::RFieldBase> child);
 
   std::string GetName() const { return fName; }
   std::string GetType() const { return fType; }
   ENTupleStructure GetStructure() const { return fStructure; }
   std::size_t GetNRepetitions() const { return fNRepetitions; }
   NTupleSize_t GetNElements() const { return fPrincipalColumn->GetNElements(); }
   RFieldBase *GetParent() const { return fParent; }
   std::vector<RFieldBase *> GetSubFields() const;
   bool IsSimple() const { return fIsSimple; }
   /// Get the field's description
   std::string GetDescription() const { return fDescription; }
   void SetDescription(std::string_view description) { fDescription = std::string(description); }
 
   DescriptorId_t GetOnDiskId() const { return fOnDiskId; }
   void SetOnDiskId(DescriptorId_t id) { fOnDiskId = id; }
 
   /// Fields and their columns live in the void until connected to a physical page storage.  Only once connected, data
   /// can be read or written.  In order to find the field in the page storage, the field's on-disk ID has to be set.
   void ConnectPageSink(RPageSink &pageSink);
   void ConnectPageSource(RPageSource &pageSource);
 
   /// Indicates an evolution of the mapping scheme from C++ type to columns
   virtual std::uint32_t GetFieldVersion() const { return 0; }
   /// Indicates an evolution of the C++ type itself
   virtual std::uint32_t GetTypeVersion() const { return 0; }
 
   RSchemaIterator begin();
   RSchemaIterator end();
 
   virtual void AcceptVisitor(RFieldVisitor &visitor) const;
};
 
} // namespace Detail
 
 
 
/// The container field for an ntuple model, which itself has no physical representation
class RFieldZero : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const override;
 
public:
   RFieldZero() : Detail::RFieldBase("", "", ENTupleStructure::kRecord, false /* isSimple */) { }
 
   void GenerateColumnsImpl() final {}
   void GenerateColumnsImpl(const RNTupleDescriptor &) final {}
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void*) override { return Detail::RFieldValue(); }
   Detail::RFieldValue CaptureValue(void*) final { return Detail::RFieldValue(); }
   size_t GetValueSize() const final { return 0; }
 
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
/// The field for a class with dictionary
class RClassField : public Detail::RFieldBase {
private:
   enum ESubFieldRole {
      kBaseClass,
      kDataMember,
   };
   struct RSubFieldInfo {
      ESubFieldRole fRole;
      std::size_t fOffset;
   };
   /// Prefix used in the subfield names generated for base classes
   static constexpr const char *kPrefixInherited{":"};
 
   TClass* fClass;
   /// Additional information kept for each entry in `fSubFields`
   std::vector<RSubFieldInfo> fSubFieldsInfo;
   std::size_t fMaxAlignment = 1;
 
private:
   RClassField(std::string_view fieldName, std::string_view className, TClass *classp);
   void Attach(std::unique_ptr<Detail::RFieldBase> child, RSubFieldInfo info);
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
   void ReadInClusterImpl(const RClusterIndex &clusterIndex, Detail::RFieldValue *value) final;
 
public:
   RClassField(std::string_view fieldName, std::string_view className);
   RClassField(RClassField&& other) = default;
   RClassField& operator =(RClassField&& other) = default;
   ~RClassField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void* where) override;
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) final;
   Detail::RFieldValue CaptureValue(void *where) final;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetValueSize() const override;
   size_t GetAlignment() const final { return fMaxAlignment; }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const override;
};
 
/// The field for a class representing a collection of elements via `TVirtualCollectionProxy`.
/// Objects of such type behave as collections that can be accessed through the corresponding member functions in
/// `TVirtualCollectionProxy`. At a bare minimum, the user is required to provide an implementation for the following
/// functions in `TVirtualCollectionProxy`: `HasPointers()`, `GetProperties()`, `GetValueClass()`, `GetType()`,
/// `Sizeof()`, `PushProxy()`, `PopProxy()`, `At()`, `Clear()`, and `Insert()`.
///
/// The collection proxy for a given class can be set via `TClass::CopyCollectionProxy()`.
class RCollectionClassField : public Detail::RFieldBase {
private:
   /// Chunk size in bytes used in `ReadGlobalImp()`. Items held in the same chunk will be inserted in
   /// a single `TVirtualCollectionProxy::Insert()` call.
   static constexpr const std::size_t kReadChunkSize = 64 * 1024;
   std::unique_ptr<TVirtualCollectionProxy> fProxy;
   std::size_t fItemSize;
   ClusterSize_t fNWritten;
 
   RCollectionClassField(std::string_view fieldName, std::string_view className, TClass *classp);
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
   std::size_t AppendImpl(const Detail::RFieldValue &value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
 
public:
   RCollectionClassField(std::string_view fieldName, std::string_view className);
   RCollectionClassField(RCollectionClassField &&other) = default;
   RCollectionClassField &operator=(RCollectionClassField &&other) = default;
   ~RCollectionClassField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) final;
   Detail::RFieldValue CaptureValue(void *where) override;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetValueSize() const override { return fProxy->Sizeof(); }
   size_t GetAlignment() const final { return alignof(std::max_align_t); }
   void CommitCluster() final;
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
   void GetCollectionInfo(NTupleSize_t globalIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const
   {
      fPrincipalColumn->GetCollectionInfo(globalIndex, collectionStart, size);
   }
   void GetCollectionInfo(const RClusterIndex &clusterIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const
   {
      fPrincipalColumn->GetCollectionInfo(clusterIndex, collectionStart, size);
   }
};
 
/// The field for an untyped record. The subfields are stored consequitively in a memory block, i.e.
/// the memory layout is identical to one that a C++ struct would have
class RRecordField : public Detail::RFieldBase {
protected:
   std::size_t fMaxAlignment = 1;
   std::size_t fSize = 0;
   std::vector<std::size_t> fOffsets;
 
   std::size_t GetItemPadding(std::size_t baseOffset, std::size_t itemAlignment) const;
 
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const override;
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
   void ReadInClusterImpl(const RClusterIndex &clusterIndex, Detail::RFieldValue *value) final;
 
   RRecordField(std::string_view fieldName, std::vector<std::unique_ptr<Detail::RFieldBase>> &&itemFields,
                const std::vector<std::size_t> &offsets, std::string_view typeName = "");
 
   template <std::size_t N>
   RRecordField(std::string_view fieldName, std::array<std::unique_ptr<Detail::RFieldBase>, N> &&itemFields,
                const std::array<std::size_t, N> &offsets, std::string_view typeName = "")
      : ROOT::Experimental::Detail::RFieldBase(fieldName, typeName, ENTupleStructure::kRecord, false /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
      for (unsigned i = 0; i < N; ++i) {
         fOffsets.push_back(offsets[i]);
         fMaxAlignment = std::max(fMaxAlignment, itemFields[i]->GetAlignment());
         fSize += GetItemPadding(fSize, itemFields[i]->GetAlignment()) + itemFields[i]->GetValueSize();
         fTraits &= itemFields[i]->GetTraits();
         Attach(std::move(itemFields[i]));
      }
   }
public:
   /// Construct a RRecordField based on a vector of child fields. The ownership of the child fields is transferred
   /// to the RRecordField instance.
   RRecordField(std::string_view fieldName, std::vector<std::unique_ptr<Detail::RFieldBase>> &&itemFields);
   RRecordField(std::string_view fieldName, std::vector<std::unique_ptr<Detail::RFieldBase>> &itemFields);
   RRecordField(RRecordField&& other) = default;
   RRecordField& operator =(RRecordField&& other) = default;
   ~RRecordField() override = default;
 
   void GenerateColumnsImpl() final {}
   void GenerateColumnsImpl(const RNTupleDescriptor &) final {}
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void* where) override;
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) override;
   Detail::RFieldValue CaptureValue(void *where) final;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetValueSize() const final { return fSize; }
   size_t GetAlignment() const final { return fMaxAlignment; }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
/// The generic field for a (nested) std::vector<Type> except for std::vector<bool>
class RVectorField : public Detail::RFieldBase {
private:
   std::size_t fItemSize;
   ClusterSize_t fNWritten;
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
 
public:
   RVectorField(std::string_view fieldName, std::unique_ptr<Detail::RFieldBase> itemField);
   RVectorField(RVectorField&& other) = default;
   RVectorField& operator =(RVectorField&& other) = default;
   ~RVectorField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void* where) override;
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) final;
   Detail::RFieldValue CaptureValue(void *where) override;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetValueSize() const override { return sizeof(std::vector<char>); }
   size_t GetAlignment() const final { return std::alignment_of<std::vector<char>>(); }
   void CommitCluster() final;
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
   void GetCollectionInfo(NTupleSize_t globalIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const {
      fPrincipalColumn->GetCollectionInfo(globalIndex, collectionStart, size);
   }
   void GetCollectionInfo(const RClusterIndex &clusterIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const {
      fPrincipalColumn->GetCollectionInfo(clusterIndex, collectionStart, size);
   }
};
 
/// The type-erased field for a RVec<Type>
class RRVecField : public Detail::RFieldBase {
private:
   /// Evaluate the constant returned by GetValueSize.
   // (we separate evaluation from the getter to avoid repeating the computation).
   std::size_t EvalValueSize() const;
 
protected:
   std::size_t fItemSize;
   ClusterSize_t fNWritten;
   std::size_t fValueSize;
 
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const override;
   std::size_t AppendImpl(const Detail::RFieldValue &value) override;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) override;
 
public:
   RRVecField(std::string_view fieldName, std::unique_ptr<Detail::RFieldBase> itemField);
   RRVecField(RRVecField &&) = default;
   RRVecField &operator=(RRVecField &&) = default;
   RRVecField(const RRVecField &) = delete;
   RRVecField &operator=(RRVecField &) = delete;
   ~RRVecField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) override;
   Detail::RFieldValue CaptureValue(void *where) override;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetValueSize() const override;
   size_t GetAlignment() const override;
   void CommitCluster() final;
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
   void GetCollectionInfo(NTupleSize_t globalIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const
   {
      fPrincipalColumn->GetCollectionInfo(globalIndex, collectionStart, size);
   }
   void GetCollectionInfo(const RClusterIndex &clusterIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const
   {
      fPrincipalColumn->GetCollectionInfo(clusterIndex, collectionStart, size);
   }
};
 
/// The generic field for fixed size arrays, which do not need an offset column
class RArrayField : public Detail::RFieldBase {
private:
   std::size_t fItemSize;
   std::size_t fArrayLength;
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
   void ReadInClusterImpl(const RClusterIndex &clusterIndex, Detail::RFieldValue *value) final;
 
public:
   RArrayField(std::string_view fieldName, std::unique_ptr<Detail::RFieldBase> itemField, std::size_t arrayLength);
   RArrayField(RArrayField &&other) = default;
   RArrayField& operator =(RArrayField &&other) = default;
   ~RArrayField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) final;
   Detail::RFieldValue CaptureValue(void *where) final;
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   size_t GetLength() const { return fArrayLength; }
   size_t GetValueSize() const final { return fItemSize * fArrayLength; }
   size_t GetAlignment() const final { return fSubFields[0]->GetAlignment(); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
/// The generic field for std::variant types
class RVariantField : public Detail::RFieldBase {
private:
   size_t fMaxItemSize = 0;
   size_t fMaxAlignment = 1;
   /// In the std::variant memory layout, at which byte number is the index stored
   size_t fTagOffset = 0;
   std::vector<ClusterSize_t::ValueType> fNWritten;
 
   static std::string GetTypeList(const std::vector<Detail::RFieldBase *> &itemFields);
   /// Extracts the index from an std::variant and transforms it into the 1-based index used for the switch column
   std::uint32_t GetTag(void *variantPtr) const;
   void SetTag(void *variantPtr, std::uint32_t tag) const;
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
 
public:
   // TODO(jblomer): use std::span in signature
   RVariantField(std::string_view fieldName, const std::vector<Detail::RFieldBase *> &itemFields);
   RVariantField(RVariantField &&other) = default;
   RVariantField& operator =(RVariantField &&other) = default;
   ~RVariantField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) final;
   Detail::RFieldValue CaptureValue(void *where) final;
   size_t GetValueSize() const final;
   size_t GetAlignment() const final { return fMaxAlignment; }
   void CommitCluster() final;
};
 
 
/// Classes with dictionaries that can be inspected by TClass
template <typename T, typename=void>
class RField : public RClassField {
public:
   static std::string TypeName() { return ROOT::Internal::GetDemangledTypeName(typeid(T)); }
   RField(std::string_view name) : RClassField(name, TypeName()) {
      static_assert(std::is_class<T>::value, "no I/O support for this basic C++ type");
   }
   RField(RField &&other) = default;
   RField &operator=(RField &&other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT &&...args)
   {
      return Detail::RFieldValue(this, static_cast<T *>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final { return GenerateValue(where, T()); }
};
 
template <typename T, typename = void>
struct HasCollectionProxyMemberType : std::false_type {
};
template <typename T>
struct HasCollectionProxyMemberType<
   T, typename std::enable_if<std::is_same<typename T::IsCollectionProxy, std::true_type>::value>::type>
   : std::true_type {
};
 
/// The point here is that we can only tell at run time if a class has an associated collection proxy.
/// For compile time, in the first iteration of this PR we had an extra template argument that acted as a "tag" to
/// differentiate the RField specialization for classes with an associated collection proxy (inherits
/// `RCollectionClassField`) from the RField primary template definition (`RClassField`-derived), as in:
/// ```
/// auto field = std::make_unique<RField<MyClass>>("klass");
/// // vs
/// auto otherField = std::make_unique<RField<MyClass, ROOT::Experimental::TagIsCollectionProxy>>("klass");
/// ```
///
/// That is convenient only for non-nested types, i.e. it doesn't work with, e.g. `RField<std::vector<MyClass>,
/// ROOT::Experimental::TagIsCollectionProxy>`, as the tag is not forwarded to the instantiation of the inner RField
/// (that for the value type of the vector).  The following two possible solutions were considered:
/// - A wrapper type (much like `ntuple/v7/inc/ROOT/RNTupleUtil.hxx:49`), that helps to differentiate both cases.
/// There we would have:
/// ```
/// auto field = std::make_unique<RField<RProxiedCollection<MyClass>>>("klass"); // Using collection proxy
/// ```
/// - A helper `IsCollectionProxy<T>` type, that can be used in a similar way to those in the `<type_traits>` header.
/// We found this more convenient and is the implemented thing below.  Here, classes can be marked as a
/// collection proxy with either of the following two forms (whichever is more convenient for the user):
/// ```
/// template <>
/// struct IsCollectionProxy<MyClass> : std::true_type {};
/// ```
/// or by adding a member type to the class as follows:
/// ```
/// class MyClass {
/// public:
///    using IsCollectionProxy = std::true_type;
/// };
/// ```
///
/// Of course, there is another possible solution which is to have a single `RClassField` that implements both
/// the regular-class and the collection-proxy behaviors, and always chooses appropriately at run time.
/// We found that less clean and probably has more overhead, as most probably it involves an additional branch + call
/// in each of the member functions.
template <typename T, typename = void>
struct IsCollectionProxy : HasCollectionProxyMemberType<T> {
};
 
/// Classes behaving as a collection of elements that can be queried via the `TVirtualCollectionProxy` interface
/// The use of a collection proxy for a particular class can be enabled via:
/// ```
/// namespace ROOT::Experimental {
///    template <> struct IsCollectionProxy<Classname> : std::true_type {};
/// }
/// ```
/// Alternatively, this can be achieved by adding a member type to the class definition as follows:
/// ```
/// class Classname {
/// public:
///    using IsCollectionProxy = std::true_type;
/// };
/// ```
template <typename T>
class RField<T, typename std::enable_if<IsCollectionProxy<T>::value>::type> : public RCollectionClassField {
public:
   static std::string TypeName() { return ROOT::Internal::GetDemangledTypeName(typeid(T)); }
   RField(std::string_view name) : RCollectionClassField(name, TypeName())
   {
      static_assert(std::is_class<T>::value, "collection proxy unsupported for fundamental types");
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<T*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, T()); }
};
 
/// The collection field is only used for writing; when reading, untyped collections are projected to an std::vector
class RCollectionField : public ROOT::Experimental::Detail::RFieldBase {
private:
   /// Save the link to the collection ntuple in order to reset the offset counter when committing the cluster
   std::shared_ptr<RCollectionNTupleWriter> fCollectionNTuple;
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final;
public:
   static std::string TypeName() { return ""; }
   RCollectionField(std::string_view name,
                    std::shared_ptr<RCollectionNTupleWriter> collectionNTuple,
                    std::unique_ptr<RNTupleModel> collectionModel);
   RCollectionField(RCollectionField&& other) = default;
   RCollectionField& operator =(RCollectionField&& other) = default;
   ~RCollectionField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   using Detail::RFieldBase::GenerateValue;
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final {
      return Detail::RFieldValue(
         Detail::RColumnElement<ClusterSize_t>(static_cast<ClusterSize_t*>(where)),
         this, static_cast<ClusterSize_t*>(where));
   }
   Detail::RFieldValue CaptureValue(void* where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<ClusterSize_t>(static_cast<ClusterSize_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(ClusterSize_t); }
   void CommitCluster() final;
};
 
/// The generic field for `std::pair<T1, T2>` types
class RPairField : public RRecordField {
private:
   TClass *fClass = nullptr;
   static std::string GetTypeList(const std::array<std::unique_ptr<Detail::RFieldBase>, 2> &itemFields);
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const override;
 
   RPairField(std::string_view fieldName, std::array<std::unique_ptr<Detail::RFieldBase>, 2> &&itemFields,
              const std::array<std::size_t, 2> &offsets);
 
public:
   RPairField(std::string_view fieldName, std::array<std::unique_ptr<Detail::RFieldBase>, 2> &itemFields);
   RPairField(RPairField &&other) = default;
   RPairField &operator=(RPairField &&other) = default;
   ~RPairField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) override;
};
 
/// The generic field for `std::tuple<Ts...>` types
class RTupleField : public RRecordField {
private:
   TClass *fClass = nullptr;
   static std::string GetTypeList(const std::vector<std::unique_ptr<Detail::RFieldBase>> &itemFields);
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const override;
 
   RTupleField(std::string_view fieldName, std::vector<std::unique_ptr<Detail::RFieldBase>> &&itemFields,
               const std::vector<std::size_t> &offsets);
 
public:
   RTupleField(std::string_view fieldName, std::vector<std::unique_ptr<Detail::RFieldBase>> &itemFields);
   RTupleField(RTupleField &&other) = default;
   RTupleField &operator=(RTupleField &&other) = default;
   ~RTupleField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   Detail::RFieldValue GenerateValue(void *where) override;
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) override;
};
 
/// Template specializations for concrete C++ types
 
 
template <>
class RField<ClusterSize_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "ROOT::Experimental::ClusterSize_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   ClusterSize_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<ClusterSize_t>(globalIndex);
   }
   ClusterSize_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<ClusterSize_t>(clusterIndex);
   }
   ClusterSize_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<ClusterSize_t>(globalIndex, nItems);
   }
   ClusterSize_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<ClusterSize_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<ClusterSize_t>(static_cast<ClusterSize_t*>(where)),
         this, static_cast<ClusterSize_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<ClusterSize_t>(static_cast<ClusterSize_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(ClusterSize_t); }
 
   /// Special help for offset fields
   void GetCollectionInfo(NTupleSize_t globalIndex, RClusterIndex *collectionStart, ClusterSize_t *size) {
      fPrincipalColumn->GetCollectionInfo(globalIndex, collectionStart, size);
   }
   void GetCollectionInfo(const RClusterIndex &clusterIndex, RClusterIndex *collectionStart, ClusterSize_t *size) {
      fPrincipalColumn->GetCollectionInfo(clusterIndex, collectionStart, size);
   }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
 
template <>
class RField<bool> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "bool"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   bool *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<bool>(globalIndex);
   }
   bool *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<bool>(clusterIndex);
   }
   bool *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<bool>(globalIndex, nItems);
   }
   bool *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<bool>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<bool>(static_cast<bool*>(where)),
         this, static_cast<bool*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, false); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<bool>(static_cast<bool*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(bool); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<float> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "float"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   float *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<float>(globalIndex);
   }
   float *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<float>(clusterIndex);
   }
   float *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<float>(globalIndex, nItems);
   }
   float *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<float>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<float>(static_cast<float*>(where)),
         this, static_cast<float*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0.0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<float>(static_cast<float*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(float); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
 
template <>
class RField<double> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "double"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   double *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<double>(globalIndex);
   }
   double *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<double>(clusterIndex);
   }
   double *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<double>(globalIndex, nItems);
   }
   double *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<double>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<double>(static_cast<double*>(where)),
         this, static_cast<double*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0.0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<double>(static_cast<double*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(double); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<char> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "char"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   char *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<char>(globalIndex);
   }
   char *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<char>(clusterIndex);
   }
   char *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<char>(globalIndex, nItems);
   }
   char *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<char>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<char>(static_cast<char*>(where)),
         this, static_cast<char*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<char>(static_cast<char*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(char); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::int8_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::int8_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::int8_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::int8_t>(globalIndex);
   }
   std::int8_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::int8_t>(clusterIndex);
   }
   std::int8_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int8_t>(globalIndex, nItems);
   }
   std::int8_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int8_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::int8_t>(static_cast<std::int8_t*>(where)),
         this, static_cast<std::int8_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::int8_t>(static_cast<std::int8_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::int8_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::uint8_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::uint8_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::uint8_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::uint8_t>(globalIndex);
   }
   std::uint8_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::uint8_t>(clusterIndex);
   }
   std::uint8_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint8_t>(globalIndex, nItems);
   }
   std::uint8_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint8_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::uint8_t>(static_cast<std::uint8_t*>(where)),
         this, static_cast<std::uint8_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::uint8_t>(static_cast<std::uint8_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::uint8_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::int16_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::int16_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::int16_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::int16_t>(globalIndex);
   }
   std::int16_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::int16_t>(clusterIndex);
   }
   std::int16_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int16_t>(globalIndex, nItems);
   }
   std::int16_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int16_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::int16_t>(static_cast<std::int16_t*>(where)),
         this, static_cast<std::int16_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::int16_t>(static_cast<std::int16_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::int16_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::uint16_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::uint16_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::uint16_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::uint16_t>(globalIndex);
   }
   std::uint16_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::uint16_t>(clusterIndex);
   }
   std::uint16_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint16_t>(globalIndex, nItems);
   }
   std::uint16_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint16_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::uint16_t>(static_cast<std::uint16_t*>(where)),
         this, static_cast<std::uint16_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::uint16_t>(static_cast<std::uint16_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::uint16_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::int32_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::int32_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::int32_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::int32_t>(globalIndex);
   }
   std::int32_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::int32_t>(clusterIndex);
   }
   std::int32_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int32_t>(globalIndex, nItems);
   }
   std::int32_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int32_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::int32_t>(static_cast<std::int32_t*>(where)),
         this, static_cast<std::int32_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::int32_t>(static_cast<std::int32_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::int32_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::uint32_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::uint32_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::uint32_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::uint32_t>(globalIndex);
   }
   std::uint32_t *Map(const RClusterIndex clusterIndex) {
      return fPrincipalColumn->Map<std::uint32_t>(clusterIndex);
   }
   std::uint32_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint32_t>(globalIndex, nItems);
   }
   std::uint32_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint32_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::uint32_t>(static_cast<std::uint32_t*>(where)),
         this, static_cast<std::uint32_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::uint32_t>(static_cast<std::uint32_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::uint32_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::uint64_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::uint64_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::uint64_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::uint64_t>(globalIndex);
   }
   std::uint64_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::uint64_t>(clusterIndex);
   }
   std::uint64_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint64_t>(globalIndex, nItems);
   }
   std::uint64_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::uint64_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::uint64_t>(static_cast<std::uint64_t*>(where)),
         this, static_cast<std::uint64_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::uint64_t>(static_cast<std::uint64_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::uint64_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::int64_t> : public Detail::RFieldBase {
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
 
public:
   static std::string TypeName() { return "std::int64_t"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, true /* isSimple */)
   {
      fTraits |= kTraitTrivialType;
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   std::int64_t *Map(NTupleSize_t globalIndex) {
      return fPrincipalColumn->Map<std::int64_t>(globalIndex);
   }
   std::int64_t *Map(const RClusterIndex &clusterIndex) {
      return fPrincipalColumn->Map<std::int64_t>(clusterIndex);
   }
   std::int64_t *MapV(NTupleSize_t globalIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int64_t>(globalIndex, nItems);
   }
   std::int64_t *MapV(const RClusterIndex &clusterIndex, NTupleSize_t &nItems) {
      return fPrincipalColumn->MapV<std::int64_t>(clusterIndex, nItems);
   }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(
         Detail::RColumnElement<std::int64_t>(static_cast<std::int64_t*>(where)),
         this, static_cast<std::int64_t*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, 0); }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */,
         Detail::RColumnElement<std::int64_t>(static_cast<std::int64_t*>(where)), this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::int64_t); }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
template <>
class RField<std::string> : public Detail::RFieldBase {
private:
   ClusterSize_t fIndex;
   Detail::RColumnElement<ClusterSize_t, EColumnType::kIndex> fElemIndex;
 
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
   std::size_t AppendImpl(const ROOT::Experimental::Detail::RFieldValue& value) final;
   void ReadGlobalImpl(ROOT::Experimental::NTupleSize_t globalIndex,
                       ROOT::Experimental::Detail::RFieldValue *value) final;
 
public:
   static std::string TypeName() { return "std::string"; }
   explicit RField(std::string_view name)
      : Detail::RFieldBase(name, TypeName(), ENTupleStructure::kLeaf, false /* isSimple */), fIndex(0),
        fElemIndex(&fIndex)
   {
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<std::string*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final { return GenerateValue(where, ""); }
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) override {
      auto str = value.Get<std::string>();
      str->~basic_string(); // TODO(jblomer) C++17 std::destroy_at
      if (!dtorOnly)
         free(str);
   }
   Detail::RFieldValue CaptureValue(void *where) override {
      return Detail::RFieldValue(true /* captureFlag */, this, where);
   }
   size_t GetValueSize() const final { return sizeof(std::string); }
   size_t GetAlignment() const final { return std::alignment_of<std::string>(); }
   void CommitCluster() final;
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
};
 
 
template <typename ItemT, std::size_t N>
class RField<std::array<ItemT, N>> : public RArrayField {
   using ContainerT = typename std::array<ItemT, N>;
public:
   static std::string TypeName() {
      return "std::array<" + RField<ItemT>::TypeName() + "," + std::to_string(N) + ">";
   }
   explicit RField(std::string_view name)
      : RArrayField(name, std::make_unique<RField<ItemT>>(RField<ItemT>::TypeName()), N)
   {}
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final {
      return GenerateValue(where, ContainerT());
   }
};
 
template <typename ItemT, std::size_t N>
class RField<ItemT[N]> : public RField<std::array<ItemT, N>> {
public:
   explicit RField(std::string_view name) : RField<std::array<ItemT, N>>(name) {}
   RField(RField &&other) = default;
   RField &operator=(RField &&other) = default;
   ~RField() override = default;
};
 
template <typename... ItemTs>
class RField<std::variant<ItemTs...>> : public RVariantField {
   using ContainerT = typename std::variant<ItemTs...>;
private:
   template <typename HeadT, typename... TailTs>
   static std::string BuildItemTypes()
   {
      std::string result = RField<HeadT>::TypeName();
      if constexpr(sizeof...(TailTs) > 0)
         result += "," + BuildItemTypes<TailTs...>();
      return result;
   }
 
   template <typename HeadT, typename... TailTs>
   static std::vector<Detail::RFieldBase *> BuildItemFields(unsigned int index = 0)
   {
      std::vector<Detail::RFieldBase *> result;
      result.emplace_back(new RField<HeadT>("_" + std::to_string(index)));
      if constexpr(sizeof...(TailTs) > 0) {
         auto tailFields = BuildItemFields<TailTs...>(index + 1);
         result.insert(result.end(), tailFields.begin(), tailFields.end());
      }
      return result;
   }
 
public:
   static std::string TypeName() { return "std::variant<" + BuildItemTypes<ItemTs...>() + ">"; }
   explicit RField(std::string_view name) : RVariantField(name, BuildItemFields<ItemTs...>()) {}
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final {
      return GenerateValue(where, ContainerT());
   }
};
 
template <typename ItemT>
class RField<std::vector<ItemT>> : public RVectorField {
   using ContainerT = typename std::vector<ItemT>;
public:
   static std::string TypeName() { return "std::vector<" + RField<ItemT>::TypeName() + ">"; }
   explicit RField(std::string_view name)
      : RVectorField(name, std::make_unique<RField<ItemT>>("_0"))
   {}
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final {
      return GenerateValue(where, ContainerT());
   }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */, this, where);
   }
   size_t GetValueSize() const final { return sizeof(ContainerT); }
};
 
// std::vector<bool> is a template specialization and needs special treatment
template <>
class RField<std::vector<bool>> : public Detail::RFieldBase {
private:
   ClusterSize_t fNWritten{0};
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      return std::make_unique<RField>(newName);
   }
   std::size_t AppendImpl(const Detail::RFieldValue& value) final;
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final;
   void GenerateColumnsImpl() final;
   void GenerateColumnsImpl(const RNTupleDescriptor &desc) final;
 
public:
   static std::string TypeName() { return "std::vector<bool>"; }
   explicit RField(std::string_view name);
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<std::vector<bool>*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final {
      return GenerateValue(where, std::vector<bool>());
   }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */, this, where);
   }
   std::vector<Detail::RFieldValue> SplitValue(const Detail::RFieldValue &value) const final;
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) final;
 
   size_t GetValueSize() const final { return sizeof(std::vector<bool>); }
   size_t GetAlignment() const final { return std::alignment_of<std::vector<bool>>(); }
   void CommitCluster() final { fNWritten = 0; }
   void AcceptVisitor(Detail::RFieldVisitor &visitor) const final;
   void GetCollectionInfo(NTupleSize_t globalIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const {
      fPrincipalColumn->GetCollectionInfo(globalIndex, collectionStart, size);
   }
   void GetCollectionInfo(const RClusterIndex &clusterIndex, RClusterIndex *collectionStart, ClusterSize_t *size) const
   {
      fPrincipalColumn->GetCollectionInfo(clusterIndex, collectionStart, size);
   }
};
 
template <typename ItemT>
class RField<ROOT::VecOps::RVec<ItemT>> : public RRVecField {
   using ContainerT = typename ROOT::VecOps::RVec<ItemT>;
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final {
      auto newItemField = fSubFields[0]->Clone(fSubFields[0]->GetName());
      return std::make_unique<RField<ROOT::VecOps::RVec<ItemT>>>(newName, std::move(newItemField));
   }
   std::size_t AppendImpl(const Detail::RFieldValue& value) final {
      auto typedValue = value.Get<ContainerT>();
      auto nbytes = 0;
      auto count = typedValue->size();
      for (unsigned i = 0; i < count; ++i) {
         auto itemValue = fSubFields[0]->CaptureValue(&typedValue->data()[i]);
         nbytes += fSubFields[0]->Append(itemValue);
      }
      Detail::RColumnElement<ClusterSize_t, EColumnType::kIndex> elemIndex(&this->fNWritten);
      this->fNWritten += count;
      fColumns[0]->Append(elemIndex);
      return nbytes + sizeof(elemIndex);
   }
   void ReadGlobalImpl(NTupleSize_t globalIndex, Detail::RFieldValue *value) final {
      auto typedValue = value->Get<ContainerT>();
      ClusterSize_t nItems;
      RClusterIndex collectionStart;
      fPrincipalColumn->GetCollectionInfo(globalIndex, &collectionStart, &nItems);
      typedValue->resize(nItems);
      for (unsigned i = 0; i < nItems; ++i) {
         auto itemValue = fSubFields[0]->CaptureValue(&typedValue->data()[i]);
         fSubFields[0]->Read(collectionStart + i, &itemValue);
      }
   }
 
public:
   RField(std::string_view fieldName, std::unique_ptr<Detail::RFieldBase> itemField)
      : RRVecField(fieldName, std::move(itemField))
   {
   }
 
   explicit RField(std::string_view name)
      : RField(name, std::make_unique<RField<ItemT>>("_0"))
   {
   }
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   void DestroyValue(const Detail::RFieldValue& value, bool dtorOnly = false) final {
      auto vec = reinterpret_cast<ContainerT*>(value.GetRawPtr());
      vec->~RVec();
      if (!dtorOnly)
         free(vec);
   }
 
   static std::string TypeName() { return "ROOT::VecOps::RVec<" + RField<ItemT>::TypeName() + ">"; }
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void* where) final {
      return GenerateValue(where, ContainerT());
   }
   Detail::RFieldValue CaptureValue(void *where) final {
      return Detail::RFieldValue(true /* captureFlag */, this, static_cast<ContainerT*>(where));
   }
   size_t GetValueSize() const final { return sizeof(ContainerT); }
   size_t GetAlignment() const final { return std::alignment_of<ContainerT>(); }
};
 
template <typename T1, typename T2>
class RField<std::pair<T1, T2>> : public RPairField {
   using ContainerT = typename std::pair<T1,T2>;
private:
   template <typename Ty1, typename Ty2>
   static std::array<std::unique_ptr<Detail::RFieldBase>, 2> BuildItemFields()
   {
      return {std::make_unique<RField<Ty1>>("_0"), std::make_unique<RField<Ty2>>("_1")};
   }
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final
   {
      std::array<std::unique_ptr<Detail::RFieldBase>, 2> items{fSubFields[0]->Clone(fSubFields[0]->GetName()),
                                                               fSubFields[1]->Clone(fSubFields[1]->GetName())};
      return std::make_unique<RField<std::pair<T1, T2>>>(newName, std::move(items));
   }
 
public:
   static std::string TypeName() {
      return "std::pair<" + RField<T1>::TypeName() + "," + RField<T2>::TypeName() + ">";
   }
   explicit RField(std::string_view name, std::array<std::unique_ptr<Detail::RFieldBase>, 2> &&itemFields)
      : RPairField(name, std::move(itemFields), {offsetof(ContainerT, first), offsetof(ContainerT, second)})
   {
      fMaxAlignment = std::max(alignof(T1), alignof(T2));
      fSize = sizeof(ContainerT);
   }
   explicit RField(std::string_view name) : RField(name, BuildItemFields<T1, T2>()) {}
   RField(RField&& other) = default;
   RField& operator =(RField&& other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT&&... args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT*>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final {
      return GenerateValue(where, ContainerT());
   }
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) final
   {
      reinterpret_cast<ContainerT *>(value.GetRawPtr())->~pair();
      if (!dtorOnly)
         free(reinterpret_cast<ContainerT *>(value.GetRawPtr()));
   }
};
 
template <typename... ItemTs>
class RField<std::tuple<ItemTs...>> : public RTupleField {
   using ContainerT = typename std::tuple<ItemTs...>;
private:
   template <typename HeadT, typename... TailTs>
   static std::string BuildItemTypes()
   {
      std::string result = RField<HeadT>::TypeName();
      if constexpr (sizeof...(TailTs) > 0)
         result += "," + BuildItemTypes<TailTs...>();
      return result;
   }
 
   template <typename HeadT, typename... TailTs>
   static void _BuildItemFields(std::vector<std::unique_ptr<Detail::RFieldBase>> &itemFields, unsigned int index = 0)
   {
      itemFields.emplace_back(new RField<HeadT>("_" + std::to_string(index)));
      if constexpr (sizeof...(TailTs) > 0)
         _BuildItemFields<TailTs...>(itemFields, index + 1);
   }
   template <typename... Ts>
   static std::vector<std::unique_ptr<Detail::RFieldBase>> BuildItemFields()
   {
      std::vector<std::unique_ptr<Detail::RFieldBase>> result;
      _BuildItemFields<Ts...>(result);
      return result;
   }
 
   template <unsigned Index, typename HeadT, typename... TailTs>
   static void _BuildItemOffsets(std::vector<std::size_t> &offsets, const ContainerT &tuple)
   {
      auto offset =
         reinterpret_cast<std::uintptr_t>(&std::get<Index>(tuple)) - reinterpret_cast<std::uintptr_t>(&tuple);
      offsets.emplace_back(offset);
      if constexpr (sizeof...(TailTs) > 0)
         _BuildItemOffsets<Index + 1, TailTs...>(offsets, tuple);
   }
   template <typename... Ts>
   static std::vector<std::size_t> BuildItemOffsets()
   {
      std::vector<std::size_t> result;
      _BuildItemOffsets<0, Ts...>(result, ContainerT());
      return result;
   }
 
protected:
   std::unique_ptr<Detail::RFieldBase> CloneImpl(std::string_view newName) const final
   {
      std::vector<std::unique_ptr<Detail::RFieldBase>> items;
      for (auto &item : fSubFields)
         items.push_back(item->Clone(item->GetName()));
      return std::make_unique<RField<std::tuple<ItemTs...>>>(newName, std::move(items));
   }
 
public:
   static std::string TypeName() { return "std::tuple<" + BuildItemTypes<ItemTs...>() + ">"; }
   explicit RField(std::string_view name, std::vector<std::unique_ptr<Detail::RFieldBase>> &&itemFields)
      : RTupleField(name, std::move(itemFields), BuildItemOffsets<ItemTs...>())
   {
      fMaxAlignment = std::max({alignof(ItemTs)...});
      fSize = sizeof(ContainerT);
   }
   explicit RField(std::string_view name) : RField(name, BuildItemFields<ItemTs...>()) {}
   RField(RField &&other) = default;
   RField &operator=(RField &&other) = default;
   ~RField() override = default;
 
   using Detail::RFieldBase::GenerateValue;
   template <typename... ArgsT>
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where, ArgsT &&...args)
   {
      return Detail::RFieldValue(this, static_cast<ContainerT *>(where), std::forward<ArgsT>(args)...);
   }
   ROOT::Experimental::Detail::RFieldValue GenerateValue(void *where) final
   {
      return GenerateValue(where, ContainerT());
   }
   void DestroyValue(const Detail::RFieldValue &value, bool dtorOnly = false) final
   {
      reinterpret_cast<ContainerT *>(value.GetRawPtr())->~tuple();
      if (!dtorOnly)
         free(reinterpret_cast<ContainerT *>(value.GetRawPtr()));
   }
};
 
} // namespace Experimental
} // namespace ROOT
 
#endif