Class NestedSelector

cpp/src/arrow/type.cc:1633–1778

// of its children.
template <typename T, bool IsFlattening = false>
class NestedSelector {
 public:
  using ArrowType = T;
  using Util = NestedSelectorUtil;

  explicit NestedSelector(const std::vector<std::shared_ptr<T>>& children)
      : parent_or_children_(&children) {}
  explicit NestedSelector(const T& parent) : parent_or_children_(&parent) {}
  explicit NestedSelector(std::shared_ptr<T> parent)
      : owned_parent_(std::move(parent)), parent_or_children_(owned_parent_.get()) {}
  template <typename Arg>
  NestedSelector(Arg&& arg, MemoryPool* pool) : NestedSelector(std::forward<Arg>(arg)) {
    if (pool) {
      pool_ = pool;
    }
  }

  // If the index is out of bounds, this returns an invalid selector rather than an
  // error.
  Result<NestedSelector> GetChild(int i) const {
    std::shared_ptr<T> child;
    if (auto parent = get_parent()) {
      const DataType* type = Util::GetType(*parent);
      // We avoid this check for schema fields since it's inconsequential (plus there are
      // tests elsewhere that rely on it not happening)
      if constexpr (!std::is_same_v<T, Field>) {
        if (ARROW_PREDICT_FALSE(type->id() != Type::STRUCT)) {
          return Util::NonStructError();
        }
      }
      // Bounds-check the index *once* using the parent's type
      if (ARROW_PREDICT_TRUE(i >= 0 && i < type->num_fields())) {
        ARROW_ASSIGN_OR_RAISE(child, GetChild(*parent, i, pool_));
      }
    } else if (auto children = get_children()) {
      if (ARROW_PREDICT_TRUE(i >= 0 && static_cast<size_t>(i) < children->size())) {
        child = (*children)[i];
      }
    }
    return NestedSelector(std::move(child), pool_);
  }

  Result<std::shared_ptr<T>> Finish() const {
    DCHECK(get_parent() && owned_parent_);
    return owned_parent_;
  }

  template <typename OStream, typename U = T>
  std::enable_if_t<std::is_same_v<U, Field>> Summarize(OStream* os) const {
    const FieldVector* fields = get_children();
    if (!fields && get_parent()) {
      fields = &get_parent()->type()->fields();
    }
    *os << "fields: { ";
    if (fields) {
      for (const auto& field : *fields) {
        *os << field->ToString() << ", ";
      }