Method GetChild

cpp/src/arrow/type.cc:1652–1673

If the index is out of bounds, this returns an invalid selector rather than an error.

  // 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_);