[ARROW-12804] [C++] Array methods IsNull and IsValid is confused for NullType

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.0
• Fix Version/s: 5.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/28541
• Assignee: Nate Clark
• Reporter: Chenxi Li
• Created: 17/May/21 04:03
• Updated: 11/Jan/23 08:28
• Resolved: 21/Jul/21 14:46

Description

I'm not sure if this is a bug or by design.

Array's method IsNull() and IsValid() is implemented as follows:

/// \brief Return true if value at index is null. Does not boundscheck
bool IsNull(int64_t i) const {
  return null_bitmap_data_ != NULLPTR &&
         !BitUtil::GetBit(null_bitmap_data_, i + data_->offset);
}

/// \brief Return true if value at index is valid (not null). Does not
/// boundscheck
bool IsValid(int64_t i) const {
  return null_bitmap_data_ == NULLPTR ||
         BitUtil::GetBit(null_bitmap_data_, i + data_->offset);
}

According to the doc, "Arrays having a 0 null count may choose to not allocate the null bitmap". But a null type array doesn't have a null bitmap. So for a null type array, IsNull() always returns false, IsValid() always returns true.

If this is by design, maybe additional comments and docs should be added.

Issue Links

• is duplicated by: ARROW-13319 [C++] NullArray::IsNull always returns false and NullArray::IsValid always returns true
• links to: GitHub Pull Request #10756