[ARROW-621] [C++] Implement an "inline visitor" template that enables visitor-pattern-like code without virtual function dispatch

• Type: New Feature
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.3.0
• Component/s: C++
• External issue URL: https://github.com/apache/arrow/issues/16249
• Assignee: Wes McKinney
• Reporter: Wes McKinney
• Created: 12/Mar/17 18:52
• Updated: 11/Jan/23 07:09
• Resolved: 23/Mar/17 16:38

Description

For performance-sensitive dynamic dispatch, the current visitor pattern may be suboptimal due to having two virtual function dispatches per invocation. For example, we might have:

ArrayVisitor* visitor = ...;
RETURN_NOT_OK(array->Accept(visitor));

For certain types of visitors, it may be possible to create a reusable pattern like:

template <typename VISITOR>
Status VisitArrayInline(const Array& array, VISITOR* visitor) {
  switch(array.type_id()) {
    case UINT8:
      return visitor->Visit(static_cast<const UInt8Array&>(array));
    ...
    default:
      break;
  }
  return Status::NotImplemented("Array type not implemented");
}

It would be useful to have an understanding of how this design pattern realistically impacts performance in canonical Arrow use cases (e.g. IPC record batch loading / unloading).