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

["Wes McKinney (JIRA)" <ji...@apache.org>]

Wes McKinney created ARROW-621:
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             Summary: [C++] Implement an "inline visitor" template that enables visitor-pattern-like code without virtual function dispatch
                 Key: ARROW-621
                 URL: https://issues.apache.org/jira/browse/ARROW-621
             Project: Apache Arrow
          Issue Type: New Feature
          Components: C++
            Reporter: Wes McKinney


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:

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

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

{code:language=c++}
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");
}
{code}

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).



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