You're getting downvoted but I don't think you're getting the benefit of understanding the disagreements. The benefit of visitor is for when you need multiple different TreeNode classes with different aspects, either different member variables or different methods/implementations themselves. In that case, the unused child ptrs are the smaller of the class modeling difficulties. For example, if you had a single base TreeNode class with the enum, after switching on the enum you need to explicitly downcast to the type. The dual-dispatch approach bakes this into the visit-accept protocol.

The last time I worked on a complex syntax tree structure, I used the approach you suggested because _editing_ a Visitor suite is a pain in the rear. Only terms that had special payload data (numbers, pointers to metadata objects, etc.) need downcasting.