Custom error message for stack overflow in C++

[HappySkeptic]

Okay, but at least you fixed that.  It is too bad it didn't speed things up.

Just note - that this is an unsafe construct in a multi-threaded program (but yours isn't, so it is fine).  With multi-threading, you have to use some sort of thread lock, not just a single variable.

Like those locks from the C++ standard library? https://en.cppreference.com/w/cpp/thread/lock

Yes.  However, getting a lock is also expensive, so in a thread-safe program you would find a way to do the static initialization at program startup, rather than test for initialization in the TreeNode constructor each time.