[millstone]

It should be changed to pass by value, and then use std::move to move it into the children array. The reason is that if the argument is a temporary, you are making an unnecessary copy of that temporary, when you could just transfer ownership.

This also gives callers the flexibility to std::move into your argument, transferring ownership.

A good rule of thumb is, if you are going to unconditionally take ownership of an object, accept it by value.