Storing temporary data for a geometry node

I’ve read this tutorial Tutorial about writing a custom geometry node for Blender and made a simple node (Imgur: The magic of the Internet).

One thing I’m confused about is where to store temporary runtime data. I only want the expression string to be saved in the .blend file. At runtime that string will be parsed and stored as some type of expression tree which I can’t put in the node’s storage structure since it will be saved to the .blend file. It seems like I could have my bNode’s runtime member point to my own derived class of bNodeRuntimeHandle, but that’s already being allocated and freed for me.

You shouldn’t create such cache. If you want to single time parse string and store this in to DNA, just create python script to generate node group, that will be DNA in general.