Psychic classes have the appearance of ignoring data provided to it in an attempt to provide you with an answer they predict is better for the situation.’? It’s impossible to look at a the data provided to an instance of the class and understand what queries on the object will return because it may think of a better answer for you, or a better piece of data to look at.

This comes from an example I ran into about a month ago. I work on an IDE and naturally deal with a lot of parse trees and tokens. Parsing everything all the time is expensive so naturally the results are cached in various places for performance reasons.

While debugging one such cache I noticed some strange behavior. The cache wasn’t returning the right tree for the input it was provided. So I decided to dig into the code a bit.

This cache takes several different forms of input which has no common base class or interface. What I noticed though is that when resetting the input of the service, it would not clear the existing cache or the previous form of input. Also because of the way the code loaded certain forms of input had precedence over others. So even an explicit clear did not guarantee the ‘correct’ input was used.

The result is a service that reads well in code, but will not always act as you expect it to. The service at times will seemingly ignore all input and pick a source it thinks is better. Take the following code as an example

ParseTree* pTree = pCache->GetTree();  

This code is very straight forward but is certainly not guaranteed to do what it appears to do.

I like to think the service is predicting the results rather than calculating them. Or better yet guessing the answer. From the perspective of a code reviewer, that’s what’s happening.

Obviously I was curious about the reason for this and did a bit of research. It’s a rather old class so I had to contact people who’d been on the team awhile back and dig through the history of the code base to understand what the purpose of this behavior was. It turns out it was done to fix a few impactful scenarios where an alternate source needed precedence over the typical source. Other devs didn’t fully understand the source semantics of the service and wrote methods that caused bad interactions. Eventually it evolved to it’s current odd state.

Thanks to Dustin for coining the term ‘psychic classes’. Other ones we considered were

  • Jedi Mind Trick classes: Weak name
  • Weatherman: It’s a prediction after all??

And yes, we fixed this issue :)

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