Unreal Engine and C++. I've long worked with Unity, but as part of a new job I'm tasked with developing our Unreal plugin. Previously I only touched C++ on occasion, so I had a lot to learn — and have a lot to learn yet — of best practices, new features available in C++11, dealing with exceptions (Unreal disables them by default), and so forth. Likewise for Unreal. Like C++ itself, it's wonderfully powerful but sometimes painfully complex.

I also continued to deepen my understanding of databases and distributed systems. My favourite read this year was Designing Data-Intensive Applications which made me more familiar with the pros and cons of the various datastores and provided a better sense of the tradeoffs that each makes. It also gave me an appreciation for the guarantees that the battle-tested relational databases provide. One of my goals for 2019 is to improve my SQL knowledge — thus far any extra effort to understand it better has payed dividends.