The static analyzer. There are so many subtle things that aren't caught by the compiler and don't cause any problems in your testing but will definitely come up once 10M users are running your app. The static analyzer can catch some of them before you even run! It's great.
The various sanitizers - Address Sanitizer, Thread Sanitizer, Undefined Behavior Sanitizer. These all also find things before you ship. They require (re)building and running but it's worth it.
Beyond that, any sort of instrumentation that can show you in real time information about your running application. You can see memory growing, even if it's not leaking. You can see performance tanking and know the exact function that's causing it before you quit.
Beyond that, for me using a framework that was well written made understanding architecture much easier. I had used MFC and CodeWarrior PowerPlant for a few years. They got the job done and were easy enough to use, but I didn't really learn anything from them. But when I moved to using Cocoa, it was so elegant and the separation of concerns was so good that it clicked and really taught me a lot about better architecture.
The various sanitizers - Address Sanitizer, Thread Sanitizer, Undefined Behavior Sanitizer. These all also find things before you ship. They require (re)building and running but it's worth it.
Beyond that, any sort of instrumentation that can show you in real time information about your running application. You can see memory growing, even if it's not leaking. You can see performance tanking and know the exact function that's causing it before you quit.
Beyond that, for me using a framework that was well written made understanding architecture much easier. I had used MFC and CodeWarrior PowerPlant for a few years. They got the job done and were easy enough to use, but I didn't really learn anything from them. But when I moved to using Cocoa, it was so elegant and the separation of concerns was so good that it clicked and really taught me a lot about better architecture.