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It is recommended to use an Integrated Development Environment that works with C++. Mixxx is a large C++ project spread across many files. Without an IDE that can help navigate around code split across multiple files, it can be difficult to understand how the code fits together.
KDAB's GammaRay is a tool for dynamically inspecting applications built with Qt. Its feature set is too long to list, but of particular use for Mixxx development is:
Debuggers allow you to inspect the state of Mixxx while its running (and after Mixxx has crashed, via a core dump file).