Many programming languages are used. Autonomous driving is not a simple system, but rather a large number of cooperating systems collaborating together. The sensors have their own programming code, the communications backbones have their own code, the navigation system has its own code, and all those bits are written by different people, different companies, and all choose their development platforms to meet the needs of the task. There is no need to standardize on a single language, and many reasons for the best languages to be chosen for each specific task.
The Algorithms will be more fascinating to look at as compared to whether it is written using C++ or Python or LISP.
Autonomous vehicle engineers on the job tend to use a variety of languages, depending on their team, their facility with different languages, the APIs their tools expose, and performance requirements.
C++ is a compiled, high-performance language, so most code that actually runs on the vehicle tends to be C++.
That said, many engineers spend most of their time prototyping algorithms in Python, Matlab, or even Java or other languages. Other engineers spend pretty much all of their time writing production code in C / C++.
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