Given that C++ is (mostly) as superset of C its not simply interesting its odd.
Did they not simply give the C program to C++? Are they claiming that the C++ compiler compiles C programs to slower lower perf binary.
There must have used some definition that is not explicit in the paper, but you can see in this code sample that the author used various C++ standard data types (std::string, std::array), iterators, classes, concurrency (std::thread). I'm no judge of C++ style, but perhaps it's "C++ as a C++ developer circa 1997 would have written it".
https://github.com/greensoftwarelab/Energy-Languages/blob/ma...