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Since `std::string` has to be dynamically constructed and destructed,
it could be accessed before initialization or after destruction in a
multithreaded context. By using constant c-strings instead, we guarantee
that the array will be valid for the whole lifetime of the program. The
use of `constexpr` also enforces this requirement.

I have run clang-format on the file to format my changes according to
CONTRIBUTING.md.