libc++-11-dev 1:11.0.1~+rc2-1 (riscv64 binary) in ubuntu hirsute

 libc++ is another implementation of the C++ standard library
 .
 Features and Goals
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   * Correctness as defined by the C++ standards.
   * Fast execution.
   * Minimal memory use.
   * Fast compile times.
   * ABI compatibility with gcc's libstdc++ for some low-level features such
     as exception objects, rtti and memory allocation.
   * Extensive unit tests.