foo.cc: In constructor 'BigInt::BigInt(std::string)':
foo.cc:24:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   24 |   for (int j = 0; j < s.size(); j++)
      |                   ~~^~~~~~~~~~

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