foo.cc: In constructor 'BigInt::BigInt(std::string)':
foo.cc:23:17: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   23 |   for(int j=0; j<s.size(); j++)
      |                ~^~~~~~~~~

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