foo.cc: In function 'long long int _read(std::__cxx11::string&, long long int&)':
foo.cc:16:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (; i < str.length() && str[i] >= '0' && str[i] <= '9'; i++)
         ~~^~~~~~~~~~~~~~
foo.cc: In member function 'long long int exp::calc(long long int)':
foo.cc:47:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while (i < str.length())
          ~~^~~~~~~~~~~~~~

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