foo.cc: In function 'long long int string_to_number(std::string)':
foo.cc:6:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |  for(int i=0;i<s.length();i++)
      |              ~^~~~~~~~~~~
foo.cc: In function 'void postfix(std::string)':
foo.cc:35:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   35 |  for(int i=0;i<s.length();i++)
      |              ~^~~~~~~~~~~

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