foo.cc: In function ‘void back(std::string&)’:
foo.cc:16:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   16 |     for (int i = 0; i < s.length() / 2; i++)
      |                     ~~^~~~~~~~~~~~~~~~
foo.cc: In function ‘int main()’:
foo.cc:34:27: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   34 |         for (int i = 0; i < s2.length(); i++)
      |                         ~~^~~~~~~~~~~~~
foo.cc:36:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   36 |             if (i >= s1.length())
      |                 ~~^~~~~~~~~~~~~~
foo.cc:54:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   54 |             if (i >= s1.length())
      |                 ~~^~~~~~~~~~~~~~