foo.cc: In function 'int main()':
foo.cc:21:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < b.length(); i++)
              ~~^~~~~~~~~~~~
foo.cc:28:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i<f.size(); i++)
              ~^~~~~~~~~
foo.cc:18:7: warning: unused variable 'tmp' [-Wunused-variable]
  char tmp;
       ^~~

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