foo.cc: In function 'int main()':
foo.cc:9:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < x.length(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:12:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < m.length(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:16:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < m.length(); i++) { 
                     ~~^~~~~~~~~~~~

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