foo.cc: In function 'int main()':
foo.cc:10:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < d.length(); i++) 
                     ~~^~~~~~~~~~~~
foo.cc:13:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < w.length(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:17:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < w.length(); i++) { 
                     ~~^~~~~~~~~~~~

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