/in/foo.cc: In function 'int main()':
/in/foo.cc:21:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < v.size(); i++)
                     ~~^~~~~~~~~~
/in/foo.cc:27:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < v.size(); i++)
                     ~~^~~~~~~~~~
/in/foo.cc:31:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < v.size(); i++)
                     ~~^~~~~~~~~~