/in/foo.cc: In function 'int main()':
/in/foo.cc:12:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < c1.size(); i++)
                     ^
/in/foo.cc:14:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < c2.size(); i++)
                     ^