foo.cc: In function ‘int main()’:
foo.cc:20:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
   20 |         for (int i=0;i<b1.size();i++)
      |                      ~^~~~~~~~~~
foo.cc:25:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
   25 |         for (int i=0;i<b2.size();i++)
      |                      ~^~~~~~~~~~