/in/foo.cc: In function 'std::__cxx11::string cf(std::__cxx11::string)':
/in/foo.cc:15:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<=a.size();i++)
               ~^~~~~~~~~~
/in/foo.cc:27:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(a.size()>len)
      ~~~~~~~~^~~~
/in/foo.cc:32:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(a.size()==len)
      ~~~~~~~~^~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:124:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<s.size();i++)
               ~^~~~~~~~~