foo.cc: In function ‘void Concat(std::string, std::string)’:
foo.cc:9:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    9 |         for(int i=0; i<s2.length(); i++)
      |                      ~^~~~~~~~~~~~
foo.cc:11:25: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   11 |                 if(s2[i]=' ')
foo.cc:17:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   17 |         for(int i=0; i<s1.length()-1; i++)
      |                      ~^~~~~~~~~~~~~~
foo.cc:21:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   21 |         for(int i=n;i<s2.length();i++)
      |                     ~^~~~~~~~~~~~