foo.cc: In function 'int main()':
foo.cc:10:18: error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'char*' for argument '1' to 'char* strncat(char*, const char*, size_t)'
  s2=strncat(s2,s1);
                  ^
foo.cc:11:18: error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'char*' for argument '1' to 'char* strncat(char*, const char*, size_t)'
  s1=strncat(s1,s2);
                  ^

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