foo.cc:2:17: error: 'stda' is not a namespace-name
 using namespace stda;
                 ^~~~
foo.cc:2:21: error: expected namespace-name before ';' token
 using namespace stda;
                     ^
foo.cc: In function 'int main()':
foo.cc:6:2: error: 'cout' was not declared in this scope
  cout<<12<<endl;
  ^~~~
foo.cc:6:2: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:6:12: error: 'endl' was not declared in this scope
  cout<<12<<endl;
            ^~~~
foo.cc:6:12: note: suggested alternative:
In file included from /usr/include/c++/7/iostream:39:0,
                 from foo.cc:1:
/usr/include/c++/7/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~