foo.cc: In function 'int main()':
foo.cc:6:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |         cin >> t;
      |         ^~~
      |         std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:75,
                 from foo.cc:1:
/usr/include/c++/11/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;           /// Linked to standard input
      |                  ^~~
foo.cc:7:9: error: 'string' was not declared in this scope
    7 |         string s;
      |         ^~~~~~
foo.cc:7:9: note: suggested alternatives:
In file included from /usr/include/c++/11/iosfwd:39,
                 from /usr/include/c++/11/ios:38,
                 from /usr/include/c++/11/istream:38,
                 from /usr/include/c++/11/sstream:38,
                 from /usr/include/c++/11/complex:45,
                 from /usr/include/c++/11/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:54,
                 from foo.cc:1:
/usr/include/c++/11/bits/stringfwd.h:79:33: note:   'std::string'
   79 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
In file included from /usr/include/c++/11/bits/locale_classes.h:40,
                 from /usr/include/c++/11/bits/ios_base.h:41,
                 from /usr/include/c++/11/ios:42,
                 from /usr/include/c++/11/istream:38,
                 from /usr/include/c++/11/sstream:38,
                 from /usr/include/c++/11/complex:45,
                 from /usr/include/c++/11/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:54,
                 from foo.cc:1:
/usr/include/c++/11/string:67:11: note:   'std::pmr::string'
   67 |     using string    = basic_string<char>;
      |           ^~~~~~
foo.cc:8:30: error: 's' was not declared in this scope
    8 |         while (t-- && cin >> s)
      |                              ^
foo.cc:10:17: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   10 |                 cout << s[0] << "\n";
      |                 ^~~~
      |                 std::cout
In file included from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:75,
                 from foo.cc:1:
/usr/include/c++/11/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;          /// Linked to standard output
      |                  ^~~~