Language: C++

#include <unordered_set>
#include <algorithm>
#include <iostream>
#include <iterator>
 
int main() {
	int size;
  	cin >> size;
  	unordered_set<int> item_set;
  	n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
  	cout << item_set.size();
  	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    8 |         cin >> size;
      |         ^~~
      |         std::cin
In file included from a.cc:3:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:9:9: error: 'unordered_set' was not declared in this scope
    9 |         unordered_set<int> item_set;
      |         ^~~~~~~~~~~~~
a.cc:9:9: note: suggested alternatives:
In file included from /usr/include/c++/14/unordered_set:41,
                 from a.cc:1:
/usr/include/c++/14/bits/unordered_set.h:104:11: note:   'std::unordered_set'
  104 |     class unordered_set
      |           ^~~~~~~~~~~~~
/usr/include/c++/14/unordered_set:65:13: note:   'std::pmr::unordered_set'
   65 |       using unordered_set
      |             ^~~~~~~~~~~~~
a.cc:9:23: error: expected primary-expression before 'int'
    9 |         unordered_set<int> item_set;
      |                       ^~~
a.cc:10:16: error: 'istream_iterator' was not declared in this scope; did you mean 'std::istream_iterator'?
   10 |         n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
      |                ^~~~~~~~~~~~~~~~
      |                std::istream_iterator
In file included from /usr/include/c++/14/iterator:65,
                 from a.cc:4:
/usr/include/c++/14/bits/stream_iterator.h:56:11: note: 'std::istream_iterator' declared here
   56 |     class istream_iterator
      |           ^~~~~~~~~~~~~~~~
a.cc:10:33: error: expected primary-expression before 'int'
   10 |         n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
      |                                 ^~~
a.cc:10:59: error: 'item_set' was not declared in this scope
   10 |         n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
      |                                                           ^~~~~~~~
a.cc:10:50: error: 'inserter' was not declared in this scope; did you mean 'std::inserter'?
   10 |         n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
      |                                                  ^~~~~~~~
      |                                                  std::inserter
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/bits/hashtable_policy.h:36,
                 from /usr/include/c++/14/bits/hashtable.h:35,
                 from /usr/include/c++/14/bits/unordered_set.h:33:
/usr/include/c++/14/bits/stl_iterator.h:1017:5: note: 'std::inserter' declared here
 1017 |     inserter(_Container& __x, typename _Container::iterator __i)
      |     ^~~~~~~~
a.cc:10:9: error: 'n_copy' was not declared in this scope
   10 |         n_copy(istream_iterator<int>(cin), size, inserter(item_set, item_set.begin()));
      |         ^~~~~~
a.cc:11:9: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   11 |         cout << item_set.size();
      |         ^~~~
      |         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~