Language: C++

#include<bits/stdc++.h>
using namespace std;
int main(){
unordered_set<string>a;
  int n;
  cin>>n;
  for(int i=0;i<n;i++){
a.insert();
  }
  cout<<a.size()<<endl;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:8:9: error: no matching function for call to 'std::unordered_set<std::__cxx11::basic_string<char> >::insert()'
    8 | a.insert();
      | ~~~~~~~~^~
In file included from /usr/include/c++/14/unordered_set:41,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:189,
                 from a.cc:1:
/usr/include/c++/14/bits/unordered_set.h:476:9: note: candidate: 'template<class _InputIterator> void std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(_InputIterator, _InputIterator) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]'
  476 |         insert(_InputIterator __first, _InputIterator __last)
      |         ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:476:9: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/unordered_set.h:428:7: note: candidate: 'std::pair<typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::iterator, bool> std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(const value_type&) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::iterator; value_type = std::__cxx11::basic_string<char>]'
  428 |       insert(const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:428:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/unordered_set.h:432:7: note: candidate: 'std::pair<typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::iterator, bool> std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(value_type&&) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::iterator; value_type = std::__cxx11::basic_string<char>]'
  432 |       insert(value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:432:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/unordered_set.h:457:7: note: candidate: 'std::unordered_set<_Value, _Hash, _Pred, _Alloc>::iterator std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(const_iterator, const value_type&) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::iterator; const_iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  457 |       insert(const_iterator __hint, const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:457:7: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/unordered_set.h:461:7: note: candidate: 'std::unordered_set<_Value, _Hash, _Pred, _Alloc>::iterator std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(const_iterator, value_type&&) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::iterator; const_iterator = std::__detail::_Insert_base<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  461 |       insert(const_iterator __hint, value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:461:7: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/unordered_set.h:487:7: note: candidate: 'void std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(std::initializer_list<typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::value_type>) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Hashtable<_Value, _Value, _Alloc, std::__detail::_Identity, _Pred, _Hash, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<std::__not_<std::__and_<std::__is_fast_hash<_Hash>, std::__is_nothrow_invocable<const _Hash&, const _Tp&> > >::value, true, true> >::value_type = std::__cxx11::basic_string<char>]'
  487 |       insert(initializer_list<value_type> __l)
      |       ^~~~~~
/usr/include/c++/14/bits/unordered_set.h:487:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/unordered_set.h:506:7: note: candidate: 'std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert_return_type std::unordered_set<_Value, _Hash, _Pred, _Alloc>::insert(node_type&&) [with _Value = std::__cxx11::basic_string<char>; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; insert_return_type = std::_Hashtable<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::insert_return_type; node_type = std::_Hashtable<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, s