Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
#define FOR(i,a,n) for(int (i)=(a);(i)<(n);++(i))
#define eFOR(i,a,n) for(int (i)=(a);(i)<=(n);++(i))
#define rFOR(i,a,n) for(int (i)=(n)-1;(i)>=(a);--(i))
#define erFOR(i,a,n) for(int (i)=(n);(i)>=(a);--(i))
#define SORT(i) sort((i).begin(),(i).end())
#define rSORT(i,a) sort((i).begin(),(i).end(),(a))
#define all(i) (i).begin(),(i).end()
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; }return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; }return 0; }
inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); }
template<class T>inline istream& operator>>(istream& input, vector<T>& v) { for (auto& elemnt : v)input >> elemnt; return input; }

int main() {
    init();
        
    vector<string> s(3); cin >> s;
    FOR(i,0,3){
        transform(all(s[i]), s[i].begin(), toupper);
        cout << s[i][0];
    }
    cout << "\n";
}


Compiler output:
a.cc: In function 'int main()':
a.cc:27:18: error: no matching function for call to 'transform(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
   27 |         transform(all(s[i]), s[i].begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:61,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algo.h:4209:5: note: candidate: 'template<class _IIter, class _OIter, class _UnaryOperation> _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)'
 4209 |     transform(_InputIterator __first, _InputIterator __last,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4209:5: note:   template argument deduction/substitution failed:
a.cc:27:18: note:   couldn't deduce template parameter '_UnaryOperation'
   27 |         transform(all(s[i]), s[i].begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4247:5: note: candidate: 'template<class _IIter1, class _IIter2, class _OIter, class _BinaryOperation> _OIter std::transform(_IIter1, _IIter1, _IIter2, _OIter, _BinaryOperation)'
 4247 |     transform(_InputIterator1 __first1, _InputIterator1 __last1,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4247:5: note:   candidate expects 5 arguments, 4 provided
In file included from /usr/include/c++/14/algorithm:86:
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _UnaryOperation)'
  156 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note:   candidate expects 5 arguments, 4 provided
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator, class _BinaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator, _BinaryOperation)'
  162 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note:   candidate expects 6 arguments, 4 provided