Language: C++

#include <bits/stdc++.h>
#define INF 2e9
#define MOD 1000000007
#define MOD9 998244353
#define intMAX 51000;
#define PI 3.14159265359
using namespace std;
typedef long long llong;
// typedef long double ldouble;

using Graph = vector<vector<int>>;

// int ctoi(const char c){
//   if('0' <= c && c <= '9') return (c-'0');
//   return -1;
// }

// int gcd(int a, int b){
//   int ma = max(a,b);
//   int mi = min(a,b);
//   if(mi == 0) return ma;

//   return gcd(mi, ma%mi);
// }

// llong POW(llong a, llong b){
//   if(b == 0) return 1;
//   else{
//     if(b%2 == 0) return POW(a * a % MOD, b / 2) % MOD;
//     else return a * POW(a * a % MOD, b / 2) % MOD;
//   }
// }

int main(){
  string A, B, C;
  cin >> A >> B >> C;
  string S;
  S[0] = A[0]; S[1] = B[0]; S[2] = C[0];
  transform(S.cbegin(), S.cend(), S.begin(), toupper);
  cout << S << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:39:12: error: no matching function for call to 'transform(std::__cxx11::basic_string<char>::const_iterator, std::__cxx11::basic_string<char>::const_iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
   39 |   transform(S.cbegin(), S.cend(), S.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:39:12: note:   couldn't deduce template parameter '_UnaryOperation'
   39 |   transform(S.cbegin(), S.cend(), S.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