Language: C++

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream> 
#define _USE_MATH_DEFINES
#include <math.h>

using namespace std;

inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }

template<class T> inline T sqr(T x) { return x*x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long long int> vll;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
typedef unsigned long long ull;

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  FOR(i,0,n)
#define P(p) cout<<(p)<<endl;
#define VEC_2D(a,b) vector<vector<int> >(a, vector<int>(b, 0))
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define INF (1100000000)
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define MOD 1000000007LL
#define FSP(a) cout << fixed << setprecision(a)


ll gcd(ll x, ll y) {
	if (y == 0) return x;
	else return gcd(y, x%y);
}
ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}
bool is_prime(int n) {
	for (int i = 2; i * i <= n; i++) {
		if (n % i == 0) return false;
	}
	return n != 1;
}
map<int, int> prime_factor(int n) {
	map<int, int> res;
	for (int i = 2; i * i <= n; i++) {
		while (n % i == 0) {
			++res[i];
			n /= i;
		}
	}
	if (n != 1) res[n] = 1;
	return res;
}
int extgcd(int a, int b, int& x, int& y) {//
	int d = a;
	if (b != 0) {
		d = extgcd(b, a%b, y, x);
		y -= (a / b)*x;
	}
	else {
		x = 1; y = 0;
	}
	return d;
}



ll mod_pow(ll x, ll n, ll mod) {
	if (n == 0) return 1;
	ll res = mod_pow(x * x % mod, n / 2, mod);
	if (n & 1) res = res * x % mod;
	return res;
}
vector<string> split(const string &str, char delim) {
	vector<string> res;
	size_t current = 0, found;
	while ((found = str.find_first_of(delim, current)) != string::npos) {
		res.push_back(string(str, current, found - current));
		current = found + 1;
	}
	res.push_back(string(str, current, str.size() - current));
	return res;
}

bool is_kadomatsu(int a, int b, int c) {
	if (a == b || a == c || b == c)return false;
	if (a > b && c > b) return true;
	if (a < b && c < b)return true;
	return false;
}


struct UF {
	int n;
	vi d;
	UF() {}
	UF(int n) :n(n), d(n, -1) {}
	int root(int v) {
		if (d[v] < 0) return v;
		return d[v] = root(d[v]);
	}
	bool unite(int x, int y) {
		x = root(x); y = root(y);
		if (x == y) return false;
		if (size(x) < size(y)) swap(x, y);
		d[x] += d[y];
		d[y] = x;
		return true;
	}
	int size(int v) { return -d[root(v)]; }
};



vector<int> divisor(int n) {
	if (n == 1) return{};
	vi res;
	for (int i = 1; i*i <= n; i++) {
		if (n%i == 0) {
			res.emplace_back(i);
			if (i != 1 && i != n / i)res.emplace_back(n / i);
		}
	}
	return res;
}
//-------------------------------------------------------------



int main() {

	string a, b, c;
	cin >> a >> b >> c;

	transform(ALL(a), a.begin(), toupper);
	transform(ALL(b), b.begin(), toupper);
	transform(ALL(c), c.begin(), toupper);

	cout << a[0] << b[0] << c[0] << endl;
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:166:18: 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>)'
  166 |         transform(ALL(a), a.begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:9:
/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:166:18: note:   couldn't deduce template parameter '_UnaryOperation'
  166 |         transform(ALL(a), a.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
a.cc:167:18: 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>)'
  167 |         transform(ALL(b), b.begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/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:167:18: note:   couldn't deduce template parameter '_UnaryOperation'
  167 |         transform(ALL(b), b.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
/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
a.cc:168:18: 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>)'
  168 |         transform(ALL(c), c.begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/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:168:18: note:   couldn't deduce template parameter '_UnaryOperation'
  168 |         transform(ALL(c), c.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
/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