Language: C++

#define rep(i,n) for(ll i=0, i##_len=(n); i<i##_len; ++i)
#define rrep(i,n) for(ll i##_len=(n) ,i=i##_len-1;i>=0;--i)
#define repi(i,a,b) for(ll i=ll(a);i<ll(b);++i)
#define fe(vec,lambda) std::for_each(vec.begin(), vec.end(), lambda );
#define printd(val) std::cout<< #val" = " <<val<<"\n";
#define printv(val) std::for_each(val.begin(),val.end(),[](auto x){cout<<x<<"\n";});
#define lim 1000000007ll
#include<bits/stdc++.h>
using ll = long long;
using pii = std::pair<ll,ll>;
using namespace std;
template<class T>T chmin(T& a,T b){if(a>b){a^=b;b^=a;a^=b;return 1;}return 0;}
template<class T>T chmax(T& b,T a){if(a>b){a^=b;b^=a;a^=b;return 1;}return 0;}
template <class T> T gcd(T a, T b){if (a < b) {a ^= b;b ^= a;a ^= b;} return b ? gcd(b, a % b) : a;}
template <class T> T lcm(T a, T b){return a * b / gcd(a, b);}
inline bool operator<(const pii &lhs,const pii &rhs){return(lhs.second<rhs.second);}
template<class T> vector<pair<T,T>> duplicator(const vector<T>& input){vector<pair<T,T>> out(input.size()-1);rep(i,input.size()-1){out[i].first =input[i  ];out[i].second=input[i+1];}return(out);}
vector<char> String_to_vector_char(const string& input){vector<char> out(input.size());rep(i,input.size()){out[i]=input[i];}return out;}


ll a,b,c,d,n;
vector<ll>v;
map<ll,ll>m;
string s[3],s2;
void input(){
    cin>>s[0]>>s[1]>>s[2];
}
void src(){
    upper_bound(s[0].begin(),s[0].end(),s[0].begin());
    upper_bound(s[1].begin(),s[1].end(),s[1].begin());
    upper_bound(s[2].begin(),s[2].end(),s[2].begin());

    cout<<s[0][0]<<s[1][0]<<s[2][0];
}

int main(int argc,char* argv[]){
    ios::sync_with_stdio(false);
 //   ifstream in( argv[1] ); cin.rdbuf(in.rdbuf());
    input();
    src();    
    return 0;
}

Compiler output:
a.cc: In function 'void src()':
a.cc:29:16: warning: ignoring return value of '_FIter std::upper_bound(_FIter, _FIter, const _Tp&) [with _FIter = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Tp = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >]', declared with attribute 'nodiscard' [-Wunused-result]
   29 |     upper_bound(s[0].begin(),s[0].end(),s[0].begin());
      |     ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:8:
/usr/include/c++/14/bits/stl_algo.h:2019:5: note: declared here
 2019 |     upper_bound(_ForwardIterator __first, _ForwardIterator __last,
      |     ^~~~~~~~~~~
a.cc:30:16: warning: ignoring return value of '_FIter std::upper_bound(_FIter, _FIter, const _Tp&) [with _FIter = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Tp = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >]', declared with attribute 'nodiscard' [-Wunused-result]
   30 |     upper_bound(s[1].begin(),s[1].end(),s[1].begin());
      |     ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:2019:5: note: declared here
 2019 |     upper_bound(_ForwardIterator __first, _ForwardIterator __last,
      |     ^~~~~~~~~~~
a.cc:31:16: warning: ignoring return value of '_FIter std::upper_bound(_FIter, _FIter, const _Tp&) [with _FIter = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Tp = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >]', declared with attribute 'nodiscard' [-Wunused-result]
   31 |     upper_bound(s[2].begin(),s[2].end(),s[2].begin());
      |     ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:2019:5: note: declared here
 2019 |     upper_bound(_ForwardIterator __first, _ForwardIterator __last,
      |     ^~~~~~~~~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:71,
                 from /usr/include/c++/14/algorithm:60:
/usr/include/c++/14/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Val_less_iter::operator()(_Value&, _Iterator) const [with _Value = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; _Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >]':
/usr/include/c++/14/bits/stl_algo.h:1993:14:   required from '_ForwardIterator std::__upper_bound(_ForwardIterator, _ForwardIterator, const _Tp&, _Compare) [with _ForwardIterator = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Tp = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Compare = __gnu_cxx::__ops::_Val_less_iter]'
 1993 |           if (__comp(__val, __middle))
      |               ~~~~~~^~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:2028:32:   required from '_FIter std::upper_bound(_FIter, _FIter, const _Tp&) [with _FIter = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >; _Tp = __gnu_cxx::__normal_iterator<char*, __cxx11::basic_string<char> >]'
 2028 |       return std::__upper_bound(__first, __last, __val,
      |              ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
 2029 |                                 __gnu_cxx::__ops::__val_less_iter());
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:29:16:   required from here
   29 |     upper_bound(s[0].begin(),s[0].end(),s[0].begin());
      |     ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/predefined_ops.h:98:22: error: no match for 'operator<' (operand types are 'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' and 'char')
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:1241:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator<(const __normal_iterator<_IteratorL, _Container>&, const __normal_iterator<_IteratorR, _Container>&)'
 1241 |     operator<(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1241:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_IteratorR, _Container>' and 'char'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1249:5: note: candidate: 'template<class _Iterator, class _Container> bool __gnu_cxx::operator<(const __normal_iterator<_Iterator, _Container>&, const __normal_iterator<_Iterator, _Container>&)'
 1249 |     operator<(const __normal_iterator<_Iterator, _Container>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1249:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   mismatched types 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>' and 'char'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'const std::pair<_T1, _T2>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'const std::reverse_iterator<_Iterator>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'const std::reverse_iterator<_Iterator>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'const std::move_iterator<_IteratorL>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   'const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'const std::move_iterator<_IteratorL>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   '__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/predefined_ops.h:98:22: note:   '__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/usr/include/c++/14/string_view:688:5: note: candidate: 't