Language: C++

#include <iostream>
#include <algorithm>
#include <string>
using namespace std;

int main(){
    string a,b,c;
    cin>>a>b>>c;

    char dif='A'-'a';
 
    cout<<(char)(a[0]+dif)<<(char)(b[0]+dif)<<(char)(c[0]+dif)<<endl;

    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:13: error: no match for 'operator>>' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    8 |     cin>>a>b>>c;
      |            ~^~~
      |            |  |
      |            |  basic_string<[...]>
      |            basic_string<[...]>
In file included from /usr/include/c++/14/string:55,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.tcc:835:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
  835 |     operator>>(basic_istream<_CharT, _Traits>& __in,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.tcc:835:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
In file included from /usr/include/c++/14/bits/memory_resource.h:38,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/cstddef:131:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator>>(byte, _IntegerType)'
  131 |     operator>>(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:131:5: note:   template argument deduction/substitution failed:
a.cc:8:12: note:   cannot convert 'b' (type 'std::string' {aka 'std::__cxx11::basic_string<char>'}) to type 'std::byte'
    8 |     cin>>a>b>>c;
      |            ^
In file included from /usr/include/c++/14/istream:1109,
                 from /usr/include/c++/14/iostream:42:
/usr/include/c++/14/bits/istream.tcc:978:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT&)'
  978 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
      |     ^~~~~~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:849:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char&)'
  849 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:849:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:854:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char&)'
  854 |     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:854:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:896:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT*)'
  896 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:896:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:939:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char*)'
  939 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:939:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:945:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char*)'
  945 |     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:945:5: note:   template argument deduction/substitution failed:
a.cc:8:15: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:1099:5: note: candidate: 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&)'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
/usr/include/c++/14/istream:1099:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/istream: In substitution of 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&) [with _Istream = std::__cxx11::basic_string<char>&; _Tp = std::__cxx11::basic_string<char>&]':
a.cc:8:15:   required from here
    8 |     cin>>a>b>>c;
      |               ^
/usr/include/c++/14/istream:1099:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~