Language: C++

#include <bits/stdc++.h>
using namespace std;
#define all(x) (x).begin(),(x).end()
 
int main(){
    string s,a="",b="";
    char c;
    int q,i;
    int t,f,flag=1;
    s.reserve(1000000);
    cin>>s>>q;
    for(i=0;i<q;i++){
        cin>>t;
        if(t==1){
            flag*=-1;
        }else{
            cin>>f>>c;
            switch(f*flag){
                case 1: 
                    a+=c;
                    break;
                case -1:
                    b+=c;
                    break;
                case 2:
                    b+=c;
                    break;
                case -2:
                    a+=c;
                    break;
            }
        }
    }
    s=reverse(a.begin(),a.end())+s+b;
    if(flag<0) reverse(s.begin(),s.end());
    cout<<s;
 
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:34:33: error: no match for 'operator+' (operand types are 'void' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
      |              |                   |
      |              void                std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:627:5: note: candidate: 'template<class _Iterator> constexpr std::reverse_iterator<_Iterator> std::operator+(typename reverse_iterator<_Iterator>::difference_type, const reverse_iterator<_Iterator>&)'
  627 |     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:627:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note: candidate: 'template<class _Iterator> constexpr std::move_iterator<_IteratorL> std::operator+(typename move_iterator<_IteratorL>::difference_type, const move_iterator<_IteratorL>&)'
 1798 |     operator+(typename move_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
In file included 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/bits/basic_string.h:3598:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3598 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3598:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3616:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3616 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3616:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const _CharT*' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3635:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3635 |     operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3635:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   deduced conflicting types for parameter '_CharT' ('void' and 'char')
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3652:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3652 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3652:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3670:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, _CharT)'
 3670 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3670:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3682:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3682 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3682:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3689:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3689 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3689:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3696:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3696 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3696:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3719:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3719 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3719:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'const _CharT*' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3726:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3726 |     operator+(_CharT __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3726:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   deduced conflicting types for parameter '_CharT' ('void' and 'char')
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3733:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const _CharT*)'
 3733 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3733:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
/usr/include/c++/14/bits/basic_string.h:3740:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, _CharT)'
 3740 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3740:5: note:   template argument deduction/substitution failed:
a.cc:34:34: note:   mismatched types 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'void'
   34 |     s=reverse(a.begin(),a.end())+s+b;
      |                                  ^
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/complex:340:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator+(const complex<_Tp>&, const complex<_Tp>&)