Language: C++

    /*  Sourav Verma IPG_2013  ABV_IIITMG 
        Task @ Codeforces & Atcoder  */

#include<bits/stdc++.h>

#define ll                    long long int
#define loop(i,a,b)           for(int i=(int)a;i<(int)b;++i)
#define rloop(i,a,b)          for(int i=(int)a;i<=(int)b;++i)
#define loopr(i,a,b)          for(int i=(int)a;i>=(int)b;--i)
#define count_1(n)            __builtin_popcountll(n)
#define pb                    push_back
#define mset(a,b,c)           loop(i,0,b) a[i]=c
#define F                     first
#define S                     second
#define mp                    make_pair
#define clr(x)                x.clear()
#define MOD                   1000000007
#define MAX                   1e9
#define MIN                   -1e9
#define itoc(c)               ((char)(((int)'0')+c))
#define vi                    vector<int>
#define vvi                   vector<vi>
#define pll                   pair<ll,ll>
#define pii                   pair<int,int>
#define all(p)                p.begin(),p.end()
#define mid(s,e)              (s+(e-s)/2)
#define mini                  INT_MIN
#define sv()                  int t; scanf("%d",&t); while(t--)
using namespace std;
ll extgcd(ll a,ll b,ll& x,ll& y){if(b==0){x=1;y=0;return a;}else{int g=extgcd(b,a%b,y,x);y-=a/b*x;return g;}}
ll lcm(int x, int y) { return x * 1ll * y / (int)__gcd(x, y); }
ll hcf(ll a, ll b) {if(b>a) return (hcf(b, a)); if(a%b==0) return b; return (hcf(b, a%b));} 
ll modpow(ll a,ll b) {ll res=1;a%=MOD;for(;b;b>>=1){if(b&1)res=res*a%MOD;a=a*a%MOD;}return res;}

void solve(std::istream &in, std::ostream &out){
    int a,b; char op; in>>a>>op>>b;
    if(op=='+') out<<a+b<"\n";
    else out<<a-b<<"\n";
}

int main()
{
    ios_base::sync_with_stdio(false); cin.tie(nullptr);
    #ifdef debug
        freopen("in.txt", "r", stdin);
    	freopen("out.txt", "w", stdout);
	#endif
    
    solve(std::cin, std::cout);
    
    #ifdef debug
        fprintf(stdout,"\nTIME: %.3lf sec\n", (double)clock()/(CLOCKS_PER_SEC));
    #endif
    
    return 0;
}

Compiler output:
a.cc: In function 'void solve(std::istream&, std::ostream&)':
a.cc:37:25: error: no match for 'operator<' (operand types are 'std::basic_ostream<char>' and 'const char [2]')
   37 |     if(op=='+') out<<a+b<"\n";
      |                 ~~~~~~~~^~~~~
      |                    |     |
      |                    |     const char [2]
      |                    std::basic_ostream<char>
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:4:
/usr/include/c++/14/bits/regex.h:1143:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1143 |     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1143:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1224:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1224 |     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1224:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1317:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1317 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1317:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1391:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1391 |     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1391:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'const char [2]'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1485:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1485 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1485:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1560:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1560 |     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1560:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'const char [2]'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/regex.h:1660:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1660 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1660:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::pair<_T1, _T2>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/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:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/string_view:688:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  688 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:688:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'const char*'
   37 |     if(op=='+') out<<a+b<"\n";
      |                          ^~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3874 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note:   template argument deduction/substitution failed:
a.cc:37:26: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::