Language: C++

#include<iostream>
#include<stack>
#include<cctype>
#include<cstdlib>
using namespace std;

int main(void){
	stack<int> st;
	string ch;
	while(cin >> ch){
		if(isdigit(ch[0])){
			st.push((int)ch[0]);
		}
		else{
			int a = st.top();
			st.pop();
			int b = st.top();
			st.pop();
			if(ch == '+') st.push(b + a);
			if(ch == '-') st.push(b - a);
			if(ch == '*') st.push(b * a);
		}
	}
	cout << st.top() << endl;
	return 0;
} 


Compiler output:
a.cc: In function 'int main()':
a.cc:19:31: error: no match for 'operator==' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'char')
   19 |                         if(ch == '+') st.push(b + a);
      |                            ~~ ^~ ~~~
      |                            |     |
      |                            |     char
      |                            std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/iosfwd:42,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/postypes.h:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::fpos<_StateT>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/string:43,
                 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:
/usr/include/c++/14/bits/allocator.h:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::allocator<_CharT>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:629:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  629 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:629:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/string_view:637:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  637 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:637:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/string_view:644:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  644 |     operator==(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:644:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'char'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/basic_string.h:3755:5: 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>&)'
 3755 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3755:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'char'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/basic_string.h:3772:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3772 |     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3772:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   mismatched types 'const _CharT*' and 'char'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
/usr/include/c++/14/bits/basic_string.h:3819:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3819 |     operator==(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3819:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   mismatched types 'const _CharT*' and 'std::__cxx11::basic_string<char>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/tuple:2558:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator==(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2558 |     operator==(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2558:5: note:   template argument deduction/substitution failed:
a.cc:19:34: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::tuple<_UTypes ...>'
   19 |                         if(ch == '+') st.push(b + a);
      |                                  ^~~
In file included from /usr/include/c++/14/bits/locale_facets.h:48,
                 from /usr/include/c++/14/bits/basic_ios.h:37,
                 from /usr/include/c++/14/ios:46:
/usr/include/c++/14/bits/streambuf_iterator.h:234:5: note: candidate: 'template<class _CharT, class _Traits> bool std::operator==(const istreambuf