print_number.cpp: In function ‘int main()’:
print_number.cpp:11:7: error: no match for ‘operator<<’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘void’)
  cout << number_string(6) << "  " << number_string(0) << endl;
  ~~~~~^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:108:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(__ostream_type& (*__pf)(__ostream_type&))
       ^~~~~~~~
/usr/include/c++/8/ostream:108:7: note:   no known conversion for argument 1 from ‘void’ to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&)’ {aka ‘std::basic_ostream<char>& (*)(std::basic_ostream<char>&)’}
/usr/include/c++/8/ostream:117:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]’
       operator<<(__ios_type& (*__pf)(__ios_type&))
       ^~~~~~~~
/usr/include/c++/8/ostream:117:7: note:   no known conversion for argument 1 from ‘void’ to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&)’ {aka ‘std::basic_ios<char>& (*)(std::basic_ios<char>&)’}
/usr/include/c++/8/ostream:127:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(ios_base& (*__pf) (ios_base&))
       ^~~~~~~~
/usr/include/c++/8/ostream:127:7: note:   no known conversion for argument 1 from ‘void’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/8/ostream:166:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(long __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:166:7: note:   no known conversion for argument 1 from ‘void’ to ‘long int’
/usr/include/c++/8/ostream:170:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(unsigned long __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:170:7: note:   no known conversion for argument 1 from ‘void’ to ‘long unsigned int’
/usr/include/c++/8/ostream:174:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(bool __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:174:7: note:   no known conversion for argument 1 from ‘void’ to ‘bool’
In file included from /usr/include/c++/8/ostream:693,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/bits/ostream.tcc:91:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]’
     basic_ostream<_CharT, _Traits>::
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from ‘void’ to ‘short int’
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:181:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(unsigned short __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:181:7: note:   no known conversion for argument 1 from ‘void’ to ‘short unsigned int’
In file included from /usr/include/c++/8/ostream:693,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/bits/ostream.tcc:105:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]’
     basic_ostream<_CharT, _Traits>::
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from ‘void’ to ‘int’
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:192:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(unsigned int __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:192:7: note:   no known conversion for argument 1 from ‘void’ to ‘unsigned int’
/usr/include/c++/8/ostream:201:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(long long __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:201:7: note:   no known conversion for argument 1 from ‘void’ to ‘long long int’
/usr/include/c++/8/ostream:205:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(unsigned long long __n)
       ^~~~~~~~
/usr/include/c++/8/ostream:205:7: note:   no known conversion for argument 1 from ‘void’ to ‘long long unsigned int’
/usr/include/c++/8/ostream:220:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(double __f)
       ^~~~~~~~
/usr/include/c++/8/ostream:220:7: note:   no known conversion for argument 1 from ‘void’ to ‘double’
/usr/include/c++/8/ostream:224:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(float __f)
       ^~~~~~~~
/usr/include/c++/8/ostream:224:7: note:   no known conversion for argument 1 from ‘void’ to ‘float’
/usr/include/c++/8/ostream:232:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(long double __f)
       ^~~~~~~~
/usr/include/c++/8/ostream:232:7: note:   no known conversion for argument 1 from ‘void’ to ‘long double’
/usr/include/c++/8/ostream:245:7: note: candidate: ‘std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]’
       operator<<(const void* __p)
       ^~~~~~~~
/usr/include/c++/8/ostream:245:7: note:   no known conversion for argument 1 from ‘void’ to ‘const void*’
In file included from /usr/include/c++/8/ostream:693,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/bits/ostream.tcc:119:5: note: candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]’
     basic_ostream<_CharT, _Traits>::
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8/bits/ostream.tcc:119:5: note:   no known conversion for argument 1 from ‘void’ to ‘std::basic_ostream<char>::__streambuf_type*’ {aka ‘std::basic_streambuf<char>*’}
In file included from /usr/include/c++/8/string:52,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6323:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator<<(basic_ostream<_CharT, _Traits>& __os,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6323:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:25: note:   mismatched types ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’ and ‘void’
  cout << number_string(6) << "  " << number_string(0) << endl;
                         ^
In file included from /usr/include/c++/8/bits/ios_base.h:46,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/system_error:217:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)’
     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
     ^~~~~~~~
/usr/include/c++/8/system_error:217:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘const std::error_code&’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:497:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)’
     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
     ^~~~~~~~
/usr/include/c++/8/ostream:497:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:25: note:   deduced conflicting types for parameter ‘_CharT’ (‘char’ and ‘void’)
  cout << number_string(6) << "  " << number_string(0) << endl;
                         ^
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:502:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)’
     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
     ^~~~~~~~
/usr/include/c++/8/ostream:502:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘char’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:508:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)’
     operator<<(basic_ostream<char, _Traits>& __out, char __c)
     ^~~~~~~~
/usr/include/c++/8/ostream:508:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘char’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:514:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)’
     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
     ^~~~~~~~
/usr/include/c++/8/ostream:514:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘signed char’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:519:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)’
     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
     ^~~~~~~~
/usr/include/c++/8/ostream:519:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘unsigned char’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:539:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)’
     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
     ^~~~~~~~
/usr/include/c++/8/ostream:539:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:25: note:   mismatched types ‘const _CharT*’ and ‘void’
  cout << number_string(6) << "  " << number_string(0) << endl;
                         ^
In file included from /usr/include/c++/8/ostream:693,
                 from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/bits/ostream.tcc:321:5: note: candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)’
     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
     ^~~~~~~~
/usr/include/c++/8/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘const char*’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:556:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)’
     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
     ^~~~~~~~
/usr/include/c++/8/ostream:556:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘const char*’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:569:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)’
     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
     ^~~~~~~~
/usr/include/c++/8/ostream:569:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘const signed char*’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:574:5: note: candidate: ‘template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)’
     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
     ^~~~~~~~
/usr/include/c++/8/ostream:574:5: note:   template argument deduction/substitution failed:
print_number.cpp:11:23: note:   cannot convert ‘number_string(6)’ (type ‘void’) to type ‘const unsigned char*’
  cout << number_string(6) << "  " << number_string(0) << endl;
          ~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/8/iostream:39,
                 from print_number.cpp:1:
/usr/include/c++/8/ostream:682:5: note: candidate: ‘template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&)’
     operator<<(_Ostream&& __os, const _Tp& __x)
     ^~~~~~~~
/usr/include/c++/8/ostream:682:5: note:   template argument deduction/substitution failed:
/usr/include/c++/8/ostream: In substitution of ‘template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = void]’:
print_number.cpp:11:25:   required from here
/usr/include/c++/8/ostream:682:5: error: forming reference to void