C++ error

error

$ g++ -o x x.cpp
x.cpp: In function ‘int main()’:
x.cpp:9:13: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘voi ’)
   std::cout << foo() << std::endl;
             ^
x.cpp:9:13: note: candidates are:
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:108:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:117:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:127:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:127:7: note:   no known conversion for argument 1 from ‘void’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:166:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:166:7: note:   no known conversion for argument 1 from ‘void’ to ‘long int’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:170:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:170:7: note:   no known conversion for argument 1 from ‘void’ to ‘long unsigned int’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:174:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:174:7: note:   no known conversion for argument 1 from ‘void’ to ‘bool’
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:612:0,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:91:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from ‘void’ to ‘short int’
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:181:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:181:7: note:   no known conversion for argument 1 from ‘void’ to ‘short unsigned int’
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:612:0,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:105:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from ‘void’ to ‘int’
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:192:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:192:7: note:   no known conversion for argument 1 from ‘void’ to ‘unsigned int’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:201:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:201:7: note:   no known conversion for argument 1 from ‘void’ to ‘long long int’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:205:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:205:7: note:   no known conversion for argument 1 from ‘void’ to ‘long long unsigned int’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:220:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:220:7: note:   no known conversion for argument 1 from ‘void’ to ‘double’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:224:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:224:7: note:   no known conversion for argument 1 from ‘void’ to ‘float’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:232:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:232:7: note:   no known conversion for argument 1 from ‘void’ to ‘long double’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:245:7: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:245:7: note:   no known conversion for argument 1 from ‘void’ to ‘const void*’
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:612:0,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:119:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:548:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:548:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘const unsigned char*’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:543:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:543:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘const signed char*’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:530:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:530:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘const char*’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:612:0,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:321:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘const char*’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:513:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:513:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   mismatched types ‘const _CharT*’ and ‘void’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:493:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:493:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘unsigned char’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:488:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:488:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘signed char’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:482:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:482:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘char’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:476:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:476:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   cannot convert ‘foo()’ (type ‘void’) to type ‘char’
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39:0,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:471:5: note: 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/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:471:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   deduced conflicting types for parameter ‘_CharT’ (‘char’ and ‘void’)
   std::cout << foo() << std::endl;
                    ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/string:52:0,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ios:42,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/iostream:39,
                 from x.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/basic_string.h:2772:5: note: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
     operator<<(basic_ostream<_CharT, _Traits>& __os,
     ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/basic_string.h:2772:5: note:   template argument deduction/substitution failed:
x.cpp:9:20: note:   mismatched types ‘const std::basic_string<_CharT, _Traits, _Alloc>’ and ‘void’
   std::cout << foo() << std::endl;

x.cpp

#include <iostream>

void foo()
{
}

int main()
{
  std::cout << foo() << std::endl;
}