overload ostream::<< to print vector of vectors

Question

Working off Chris's answer in this post How to print out the contents of a vector? I am trying to overload ostream << to print a vector vectors. Here's what I'm reading in from a file, where each line becomes a vector:

1, 2, 3
4, 4, 4, 4, 4

7, 7
41, 52, 632

So the structure looks something like

v[0] = [1, 2, 3]

and so on. I thought Chris's solution should apply perfectly to this situation, and it certainly works for a 1d vector, but I'm trying to understand why it doesn't work for a 2d vector. The code's below:

template <typename T>
std::ostream& operator << (std::ostream& out, const std::vector<T>& v){

  if(! v.empty()){
    std::cout<< '[';
    std::copy(v.begin(), v.end(), std::ostream_iterator<T>(std::cout, " "));
    std::cout<< ']' << std::endl;

  }
  else{
    std::cout << "[ ]" << std::endl;
  }  
}

I'm thinking I need to replace the function in std::ostream_iterator to be << since that is what I am overriding in the first place, but I have tried << and ostream::<< and std:: << and these all yield compile-time errors. What would be the proper way to do this?

If I try to run the code above as is on a vector of vectors of ints, I have the following compile-time error:

In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h: In instantiation of ‘std::ostream_iterator<_Tp, _CharT, _Traits>& std::ostream_iterator<_Tp, _CharT, _Traits>::operator=(const _Tp&) [with _Tp = std::vector<int>; _CharT = char; _Traits = std::char_traits<char>]’:
/usr/include/c++/4.9/bits/stl_algobase.h:336:18:   required from ‘static _OI std::__copy_move<false, false, std::random_access_iterator_tag>::__copy_m(_II, _II, _OI) [with _II = const std::vector<int>*; _OI = std::ostream_iterator<std::vector<int>, char, std::char_traits<char> >]’
/usr/include/c++/4.9/bits/stl_algobase.h:396:70:   required from ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const std::vector<int>*; _OI = std::ostream_iterator<std::vector<int>, char, std::char_traits<char> >]’
/usr/include/c++/4.9/bits/stl_algobase.h:434:38:   required from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = __gnu_cxx::__normal_iterator<const std::vector<int>*, std::vector<std::vector<int> > >; _OI = std::ostream_iterator<std::vector<int>, char, std::char_traits<char> >]’
/usr/include/c++/4.9/bits/stl_algobase.h:466:17:   required from ‘_OI std::copy(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<const std::vector<int>*, std::vector<std::vector<int> > >; _OI = std::ostream_iterator<std::vector<int>, char, std::char_traits<char> >]’
ctest.cpp:15:75:   required from ‘std::ostream& operator<<(std::ostream&, const std::vector<T>&) [with T = std::vector<int>; std::ostream = std::basic_ostream<char>]’
ctest.cpp:70:18:   required from here
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: error: no match for ‘operator<<’ (operand types are ‘std::ostream_iterator<std::vector<int>, char, std::char_traits<char> >::ostream_type {aka std::basic_ostream<char>}’ and ‘const std::vector<int>’)
  *_M_stream << __value;
             ^
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note: candidates are:
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:108:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ 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++/4.9/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/include/c++/4.9/ostream:117:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ 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++/4.9/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/include/c++/4.9/ostream:127:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:166:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘long int’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:170:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘long unsigned int’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:174:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘bool’
In file included from /usr/include/c++/4.9/ostream:612:0,
                 from /usr/include/c++/4.9/iostream:39,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘short int’
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:181:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘short unsigned int’
In file included from /usr/include/c++/4.9/ostream:612:0,
                 from /usr/include/c++/4.9/iostream:39,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘int’
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:192:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘unsigned int’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:201:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘long long int’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:205:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘long long unsigned int’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:220:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘double’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:224:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘float’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:232:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘long double’
/usr/include/c++/4.9/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/include/c++/4.9/ostream:245:7: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘const void*’
In file included from /usr/include/c++/4.9/ostream:612:0,
                 from /usr/include/c++/4.9/iostream:39,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/bits/ostream.tcc:119:5: note:   no known conversion for argument 1 from ‘const std::vector<int>’ to ‘std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
In file included from /usr/include/c++/4.9/string:52:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/bits/basic_string.h:2772:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   ‘const std::vector<int>’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:471:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   deduced conflicting types for parameter ‘_CharT’ (‘char’ and ‘std::vector<int>’)
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:476:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘char’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:482:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘char’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:488:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘signed char’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:493:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘unsigned char’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:513:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   mismatched types ‘const _CharT*’ and ‘std::vector<int>’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/ostream:612:0,
                 from /usr/include/c++/4.9/iostream:39,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘const char*’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:530:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘const char*’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:543:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘const signed char*’
  *_M_stream << __value;
             ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from ctest.cpp:1:
/usr/include/c++/4.9/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/include/c++/4.9/ostream:548:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/4.9/iterator:66:0,
                 from ctest.cpp:6:
/usr/include/c++/4.9/bits/stream_iterator.h:198:13: note:   cannot convert ‘__value’ (type ‘const std::vector<int>’) to type ‘const unsigned char*’
  *_M_stream << __value;
             ^

And this is the full file:

   template <typename T>
    std::ostream& operator << (std::ostream& out, const std::vector<T>& v){

      if(! v.empty()){
        std::cout<< '[';
        std::copy(v.begin(), v.end(), std::ostream_iterator<T>(std::cout, " "));
        std::cout<< ']' << std::endl;

      }
      else{
        std::cout << "[ ]" << std::endl;
      } 
    }

    int main()
    {

      std::ifstream infile("filetest.txt");

      std::string output;
      std::string line;

      std::vector<std::vector<int> > vec;

      while(std::getline(infile, line))
        {
          std::vector<int> subvec;
          int i;

          std::istringstream iss(line);

          while(iss >> i)
        {
          subvec.push_back(i);

          if(iss.peek()==','|| iss.peek()== ' ')
            iss.ignore();     
        }

          std::cout << subvec;
          vec.push_back(subvec);
        }

        std::cout << vec;
    }

Answer 1

It depends heavily upon how you want your output formatted. You can just use the operator as-is, and it'll work (though you may or may not like the formatting):

#include <vector>
using std::vector;
#include <iostream>
using std::ostream;

template<typename T>
ostream& operator<< (ostream& out, const vector<T>& v) {
    out << "[";
    size_t last = v.size() - 1;
    for (size_t i = 0; i < v.size(); ++i) {
        out << v[i];
        if (i != last)
            out << ", ";
    }
    out << "]";
    return out;
}

int main() {
    std::vector<std::vector<int> > x { { 1, 2, 3 }, { 4, 5, 6 } };

    std::cout << x;
}

Produces:

[[1, 2, 3], [4, 5, 6]]

Things only get more difficult if you need/want to change the formatting between one instance and another, such as having commas between the numbers, and new-lines between the rows. This is somewhat non-trivial to solve well, but I'll leave it at this for now, since it's not at all clear you really even want that.

Answer 2

This idea uses std::enable_if to iterate through members of what we choose to define as is_container. I wrote the code more out of curiosity than out of practicality though.

#include <iostream>
#include <type_traits>
#include <vector>

// by default you declare that no type is a container
template<typename T>
struct is_container {
  static constexpr bool value = false;
};

// then you specify which types are actually containers.. I'm just declaring
// std::vector<T, A>, but you could make a similar declaration for list, array,
// and so on.
template<typename T, typename Allocator>
struct is_container<std::vector<T, Allocator> > {
  static constexpr bool value = true;
};

// then you enable a print function to print all elements in a container (you
// can modify the actual formatting to your liking)
template<typename T>
typename std::enable_if<
  is_container<T>::value,
  std::ostream& >::type
inline operator<<(std::ostream& os, const T& container) {
  const auto N = container.size();
  std::size_t current = 0;
  os << "[";
  for(const auto & item : container) {
    const char* separator = current++ == N - 1 ? "" : ", ";
    os << item << separator;
  }
  os << "]";
  return os;
}

// then the code will work for any level of nesting
int main() {
  int scalar = 1;
  std::cout << "Scalar: " << scalar << std::endl;

  std::vector<int> vector {1, 2, 3, 4};
  std::cout << "Vector: " << vector << std::endl;

  std::vector< std::vector<int> > vvector {
    {1, 2, 3, 4},
    {5, 6, 7},
    {8, 9, 10, 11, 12}
  };

  std::cout << "Vector<Vector>: " << vvector << std::endl;

  std::vector< std::vector< std::vector<int> > > vvvector {
    {
      {1, 2, 3, 4},
      {5, 6, 7},
      {8, 9, 10, 11, 12}
    },
    {
      {-1, -2, -3, -4},
      {-5, -6, -7},
      {-8, -9, -10}
    }
  };

  std::cout << "Vector<Vector<Vector>>: " << vvvector << std::endl;

  // you get the idea...

  return 0;
}

Sample run:

$ g++ example.cpp -std=c++14 -Wall -Wextra -O3
$ ./a.out
Scalar: 1
Vector: [1, 2, 3, 4]
Vector<Vector>: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10, 11, 12]]
Vector<Vector<Vector>>: [[[1, 2, 3, 4], [5, 6, 7], [8, 9, 10, 11, 12]], [[-1, -2, -3, -4], [-5, -6, -7], [-8, -9, -10]]]