unordered_map iterator printing using cout - c++

Question

I'm trying to use an iterator to output an integer in a multilayered unordered_map and I'm having trouble with it, the error is below the code.
#include <boost/unordered_map.hpp>
#include <iostream>
using namespace std;
int main()
{
typedef boost::unordered_map <int, boost::unordered_map<int, int> > _map;
_map MAP;
boost::unordered_map<int, int>::iterator map_it;
MAP[0][0] = 10;
map_it = MAP[0].begin();
cout<<*map_it<<endl;
return 0;
}
error below:
unordered_map_iterator\unordered_map_iterator.cpp||In function 'int main()':|
unordered_map_iterator\unordered_map_iterator.cpp|16|error: no match for 'operator<<' in 'std::cout << map_it.boost::unordered_detail::hash_iterator<A, G>::operator* [with A = std::allocator<std::pair<const int, int> >, G = boost::unordered_detail::ungrouped]()'|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|108|note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>& (*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|117|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>& (*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|127|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|165|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|169|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|173|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|91|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|180|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|105|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|191|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|200|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|204|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|209|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|213|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|221|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|225|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>]|
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|119|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>]|
||=== Build finished: 1 errors, 0 warnings ===|

Answer 1

To be able to do this:
cout<<*map_it<<endl;
You will have to overload a << operator to support your class _map.
<< is designed to work with standard data types and not custom data type like your class.
<< does not understand the custom class type _map and hence the error.
So to do it with the statement you mention, you will have to overload << to accept _map as an parameter.
Alternatively, You can call << with arguments it can understand and the ones it is implemented for,
cout<<map_it->second<<endl;
This ensures that the argument being passed to << is a data type it understands and hence it will work.

Answer 2

boost::unordered_map's iterator is a std::pair. Since you have two levels you have to get the proper iterator to each level like this:
typedef boost::unordered_map <int, boost::unordered_map<int, int> > _map;
_map MAP;
MAP[0][0] = 10;
_map::iterator map_it = MAP.begin();
// map_it is an iterator, where first is the key (0), and
// second is the value (boost::unordered_map<int, int>)
boost::unordered_map<int, int>::iterator internal_map_it = map_it->second.begin();
// internal_map_it is an iterator, where first is the key (also 0)
// and second is the value (10)
cout<< internal_map_it->second <<endl;

Answer 3

boost::unordered_map::iterator points to a pair of values, the first of which is the key, and the second the value. So, this line
cout << *map_it << endl;
is trying to print an std::pair.
If you want to print the contents of MAP[0][0] use
cout << map_it->second << endl;