unordered_map iterator printing using cout

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;