Problems using a map with a bitset as a key

Question

I am trying to create a map in C++ with bitset as a key. However the compiler generates the following error messages

In file included from /usr/include/c++/4.6/string:50:0,
                 from /usr/include/c++/4.6/bits/locale_classes.h:42,
                 from /usr/include/c++/4.6/bits/ios_base.h:43,
                 from /usr/include/c++/4.6/ios:43,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from test2.cpp:1:
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = std::bitset<8u>]’:
/usr/include/c++/4.6/bits/stl_map.h:452:2:   instantiated from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::bitset<8u>, _Tp = int, _Compare = std::less<std::bitset<8u> >, _Alloc = std::allocator<std::pair<const std::bitset<8u>, int> >, std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int, std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::bitset<8u>]’
test2.cpp:22:30:   instantiated from here
/usr/include/c++/4.6/bits/stl_function.h:236:22: error: no match for ‘operator<’ in ‘__x < __y’
/usr/include/c++/4.6/bits/stl_function.h:236:22: note: candidates are:
/usr/include/c++/4.6/bits/stl_pair.h:207:5: note: template<class _T1, class _T2> bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
/usr/include/c++/4.6/bits/stl_iterator.h:291:5: note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.6/bits/stl_iterator.h:341:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.6/bits/basic_string.h:2510:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/basic_string.h:2522:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
/usr/include/c++/4.6/bits/basic_string.h:2534:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/stl_tree.h:856:5: note: template<class _Key, class _Val, class _KeyOfValue, class _Compare, class _Alloc> bool std::operator<(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&, const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_set.h:713:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::set<_Key, _Compare, _Alloc>&, const std::set<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_multiset.h:696:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::multiset<_Key, _Compare, _Alloc>&, const std::multiset<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_map.h:894:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::map<_Key, _Tp, _Compare, _Alloc>&, const std::map<_Key, _Tp, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_multimap.h:812:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::multimap<_Key, _Tp, _Compare, _Alloc>&, const std::multimap<_Key, _Tp, _Compare, _Alloc>&)

The progam code is given below I am trying to use bitset as a key to a map in C++. However, everytime I run the code below I run into errros.

#include <iostream>
#include <algorithm>
#include <string>
#include <bitset>
#include <set>
#include <utility>

using namespace std;

int main(int argc, char *argv[])
{
    bitset<8> test;
    test = 9;
    cout<<"Set to 9"<<endl;
    map <bitset<8> , int> mymap;
    pair <biset<8> , int> p;
    p.first = test;
    p.second = 9;
    string teststring;
    teststring = test.to_string<char,char_traits<char>,allocator<char> >();
    cout<<teststring<<temymap[test]<<endl;
    return 0;
}

Answer 1

Just use your own comparator class:

struct Comparer {
    bool operator() (const bitset<8> &b1, const bitset<8> &b2) const {
        return b1.to_ulong() < b2.to_ulong();
    }
};
/* ... */
map <bitset<8> , int, Comparer> mymap;

Note that you can extend this solution to support arbitrary length bitsets, as long as they are small enough to be converted to an unsigned long:

template<size_t sz> struct bitset_comparer {
    bool operator() (const bitset<sz> &b1, const bitset<sz> &b2) const {
        return b1.to_ulong() < b2.to_ulong();
    }
};
map <bitset<8> , int, bitset_comparer<8> > mymap;
map <bitset<16> , int, bitset_comparer<16> > mymap16;