Struggling to create a Boost-Bimap containing std::bitset - c++

Question

I have a number of strings and their bitset equivalents. I need to be able to look up equivalents in both directions, i.e. "str to bitset" and "bitset to str". I believe boost-bimap would be the right container for this job.
I managed to get this to work with strings and integers but my string / bitset bimap does not compile. I am using VS2019 with the latest boost release.
Integer example works:
#include <boost/bimap.hpp>
#include <string>
#include <iostream>
int main()
{
typedef boost::bimap<std::string, int> bimap_str_int_t;
bimap_str_int_t bimap1;
bimap1.insert(bimap_str_int_t::value_type("A", 1));
std::cout << bimap1.left.at("A") << '\n'; //prints 1
std::cout << bimap1.right.at(1) << '\n'; // prints A
}
Bitset example fails to compile:
#include <boost/bimap.hpp>
#include <string>
#include <iostream>
#include <bitset>
int main()
{
typedef std::bitset<3> bitset_t;
typedef boost::bimap<std::string, bitset_t> bimap_str_bitset_t;
bimap_str_bitset_t bimap2;
bitset_t bits{ "010" };
bimap2.insert(bimap_str_bitset_t::value_type("A", bits));
std::cout << bimap2.left.at("A") << '\n';
std::cout << bimap2.right.at(bits) << '\n';
}
The bitset example creates the following compiler error:
boost_test.cpp(20): message : see reference to class template instantiation 'boost::bimaps::bimap' being compiled
I am not sure how to fix this and would greatly appreciate any hints.

Answer 1

The issue is that std::bitset has no operator< - one of the requirements of any STL-like ordered collection.
To fix this, you need to supply a comparison function - here's one way you might try:
#include <boost/bimap.hpp>
#include <string>
#include <iostream>
#include <bitset>
typedef std::bitset<3> bitset_t;
struct compare_bitset {
bool operator()(const bitset_t& x, const bitset_t& y) const {
return x.to_ulong() < y.to_ulong();
}
};
int main()
{
using bitset_set = boost::bimaps::set_of<bitset_t, compare_bitset>;
typedef boost::bimap < std::string, bitset_set> bimap_str_bitset_t;
bimap_str_bitset_t bimap2;
bitset_t bits{ "010" };
bimap2.insert(bimap_str_bitset_t::value_type("A", bits));
std::cout << bimap2.left.at("A") << '\n';
std::cout << bimap2.right.at(bits) << '\n';
}