#include <iostream>
#include <boost/intrusive/set.hpp>

namespace bi = boost::intrusive;

struct A
    : public bi::set_base_hook<>
{
    int _val;
    int _extra;

    A(int val = 0, int extra = 0) : _val(val), _extra(extra) {}

    friend std::ostream& operator << (std::ostream& os, const A& rhs)
    {
        os << "(val=" << rhs._val << ", extra=" << rhs._extra << ")";
        return os;
    }

    friend bool operator < (const A& lhs, const A& rhs)
    {
        return lhs._val < rhs._val;
    }
};

template < typename Cont >
std::ostream& print_cont(std::ostream& os, const Cont& l)
{
    bool first = true;
    for (const auto& e : l)
    {
        if (not first)
        {
            os << ", ";
        }
        os << e;
        first = false;
    }
    return os;
}

int main()
{
    std::vector< A > v = { {2, 5}, {2, 7} };
    bi::multiset< A > s;
    s.insert(v[0]);
    s.insert(v[1]);

    std::cout << "s: size=" << s.size() << ", elems={";
    print_cont(std::cout, s);
    std::cout << "}\n";

    s.erase(v[0]);

    std::cout << "s: size=" << s.size() << ", elems={";
    print_cont(std::cout, s);
    std::cout << "}\n";
}