Merge two boost intrusive sets in C++?

Question

I have two boost intrusive sets which I need to merge it together. I have map_old.m_old_attributes boost intrusive set which I need to merge it to m_map_new_attributes boost intrusive set

void DataTest::merge_set(DataMap& map_old)
{

    // merge map_old.m_old_attributes set into m_map_new_attributes
}

What is the best way to do this? I am not able to find a function which can do the merge for me? I recently started working with boost intrusive sets and I am not able to find predefined methods which can do the merge, or may be I am wrong?

Answer 1

Indeed intrusive sets are a different beast. They don't manage their element allocations.

So when merging you need to decide what that means. I'd say a reasonable interpretation would be that you want to move the elements contained inside the map_old container into the DataMap.

This would leave map_old empty. Here's such an algorithm:

template <typename Set>
void merge_into(Set& s, Set& into) {
    std::vector<std::reference_wrapper<Element> > tmp(s.begin(), s.end());
    s.clear(); // important! unlinks the existing hooks
    into.insert(tmp.begin(), tmp.end());
}

Update Alternately you can do it with O(1) memory complexity by using the slightly trickier iterater-erase-loop (beware of iterating mutating containers): Live On Coliru as well

    for (auto it = s.begin(); it != s.end();) {
        auto& e = *it;
        it = s.erase(it);
        into.insert(e);
    }

See it Live On Coliru

#include <boost/intrusive/set.hpp>
#include <boost/intrusive/set_hook.hpp>
#include <string>
#include <vector>
#include <functional>
#include <iostream>

namespace bive = boost::intrusive;

struct Element : bive::set_base_hook<> {
    std::string data;

    Element(std::string const& data = "") : data(data) {}

    bool operator< (Element const& rhs) const  { return data < rhs.data; }
    bool operator==(Element const& rhs) const  { return data ==rhs.data; }
};

using Set = bive::set<Element>;

template <typename Set>
void merge_into(Set& s, Set& into) {
    std::vector<std::reference_wrapper<Element> > tmp(s.begin(), s.end());
    s.clear(); // important! unlinks the existing hooks
    into.insert(tmp.begin(), tmp.end());
}

int main() {
    std::vector<Element> va {{"one"},{"two"},{"three"},{"four"},{"five"},{"six"},{"seven"},{"eight"},{"nine"} };
    Set a;
    for(auto& v : va) a.insert(v);

    std::vector<Element> vb {{"two"},{"four"},{"six"},{"eight"},{"ten"} };
    Set b;
    for(auto& v : vb) b.insert(v);

    assert(9==a.size());
    assert(5==b.size());

    merge_into(a, b);

    assert(a.empty());
    assert(10==b.size());
}

Of course you can come up with different semantics for the merge operation (which would be more akin to 'copy' instead of 'move')