Index retrieval as a function of the boost::multi_index::sequenced<> offset

I am in a situation where I am forced to use a std::vector container as my underlying data structure. I'm trying to exploit boost::multi_index::sequenced<> to shadow the vector offset, and provide a mechanism for richer data queries. I really don't want to unnecessarily copy all the data from one container to another.

In the example code snippet below I have a class BarInterface that manages the insertion and removal of elements to the Bar::foos container. Initially I tried to store references to the vector elements as elements of the typedef boost::multi_index_container, but these are not stable against insertions.

What I'd like to do is have a custom key extractor that is a function of the _bar container and the boost::multi_index::sequenced<> index. So, for example to get the name of an element, I'd find the sequence offset x, and then _bar.foos[x].name. So, I'm really just using boost::multi_index as a proxy for richer queries against a vector of variable size.

struct Foo {
    std::string name;
};

struct Bar {
    std::vector<Foo> foos;
};

class BarInterface {
public:
    struct Dummy {};
    BarInterface(Bar& bar) : _bar(bar) {
        for (auto& foo : bar.foos) {
            _idx.get<1>().insert(Dummy{});
        }
    }

    // Index
    struct Name {};
    typedef boost::multi_index_container<
        Dummy, // We're not actually storing anything here
        boost::multi_index::indexed_by<
            boost::multi_index::sequenced<>,   // Kept inline with vector order!
            boost::multi_index::ordered_unique<
                boost::multi_index::tag<Name>,
                // I need something here that takes the boost::multi_index::sequenced<>
                // key to get the offset x, and then returns this->_bar.foos[x].name!
            >
        >
    > MultiIndex;

    // Insert
    void insert(const Foo& foo) {
        _bar.foos.push_back(foo);
        _idx.get<1>().insert(Dummy{});
    }

    // Remove
    template <typename T>
    void remove(T it) {
        _bar.foos.erase(_bar.foos.begin() + std::distance(_idx.begin(), _idx.project<0>(it)));
        _idx.erase(_idx.project<0>(it));
    }

protected:
    Bar& _bar;
    MultiIndex _idx;
};

I know that boost::multi_index supports all sorts of key extractors -- for member variables, member functions, global function, etc. However, I can't seem to find an example showing how to generate a key as a function of a boost::multi_index::sequenced<> index. Is this possible, or is there an elegant alternative?

This is extremely brittle and I wouldn't recommend going to production with such code, but since you asked for it:

Live Coliru Demo

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/random_access_index.hpp>
#include <iostream>
#include <string>
#include <vector>

struct Foo {
    std::string name;
};

struct Bar {
    std::vector<Foo> foos;
};

class BarInterface;

struct Dummy
{
    Dummy(const Foo& x): p(&x){}
    Dummy(const Dummy&): p(nullptr){}
    
    const Foo* p;
};

struct NameExtractor
{
    NameExtractor(BarInterface* p): p(p){}
    
    using result_type=std::string;
    
    const result_type& operator()(const Dummy& d)const;

    BarInterface* p;
};

class BarInterface {
public:
    BarInterface(Bar& bar) :
        _idx(boost::make_tuple(
            boost::tuple<>(),
            boost::make_tuple(NameExtractor(this), std::less<std::string>())
        )),
        _bar(bar)
    {
        for (auto& foo : bar.foos) {
            _idx.get<1>().insert(bar.foos.back());
        }
    }

    // Index
    struct Name {};
    typedef boost::multi_index_container<
        Dummy, // We're not actually storing anything here
        boost::multi_index::indexed_by<
            boost::multi_index::random_access<>,   // Kept inline with vector order!
            boost::multi_index::ordered_unique<
                boost::multi_index::tag<Name>,
                NameExtractor
            >
        >
    > MultiIndex;

    // Insert
    void insert(const Foo& foo) {
        _bar.foos.push_back(foo);
        _idx.get<1>().insert(_bar.foos.back());
    }

    // Remove
    template <typename T>
    void remove(T it) {
        _bar.foos.erase(_bar.foos.begin() + std::distance(_idx.begin(), _idx.project<0>(it)));
        _idx.erase(_idx.project<0>(it));
    }
    
    void remove(const char* name) {
        remove(_idx.get<1>().find(name));
    }
    
    void print()const
    {
        auto key=_idx.get<1>().key_extractor();
      
        for(const auto& d: _idx.get<1>()){
            std::cout<<key(d)<<" ";
        }
        std::cout<<"\n";
    }

protected:
    friend NameExtractor;
  
    Bar& _bar;
    MultiIndex _idx;
};

const NameExtractor::result_type& NameExtractor::operator()(const Dummy& d)const
{
    if(d.p){
        return d.p->name;
    }
    else{
        std::size_t offset=p->_idx.iterator_to(d)-p->_idx.begin();
        return p->_bar.foos[offset].name;
    }
}

int main()
{
    Bar bar;
    BarInterface bi(bar);
    
    bi.insert(Foo{"hello"});
    bi.insert(Foo{"bye"});
    bi.insert(Foo{"Boost"});
    
    bi.print();
    

    bi.remove("bye");

    bi.print();

    bi.insert(Foo{"MultiIndex"});
    
    bi.print();
}

Output

Boost bye hello 
Boost hello 
Boost MultiIndex hello