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

Question

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?

Answer 1

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