C++ map — Self referencing iterator

Question

Is there a way to declare a std::map whose value type is an iterator to itself?

map::iterator> myMap;


        

Answer 1

Such definition is not possible, since the value type and the iterator type would be mutually infinitely recursive.

It is possible to work around this using a bit of indirection. It is even possible to avoid the dynamic allocation of std::any, and the fact that std::map<K,V> is undefined unless V is complete.

But the solution is a bit tricky, and relies on some assumptions which are reasonable, but not specified by the standard. See comments in the implementation. The main trick is to defer definition of a member variable type until after definition of the enveloping class. This is achieved by reusing raw storage.

Usage first:

int main()
{
    Map map;
    auto [it, _] = map.emplace("first", iter_wrap{});
    map.emplace("maps to first", conv::wrap(it));
    // erase first mapping by only looking
    // up the element that maps to it
    map.erase(conv::it(map.find("maps to first")));
}

Definition

struct NoInitTag {} noInitTag;

class iter_wrap
{
public:
    iter_wrap();
    ~iter_wrap();
    iter_wrap(const iter_wrap&);
    iter_wrap(iter_wrap&&);
    const iter_wrap& operator=(const iter_wrap&);
    const iter_wrap& operator=(iter_wrap&&);

private:
    // We rely on assumption that all map iterators have the same size and alignment.
    // Compiler should hopefully warn if our allocation is insufficient.
    using dummy_it = std::map<int, int>::iterator;
    static constexpr auto it_size = sizeof(dummy_it);
    static constexpr auto it_align = alignof(dummy_it);
    alignas(it_align) std::byte store[it_size];

    explicit iter_wrap(NoInitTag){}
    friend struct conv;
};

using Map = std::map<std::string, iter_wrap>;
using It = Map::iterator;

struct conv {
    static constexpr It&
    it(iter_wrap&& wrap) noexcept {
        return *std::launder(reinterpret_cast<It*>(wrap.store));
    }
    static constexpr const It&
    it(const iter_wrap& wrap) noexcept {
        return *std::launder(reinterpret_cast<const It*>(wrap.store));
    }
    template<class It>
    static const iter_wrap
    wrap(It&& it) {
        iter_wrap iw(noInitTag);
        create(iw, std::forward<It>(it));
        return iw;
    }
    template<class... Args>
    static void
    create(iter_wrap& wrap, Args&&... args) {
        new(wrap.store) It(std::forward<Args>(args)...);
    }
    static constexpr void
    destroy(iter_wrap& wrap) {
        it(wrap).~It();
    }
};

iter_wrap::iter_wrap() {
    conv::create(*this);
}
iter_wrap::iter_wrap(const iter_wrap& other) {
    conv::create(*this, conv::it(other));
}
iter_wrap::iter_wrap(iter_wrap&& other) {
    conv::create(*this, std::move(conv::it(other)));
}
const iter_wrap& iter_wrap::operator=(const iter_wrap& other) {
    conv::destroy(*this);
    conv::create(*this, conv::it(other));
    return *this;
}
const iter_wrap& iter_wrap::operator=(iter_wrap&& other) {
    conv::destroy(*this);
    conv::create(*this, std::move(conv::it(other)));
    return *this;

}
iter_wrap::~iter_wrap() {
    conv::destroy(*this);
}

---

Old answer; This assumed that it was not an important feature to avoid lookups while traversing stored mappings.

It appears that the data structure that you attempt to represent is a set of keys (strings), where each key maps to another key of the set. Easier way to represent that is to separate those two aspects:

using Set = std::set<std::string>;
using Map = std::map<Set::iterator, Set::iterator>;

Note that these two data structures do not automatically stay in sync. An element added to the set doesn't automatically have a mapping to another, and an element erased from the set leaves dangling iterators to the map. As such, it would be wise to write a custom container class that enforces the necessary invariants.

Answer 2

Only through type erasure. For example, you could use std::any

std::map<std::string, std::any> myMap;
auto inserted = myMap.emplace("foo", std::any());

// how it can be populated:
inserted.first->second = inserted.first;
using it_type = decltype(myMap.begin());

// how values can be extracted:
auto it = std::any_cast<it_type>(myMap["foo"]);

EDIT: The following also seems to work (clang-7.0.0 and gcc-8.2), but it is illegal (basically std::map does not specify that incomplete types are allowed):

struct Iter;
using Map = std::map<std::string, Iter>;
struct Iter {
    Map::iterator it;
};