Infinitely nest maps with variant

Question

So, I am trying to make maps, which are infinitely nestable and I could use strings, ints, bools, etc. in it.

This is what I tried:

struct NMap;
struct NMap : std::map<std::string, std::variant<NMAP*, std::string, std::any>> {};
// ...
NMap* something;
something["lorem"]["ipsum"] = "Test";
                  ^ - No such operator []

Which is logical, std::variant doesn't have [] operator. Is there anyway to use std::variant in Nestable maps?

You have multiple problems, the first which is that `something` is a *pointer*. — Some programmer dude, Oct 19 '18 at 12:37

YSC · Accepted Answer · 2018-10-19T13:50:14.703

Something simple and a bit weird:

#include <map>
#include <string>
#include <optional>

struct rmap : std::map<std::string, rmap>
{
    std::optional<std::string> value; // could be anything (std::variant, std::any, ...)
};

With a bit of sugar and some other tasteful adjustments, you can use it like you intend to:

#include <map>
#include <string>
#include <optional>
#include <iostream>

struct rmap : std::map<std::string, rmap>
{
    using value_type = std::optional<std::string>;
    value_type value;
    operator const value_type&() const { return value; }
    rmap& operator=(value_type&& v) { value = v; return *this; }
    friend std::ostream& operator<<(std::ostream& os, rmap& m) { return os << (m.value ? *m.value : "(nil)"); }
};

int main()
{
    rmap m;
    m["hello"]["world"] = std::nullopt;
    m["vive"]["le"]["cassoulet"] = "Obama";

    std::cout << m["does not exist"]  << '\n';          // nil
    std::cout << m["hello"]["world"]  << '\n';          // nil
    std::cout << m["vive"]["le"]["cassoulet"]  << '\n'; // Obama
}

You can adjust to your taste with some syntactic sugar.

Hmm, it seems I can't do `std::cout << map["lorem"]["ipsum"];`, neither `std::cout << map["lorem"]["ipsum"].value;`. — Mr.DeleteMyMessages, Oct 19 '18 at 12:48
@Mr.DeleteMyMessages that's because you lack a `<<` for `std::optional` — Caleth, Oct 19 '18 at 13:04

score 0 · Answer 2 · answered Oct 19 '18 at 12:37

0

I suppose it would have to be something like:

(*std::get<NMap *>(something["lorem"]))["ipsum"] = "Test";

answered Oct 19 '18 at 12:37

jdehesa

58,456
7
77
121

score 0 · Answer 3 · answered Oct 19 '18 at 13:25

0

std::variant does not support recursive use, see this long answer. Note: it is not a beginner friendly topic.

answered Oct 19 '18 at 13:25

NoSenseEtAl

28,205
28
128
277

This is a very interesting _comment_. – YSC Oct 19 '18 at 13:46

Infinitely nest maps with variant

3 Answers3