basic autovivifying variables in c++, similar to perl/gawk/...

vivi.cpp

#include <string>
#include <unordered_map>
#include <variant>

template <typename keytype = std::string, typename valtype = std::string>
struct autovivi {
    using maptype = std::unordered_map<keytype, autovivi>;
    std::variant<std::monostate, valtype, maptype> valueormap;

    autovivi() = default;
    autovivi& operator=(const valtype& value) {
        valueormap = value;
        return *this;
    }
    autovivi& operator[](const keytype& key) {
        if (! std::holds_alternative<maptype>(valueormap))
            valueormap = maptype{};
        return std::get<maptype>(valueormap)[key];
    }
    operator valtype() const {
        if (std::holds_alternative<std::monostate>(valueormap))
            return valtype{};
        return std::get<valtype>(valueormap);
    }
    friend std::ostream& operator<<(std::ostream& os, const autovivi& av) {
        return os << valtype{av};
    }
};

#include <iostream>
int main() {

#define show(...) std::cout << #__VA_ARGS__ " = " << __VA_ARGS__ << '\n';

    autovivi var;
    var["meow"] = "moo";
    var["foo"]["bar"] = "baz";
    show(var["meow"]);
    show(var["foo"]["bar"]);
    show(var["a"]["b"]["c"]);
    var["a"]["b"]["c"] = "blah";
    show(var["a"]["b"]["c"]);
    show(var["a"]["b"]["c"]["d"]);
    try {
        show(var["a"]["b"]["c"]);
    } catch (const std::bad_variant_access& e) {
        std::cerr << e.what() << '\n';
    }
    var["a"]["b"]["c"] = "blah";
    show(var["a"]["b"]["c"]);

    autovivi<int, int> ints;
    ints[2] = 2;
    ints[1][2][3][4] = 10;
    show(ints[2]);
    show(ints[1][2][3][4]);
    show(ints[99][17]);
}