#include <iostream>
#include <thread>
#include <functional>
#include <vector>
#include <string>
#include <algorithm>
#include <cassert>
#include <sstream>

struct Value {
    virtual std::string ToString() const = 0;
    virtual ~Value() {}
};

struct Int : public Value {
    Int(int value) : value(value) {}

    std::string ToString() const override {
        return std::to_string(value);
    }

    int value;
};

struct List : public Value {
    std::string ToString() const override {
        std::stringstream ss;
        ss << "[";
        for (const auto& e : list) {
            ss << e->ToString() << ", ";
        }
        ss << "]";
        return ss.str();
    }

    std::vector<std::unique_ptr<Value>> list;
};

int main() {
    auto l1 = std::make_unique<List>();
    l1->list.emplace_back(std::make_unique<Int>(5));
    auto l2 = std::make_unique<List>();
    l2->list.emplace_back(std::make_unique<Int>(2));
    l2->list.emplace_back(std::move(l1));
    std::cout << l2->ToString() << "\n";
}