#include <cassert>
#include <vector>

class Layer {
public:
    explicit Layer(std::vector<int>& events) { events.push_back(level()); }
    virtual ~Layer() = default;
    virtual int level() const noexcept { return 10; }
};

class Ready final : public Layer {
public:
    explicit Ready(std::vector<int>& events) : Layer(events) {
        events.push_back(level());
    }
    int level() const noexcept override { return 20; }
};

int main() {
    std::vector<int> events;
    Ready object(events);
    const Layer& view = object;
    events.push_back(view.level());
    assert((events == std::vector<int>{10, 20, 20}));
}