#include <array>
#include <cassert>
#include <cstddef>
#include <memory>
#include <type_traits>

struct Log {
    std::array<int, 2> entries{};
    std::size_t used = 0;
    void add(int n) noexcept { entries[used++] = n; }
};
struct Owned {
    Log& log;
    explicit Owned(Log& l) : log(l) {}
    virtual ~Owned() { log.add(2); }
};
struct Impl final : Owned {
    explicit Impl(Log& l) : Owned(l) {}
    ~Impl() override { log.add(1); }
};

class Borrowed {
public:
    virtual int value() const = 0;
protected:
    ~Borrowed() = default;
};
struct Concrete final : Borrowed {
    int value() const override { return 7; }
};

int main() {
    Log log;
    { std::unique_ptr<Owned> object = std::make_unique<Impl>(log); }
    const std::array<int, 2> expected{{1, 2}};
    assert(log.entries == expected && log.used == 2);
    static_assert(!std::is_destructible<Borrowed>::value, "external deletion forbidden");
    Concrete concrete;
    const Borrowed& view = concrete;
    assert(view.value() == 7);
}