DeveloperNote.com

class SharedGadget
{
    Q_GADGET

    Q_PROPERTY(QObject* p READ pointer);

public:

    SharedGadget() = default;

    SharedGadget(std::shared_ptr<QObject> p) : m_p(std::move(p)) {}

    template <class T> requires std::is_base_of_v<QObject, T>
    SharedGadget(std::shared_ptr<T> p_t) : SharedGadget(std::static_pointer_cast<QObject>(p_t)) {}

private:

    QObject* pointer() const
    {
        return m_p.get();
    }

    std::shared_ptr<QObject> m_p;
};

Q_DECLARE_METATYPE(SharedGadget)

class WeakGadget;
        
class SharedGadget
{
    Q_GADGET

    Q_PROPERTY(QObject* p READ pointer CONSTANT);
    Q_PROPERTY(bool empty READ empty CONSTANT);
    Q_PROPERTY(int useCount READ useCount);

public:

    SharedGadget() = default;

    SharedGadget(std::shared_ptr<QObject> p) : m_p(std::move(p)) {}

    template <class T> requires std::is_base_of_v<QObject, T>
    SharedGadget(std::shared_ptr<T> p_t) : SharedGadget(std::static_pointer_cast<QObject>(p_t)) {}

    SharedGadget(const SharedGadget& other) = default;
    SharedGadget(SharedGadget&& other) = default;

    SharedGadget& operator = (const SharedGadget& other) = default;
    SharedGadget& operator = (SharedGadget&& other) = default;

    Q_INVOKABLE SharedGadget clone() const
    {
        return *this;
    }

    Q_INVOKABLE WeakGadget weak() const;

    //For accessing from C++
    const std::shared_ptr<QObject>& sp() const
    {
        return m_p;
    }

    template <class T>
    std::shared_ptr<T> cast() const
    {
        return std::dynamic_pointer_cast<T>(m_p);
    }

private:

    QObject* pointer() const
    {
        return m_p.get();
    }

    bool empty() const
    {
        return m_p == nullptr;
    }

    int useCount() const
    {
        return m_p.use_count();
    }

    std::shared_ptr<QObject> m_p;
};

class WeakGadget
{
    Q_GADGET

    Q_PROPERTY(bool empty READ empty CONSTANT);
    Q_PROPERTY(int useCount READ useCount);
    Q_PROPERTY(bool expired READ expired);

public:

    WeakGadget() = default;

    WeakGadget(std::weak_ptr<QObject> p) : m_p(std::move(p)) {}

    template <class T> requires std::is_base_of_v<QObject, T>
    WeakGadget(std::weak_ptr<T> p_t) : SharedGadget(std::static_pointer_cast<QObject>(p_t)) {}

    WeakGadget(const WeakGadget& other) = default;
    WeakGadget(WeakGadget&& other) = default;

    WeakGadget& operator = (const WeakGadget& other) = default;
    WeakGadget& operator = (WeakGadget&& other) = default;

    Q_INVOKABLE WeakGadget clone() const
    {
        return *this;
    }

    Q_INVOKABLE SharedGadget lock() const
    {
        return m_p.lock();
    }

    Q_INVOKABLE SharedGadget safeLock() const
    {
        std::shared_ptr<QObject> p = m_p.lock();

        if (!p)
        {
            throw std::runtime_error("Non-null pointer expected.");
        }

        return p;
    }

    //For accessing from C++
    const std::weak_ptr<QObject>& sp() const
    {
        return m_p;
    }

    template <class T>
    std::weak_ptr<T> cast() const
    {
        return std::dynamic_pointer_cast<T>(m_p);
    }

private:

    bool empty() const
    {
        using wt = std::weak_ptr<QObject>;
        return !m_p.owner_before(wt{}) && !wt{}.owner_before(m_p);
    }

    int useCount() const
    {
        return m_p.use_count();
    }

    bool expired() const
    {
        return m_p.expired();
    }

    std::weak_ptr<QObject> m_p;
};

inline WeakGadget SharedGadget::weak() const
{
    return WeakGadget(m_p);
}

Q_DECLARE_METATYPE(SharedGadget)
Q_DECLARE_METATYPE(WeakGadget)