Problems with shared_ptr re-entry

[lukacu]

In a project where I am using pybind11 I had a problem with C++ objects wrapped in shared_ptr that were handed over to Python and then re-entered C++. With default approach (using PYBIND11_DECLARE_HOLDER_TYPE) this results in two smart pointers that do not know about each other. Using std::enable_shared_from_this template in my objects I have modified the code generated by PYBIND11_DECLARE_HOLDER_TYPE to something like (basically there is only one small difference at line 7):

template <class T> class type_caster<std::shared_ptr<T>> : public type_caster_holder<T, std::shared_ptr<T>> { 
    typedef type_caster<T> parent;
public:
    bool load(PyObject *src, bool convert) {
        if (!parent::load(src, convert))
            return false;
        holder = ((T *) parent::value)->shared_from_this();
        return true;
    }
    explicit operator T*() { return this->value; }
    explicit operator T&() { return *(this->value); }
    explicit operator std::shared_ptr<T>&() { return holder; }
    explicit operator std::shared_ptr<T>*() { return &holder; }
    using type_caster<T>::cast;
    static PyObject *cast(const std::shared_ptr<T> &src, return_value_policy policy, PyObject *parent) {
        return type_caster<T>::cast(src.get(), policy, parent);
    }
protected:
    std::shared_ptr<T> holder;
};

I hope that this will save some time to anyone who will have the same scenario. If there is a good way to support this by default in some way it would be also very nice. I am not that familiar with C++ templates, but I guess the template could be limited to classes that inherit from std::enable_shared_from_this.

[wjakob]

Hi Luka,

thanks for tracking that down -- the fix ended up being a bit more involved. Please double-check that it works for you and let me know in case it doesn't.

Thanks,
Wenzel

[lukacu]

Wow, that was fast. I will check it out as soon as I can, but just looking at the code it appears that the fix is integrated well. Thanks.