PySharedPtr.h

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// -*- lsst-c++ -*-
/*
 * LSST Data Management System
 * See COPYRIGHT file at the top of the source tree.
 *
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 *
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#ifndef LSST_CPPUTILS_PYTHON_PYSHAREDPTR_H
#define LSST_CPPUTILS_PYTHON_PYSHAREDPTR_H
 
#include "pybind11/pybind11.h"
 
#include <memory>
 
namespace lsst {
namespace cpputils {
namespace python {
 
/* Workaround for C++ objects outliving Python objects
 * by Xfel and florianwechsung, https://github.com/pybind/pybind11/issues/1389
 */
template <typename T>
class PySharedPtr final {
public:
    using element_type = T;
 
    explicit PySharedPtr(T* const ptr) : _impl() {
        if (ptr != nullptr) {
            // `cast` returns new PyObject only if `*ptr` not yet associated with one
            PyObject* pyObj = pybind11::cast(ptr).ptr();
            Py_INCREF(pyObj);
            // if any operation with shared_ptr fails, pyObj will get decremented correctly
            std::shared_ptr<PyObject> manager(pyObj, [](PyObject* const obj) noexcept {
                if (obj != nullptr) Py_DECREF(obj);
            });
            _impl = std::shared_ptr<T>(manager, ptr);
        }
    }
 
    PySharedPtr(PySharedPtr const&) noexcept = default;
    PySharedPtr(PySharedPtr&&) noexcept = default;
    PySharedPtr& operator=(PySharedPtr const&) noexcept = default;
    PySharedPtr& operator=(PySharedPtr&&) noexcept = default;
    ~PySharedPtr() noexcept = default;
 
    PySharedPtr(std::shared_ptr<T> r) noexcept : _impl(std::move(r)) {}
    operator std::shared_ptr<T>() noexcept { return _impl; }
 
    T* get() const noexcept { return _impl.get(); }
 
private:
    std::shared_ptr<T> _impl;
};
 
}  // namespace python
}  // namespace cpputils
}  // namespace lsst
 
// Macro must be called in the global namespace
PYBIND11_DECLARE_HOLDER_TYPE(T, lsst::cpputils::python::PySharedPtr<T>);
 
#endif