3rdparty/pybind11/docs/advanced/smart_ptrs.rst


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Smart pointers
##############

std::unique_ptr
===============

Given a class ``Example`` with Python bindings, it's possible to return
instances wrapped in C++11 unique pointers, like so

.. code-block:: cpp

    std::unique_ptr<Example> create_example() { return std::unique_ptr<Example>(new Example()); }

.. code-block:: cpp

    m.def("create_example", &create_example);

In other words, there is nothing special that needs to be done. While returning
unique pointers in this way is allowed, it is *illegal* to use them as function
arguments. For instance, the following function signature cannot be processed
by pybind11.

.. code-block:: cpp

    void do_something_with_example(std::unique_ptr<Example> ex) { ... }

The above signature would imply that Python needs to give up ownership of an
object that is passed to this function, which is generally not possible (for
instance, the object might be referenced elsewhere).

std::shared_ptr
===============

The binding generator for classes, :class:`class_`, can be passed a template
type that denotes a special *holder* type that is used to manage references to
the object.  If no such holder type template argument is given, the default for
a type named ``Type`` is ``std::unique_ptr<Type>``, which means that the object
is deallocated when Python's reference count goes to zero.