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Wenzel Jakob 2016-06-30 14:59:23 +02:00
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@ -468,7 +468,7 @@ functions. The default policy is :enum:`return_value_policy::automatic`.
| :enum:`return_value_policy::take_ownership` | Reference an existing object (i.e. do not create a new copy) and take | | :enum:`return_value_policy::take_ownership` | Reference an existing object (i.e. do not create a new copy) and take |
| | ownership. Python will call the destructor and delete operator when the | | | ownership. Python will call the destructor and delete operator when the |
| | object's reference count reaches zero. Undefined behavior ensues when the | | | object's reference count reaches zero. Undefined behavior ensues when the |
| | C++ side does the same. | | | C++ side does the same. |
+--------------------------------------------------+----------------------------------------------------------------------------+ +--------------------------------------------------+----------------------------------------------------------------------------+
| :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python. | | :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python. |
| | This policy is comparably safe because the lifetimes of the two instances | | | This policy is comparably safe because the lifetimes of the two instances |
@ -526,14 +526,13 @@ The following example snippet shows a use case of the
non-determinism and segmentation faults, hence it is worth spending the non-determinism and segmentation faults, hence it is worth spending the
time to understand all the different options in the table above. time to understand all the different options in the table above.
.. warning:: It is worth highlighting one common issue where a method (e.g. a getter)
returns a reference (or pointer) to the first attribute of a class. In this
pybind11 tries to eliminate duplicate addresses by returning the same reference object. case, the class and attribute will be located at the same address in
If two addresses are the same, though they do not point to the same object semantically, memory, which pybind11 will recongnize and return the parent instance
this may cause unexpected behaviour. An explicit policy should be used instead of instead of creating a new Python object that represents the attribute.
relying on `automatic`. Here, the :enum:`return_value_policy::reference_internal` policy should be
A common example is a reference to the first member of a class which has the same memory used rather than relying on the automatic one.
location as its owning class.
.. note:: .. note::