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+****************
+Object ownership
+****************
+
+One of the main things a binding developer should have in mind is
+how the C++ instances lives will cope with Python's reference count.
+The last thing you want is to crash a program due to a segfault
+when your C++ instance was deleted and the
+wrapper object tries to access the invalid memory there.
+
+In this section we'll show how |project| deals with object ownership
+and parentship, taking advantage of the information provided by the
+APIExtractor.
+
+Ownership basics
+================
+
+As any python binding, |project|-based bindings uses reference counting
+to handle the life of the wrapper object (the Python object that contains the
+C++ object, do not confuse with the *wrapped* C++ object).
+When a reference count reaches zero, the wrapper is deleted by Python garbage
+collector and tries to delete the wrapped instance, but sometimes the wrapped
+C++ object is already deleted, or maybe the C++ object should not be freed after
+the Python wrapper go out of scope and die, because C++ is already taking care of
+the wrapped instance.
+
+In order to handle this, you should tell the
+generator whether the instance's ownership belongs to the binding or
+to the C++ Library. When belonging to the binding, we are sure that the C++ object
+won't be deleted by C++ code and we can call the C++ destructor when the refcount
+reaches 0. Otherwise, instances owned by C++ code can be destroyed arbitrarily,
+without notifying the Python wrapper of its destruction.
+
+Invalidating objects
+====================
+
+To prevent segfaults and double frees, the wrapper objects are invalidated.
+An invalidated can't be passed as argument or have an attributte or method accessed.
+Trying to do this will raise RuntimeError.
+
+The following situations can invalidate an object:
+
+C++ taking ownership
+--------------------
+
+ When an object is passed to a function or method that takes ownership of it, the wrapper
+ is invalidated as we can't be sure of when the object is destroyed, unless it has a
+ :ref:`virtual destructor <ownership-virt-method>` or the transfer is due to the special case
+ of :ref:`parent ownership <ownership-parent>`.
+
+ Besides being passed as argument, the callee object can have its ownership changed, like
+ the `setParent` method in Qt's `QObject`.
+
+Invalidate after use
+--------------------
+
+ Objects marked with *invalidate-after-use* in the type system description always are
+ virtual method arguments provided by a C++ originated call. They should be
+ invalidated right after the Python function returns.
+
+.. _ownership-virt-method:
+
+Objects with virtual methods
+----------------------------
+
+ A little bit of implementation details:
+ virtual methods are supported by creating a C++ class, the **shell**, that inherits
+ from the class with virtual methods, the native one, and override those methods to check if
+ any derived class in Python also override it.
+
+ If the class has a virtual destructor (and C++ classes with virtual methods should have), this
+ C++ instance invalidates the wrapper only when the overriden destructor is called.
+
+ One exception to this rule is when the object is created in C++, like in a
+ factory method. This way the wrapped object is a C++ instance of the native
+ class, not the shell one, and we cannot know when it is destroyed.
+
+.. _ownership-parent:
+
+Parent-child relationship
+=========================
+
+One special type of ownership is the parent-child relationship.
+Being a child of an object means that when the object's parent dies,
+the C++ instance also dies, so the Python references will be invalidated.
+Qt's QObject system, for example, implements this behavior, but this is valid
+for any C++ library with similar behavior.
+
+.. _ownership-parent-heuristics:
+
+Parentship heuristics
+---------------------
+
+ As the parent-child relationship is very common, |project| tries to automatically
+ infer what methods falls into the parent-child scheme, adding the extra
+ directives related to ownership.
+
+ This heuristic will be triggered when generating code for a method and:
+
+ * The function is a constructor.
+ * The argument name is `parent`.
+ * The argument type is a pointer to an object.
+
+ When triggered, the heuristic will set the argument named "parent"
+ as the parent of the object being created by the constructor.
+
+ The main focus of this process was to remove a lot of hand written code from
+ type system when binding Qt libraries. For Qt, this heuristic works in all cases,
+ but be aware that it might not when binding your own libraries.
+
+ To activate this heuristic, use the :ref:`--enable-parent-ctor-heuristic <option-heuristic>`
+ command line switch.
+
+Common pitfalls
+===============
+
+Not saving unowned objects references
+-------------------------------------
+
+ Sometimes when you pass an instance as argument to a method and the receiving
+ instance will need that object to live indifinitely, but will not take ownership
+ of the argument instance. In this case, you should hold a reference to the argument
+ instance.
+
+ For example, let's say that you have a renderer class that will use a source class
+ in a setSource method but will not take ownership of it. The following code is wrong,
+ because when `render` is called the `Source` object created during the call to `setSource`
+ is already destroyed.
+
+ .. code-block:: python
+
+ renderer.setModel(Source())
+ renderer.render()
+
+ To solve this, you should hold a reference to the source object, like in
+
+ .. code-block:: python
+
+ source = Source()
+ renderer.setSource(source)
+ renderer.render()
+
+