From 170d47f92d03b81e74e8623cf15db9282957452d Mon Sep 17 00:00:00 2001 From: Oswald Buddenhagen Date: Mon, 22 May 2017 17:50:30 +0200 Subject: move everying into sources/shiboken2 in preparation for a subtree merge. this should not be necessary to do in a separate commit, but git is a tad stupid about following history correctly without it. --- sources/shiboken2/doc/ownership.rst | 153 ++++++++++++++++++++++++++++++++++++ 1 file changed, 153 insertions(+) create mode 100644 sources/shiboken2/doc/ownership.rst (limited to 'sources/shiboken2/doc/ownership.rst') diff --git a/sources/shiboken2/doc/ownership.rst b/sources/shiboken2/doc/ownership.rst new file mode 100644 index 000000000..69791f855 --- /dev/null +++ b/sources/shiboken2/doc/ownership.rst @@ -0,0 +1,153 @@ +**************** +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 ` or the transfer is due to the special case + of :ref:`parent ownership `. + + 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 ` + command line switch. + +.. _return-value-heuristics: + +Return value heuristics +----------------------- + + When enabled, object returned as pointer in C++ will become child of the object on which the method + was called. + + To activate this heuristic, use the :ref:`--enable-return-value-heuristic ` + +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() + + -- cgit v1.2.3