9. 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 Shiboken deals with object ownership and parentship, taking advantage of the information provided by the APIExtractor.

9.1. Ownership basics

As any python binding, Shiboken-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.

9.2. Invalidating objects

To prevent segfaults and double frees, the wrapper objects are invalidated. An invalidated can’t be passed as argument or have an attribute or method accessed. Trying to do this will raise RuntimeError.

The following situations can invalidate an object:

9.2.1. 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 virtual destructor or the transfer is due to the special case of parent ownership.

Besides being passed as argument, the called object can have its ownership changed, like the setParent method in Qt’s QObject.

9.2.2. 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.

9.2.3. 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 overridden 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.

9.3. 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.

9.3.1. Parentship heuristics

As the parent-child relationship is very common, Shiboken 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 –enable-parent-ctor-heuristic command line switch.

9.3.2. 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 –enable-return-value-heuristic

9.4. Common pitfalls

9.4.1. 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 indefinitely, 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.

renderer.setModel(Source())
renderer.render()

To solve this, you should hold a reference to the source object, like in

source = Source()
renderer.setSource(source)
renderer.render()