@@ -81,6 +81,82 @@ C++-11-specific features:
overused.
* Variadic class and function templates
+Object Ownership
+~~~~~~~~~~~~~~~~
+
+libcamera creates and destroys many objects at runtime, for both objects
+internal to the library and objects exposed to the user. To guarantee proper
+operation without use after free, double free or memory leaks, knowing who owns
+each object at any time is crucial. The project has enacted a set of rules to
+make object ownership tracking as explicit and fool-proof as possible.
+
+In the context of this section, the terms object and instance are used
+interchangeably and both refer to an instance of a class. The term reference
+refers to both C++ references and C++ pointers in their capacity to refer to an
+object. Passing a reference means offering a way to a callee to obtain a
+reference to an object that the caller has a valid reference to. Borrowing a
+reference means using a reference passed by a caller without ownership transfer
+based on the assumption that the caller guarantees the validity of the
+reference for the duration of the operation that borrows it.
+
+#. Single Owner Objects
+
+ * By default an object has a single owner at any time.
+ * Storage of single owner objects varies depending on how the object
+ ownership will evolve through the lifetime of the object.
+
+ * Objects whose ownership needs to be transferred shall be stored as
+ std::unique_ptr<> as much as possible to emphasize the single ownership.
+ * Objects whose owner doesn't change may be embedded in other objects, or
+ stored as pointer or references. They may be stored as std::unique_ptr<>
+ for automatic deletion if desired.
+
+ * Ownership is transferred by passing the reference as a std::unique_ptr<>
+ and using std::move(). After ownership transfer the former owner has no
+ valid reference to the object anymore and shall not access it without first
+ obtaining a valid reference.
+ * Objects may be borrowed by passing an object reference from the owner to
+ the borrower, providing that
+
+ * the owner guarantees the validity of the reference for the whole duration
+ of the borrowing, and
+ * the borrower doesn't access the reference after the end of the borrowing.
+
+ When borrowing from caller to callee for the duration of a function call,
+ this implies that the callee shall not keep any stored reference after it
+ returns. These rules apply to the callee and all the functions it calls,
+ directly or indirectly.
+
+ When the object is stored in a std::unique_ptr<>, borrowing passes a
+ reference to the object, not to the std::unique_ptr<>, as
+
+ * a 'const &' when the object doesn't need to be modified and may not be
+ null.
+ * a pointer when the object may be modified or may be null. Unless
+ otherwise specified, pointers passed to functions are considered as
+ borrowed references valid for the duration of the function only.
+
+#. Shared Objects
+
+ * Objects that may have multiple owners at a given time are called shared
+ objects. They are reference-counted and live as long as any references to
+ the object exist.
+ * Shared objects are created with std::make_shared<> or
+ std::allocate_shared<> and stored in an std::shared_ptr<>.
+ * Ownership is shared by creating and passing copies of any valid
+ std::shared_ptr<> reference. Ownership is released by destroying the
+ corresponding std::shared_ptr<>.
+ * Borrowed references to shared objects are passed as references to the
+ object itself, not to the std::shared_ptr<>, with the same rules as for
+ single owner objects.
+
+.. attention:: Long term borrowing of single owner objects is allowed. Example
+ use cases are implementation of the singleton pattern (where the singleton
+ guarantees the validity of the reference forever), or returning references
+ to global objects whose lifetime matches the lifetime of the application. As
+ long term borrowing isn't marked through language constructs, it shall be
+ documented explicitly in details in the API.
+
Tools
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