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diff --git a/Documentation/coding-style.rst b/Documentation/coding-style.rst index 129086be..30a1455d 100644 --- a/Documentation/coding-style.rst +++ b/Documentation/coding-style.rst @@ -82,6 +82,90 @@ C++-11-specific features: * Variadic class and function templates * rvalue references, move constructor and move assignment +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<>. Ownership is released by destroying the corresponding + std::shared_ptr<>. + * When passed to a function, std::shared_ptr<> are always passed by value, + never by reference. The caller can decide whether to transfer its ownership + of the std::shared_ptr<> with std::move() or retain it. The callee shall + use std::move() if it needs to store the shared pointer. + * Borrowed references to shared objects are passed as references to the + objects themselves, not to the std::shared_ptr<>, with the same rules as + for single owner objects. + +These rules match the `object ownership rules from the Chromium C++ Style Guide`_. + +.. _object ownership rules from the Chromium C++ Style Guide: https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md#object-ownership-and-calling-conventions + +.. 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 ----- |