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+.. sectnum::
+
+TeachOS C++ Code Style Guide
+============================
+
+This document codifies the C++ coding idioms used throughout the TeachOS kernel.
+It covers language usage, ownership and lifetime models, algorithm selection, error propagation, class design, and naming conventions.
+It does **not** cover token-level formatting, which is enforced automatically by the `.clang-format` configuration.
+
+Language Standard and Vocabulary
+--------------------------------
+
+TeachOS targets **C++23** without compiler extensions.
+Standard library features may be used freely in tests only.
+All other parts of the codebase rely on a freestanding variant of the standard library as shipped with the toolchain.
+Types that are not part of the toolchain's freestanding standard library are provided by the `kstd` library.
+Below is an overview of `kstd` replacements to be used in the **non-test** kernel code, including the bundled support libraries.
+This list is does not claim completeness.
+
+
+===================  ========================= =======================================
+Concept              Preferred                 Replaces
+===================  ========================= =======================================
+Dynamic array        ``kstd::vector<T>``       ``std::vector<T>``
+Strings              ``kstd::string``          ``std::string``
+Non-owning pointer   ``kstd::observer_ptr<T>`` raw ``T*`` for ownership-neutral access
+Shared ownership     ``kstd::shared_ptr<T>``   ``std::shared_ptr<T>``
+Unique ownership     ``kstd::unique_ptr<T>``   ``std::unique_ptr<T>``
+Printing             ``kstd::println(...)``    ``std::println(...)``, ...
+String Formatting    ``kstd::format(...)``     ``std::format(...)``, ...
+===================  ========================= =======================================
+
+Common standard library parts that are available in the freestanding implementation include but are not limited to:
+
+- ``std::string_view``
+- ``std::span``
+- ``std::array``
+- ``std::optional``
+- ``std::byte``
+- ``std::ranges``
+- ``std::views``
+
+These parts may be used freely throughout the codebase.
+
+Function Declarations — Trailing Return Types
+---------------------------------------------
+
+**All** functions and member functions use trailing return type syntax, including those returning `void`.
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   auto device_registry::get() -> device_registry &;
+   auto bitmap_is_set(std::span<std::byte const> bitmap, std::size_t index) -> bool;
+   auto init() -> void;
+
+   // Wrong
+   device_registry & device_registry::get();
+   bool bitmap_is_set(std::span<std::byte const> bitmap, std::size_t index);
+   void init();
+
+This rule applies to: free functions, member functions, lambdas with explicit return types, and virtual functions.
+The only exception is constructors and destructors, which have no return type at all.
+
+Parameter Passing Conventions
+-----------------------------
+
+The choice of passing convention encodes intent and must be consistent.
+
+View and cheaply copyable types — pass by value
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Types that are designed to be non-owning views or are trivially copyable must be passed and returned **by value**.
+Passing them by ``const &`` is redundant and adds a pointer indirection with no benefit.
+
+This includes but is not limited to:
+
+- ``std::string_view``
+- ``std::span<T>``
+- ``kstd::observer_ptr<T>``
+- ``kstd::bytes`` and similar unit types
+- ``kapi::capabilities::facet_id``
+- ``kapi::memory::page``
+- ``kapi::memory::frame``
+- ``kapi::memory::physical_address``
+- ``kapi::memory::linear_address``
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   auto resolve(kapi::capabilities::facet_id id, std::string_view name) -> std::observer_ptr<void>;
+   auto has(std::span<std::byte const> data) -> bool;
+
+   // Wrong
+   auto resolve(kapi::capabilities::facet_id const & id, std::string_view const & name) -> std::observer_ptr<void>;
+
+Large or non-trivially copyable types — pass by ``const &``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For types that own heap memory or are non-trivially copyable, use ``const &`` when the callee does not take ownership.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   auto do_publish(kstd::string const & name) -> kstd::result<void>;
+   auto add_child(kstd::string const & child_name) -> void;
+
+Sink parameters — pass by value and move
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a function is designed to take **ownership** of an argument, accept it by value and move it into its destination.
+This makes the transfer explicit at the call site.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // In the header
+   auto add_child(kstd::shared_ptr<device> child) -> void;
+
+   // In the implementation
+   auto bus::add_child(kstd::shared_ptr<device> child) -> void
+   {
+     m_devices.push_back(std::move(child));  // ownership transferred here
+   }
+
+Mutable subsystem references — pass by non-const reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Services and subsystems that are mutated in-place (e.g., ``page_mapper &``, ``driver_state &``, ``kapi::devices::bus &``) are passed by non-const reference.
+This expresses that the function operates on a shared, mutable context.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   auto remap_kernel(kapi::memory::page_mapper & mapper) -> void;
+   auto add_directory_entry(inode & directory, inode & child, driver_state & state, write_batch & batch) -> kstd::result<void>;
+
+Error Handling
+--------------
+
+TeachOS kernel code, and all library code used by it, cannot make use of exceptions.
+Use of exception related keywords, `try`, `catch`, `throw` in kernel code will cause compilation to fail.
+However, exceptions are allowed in test code.
+
+Recoverable errors — ``kstd::result<T>``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Functions that can fail in an expected, recoverable way must return ``kstd::result<T>``.
+Use the ``kstd::success()`` and ``kstd::failure()`` helpers consistently.
+Define per-subsystem error code if necessary.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   auto mount(kstd::shared_ptr<inode> parent) -> kstd::result<std::pair<kstd::shared_ptr<inode>, state *>>;
+
+   // In the implementation
+   if (!device)
+   {
+     return kstd::failure(make_error_code(kstd::errc::invalid_argument));
+   }
+   return kstd::success(result_value);
+
+Callers must check the result before using its value. The idiomatic check is:
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   auto result = some_function();
+   if (!result)
+   {
+     return kstd::failure(result.error());  // propagate
+   }
+   // use *result
+
+Monadic composition (``transform``, ``and_then``, ``or_else``) is preferred over manual if-check-and-return chains when it produces clearer code.
+
+Unrecoverable errors — ``kapi::system::panic``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Violations of kernel invariants (e.g., a subsystem used before being initialized, OOM during boot) call ``kapi::system::panic(...)``.
+The ``panic`` function is ``[[noreturn]]``, meaning control will never return from it.
+It must not be used for recoverable errors.
+
+Log prefix convention: ``[SUBSYSTEM:TAG] message``. Examples: ``[OS:DEV]``, ``[OS:VFS]``, ``[ARCH:DRV]``.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   if (!instance)
+   {
+     system::panic("[OS:DEV] Device registry has not been initialized.");
+   }
+
+Ownership and Lifetime
+----------------------
+
+Shared ownership — ``kstd::shared_ptr``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Use ``kstd::shared_ptr<T>`` when a resource is co-owned by multiple subsystems and its lifetime must be extended by any of them (e.g., ``device``, ``inode``, ``dentry``).
+Weak back-references that must not extend lifetime use ``kstd::weak_ptr<T>``.
+
+Non-owning references — ``kstd::observer_ptr`` and raw references
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Use ``kstd::observer_ptr<T>`` to express a non-owning pointer where null is a valid state and the holder has no say in the lifetime of the target.
+Use a raw reference (``T &`` or ``T const &``) when null is not valid and the reference is short-lived (i.e. a function parameter or a local alias).
+
+Never use raw ``T *`` to mean "sometimes I own this, sometimes I don't".
+Ownership must be expressed unambiguously through the pointer type.
+
+Device driver data — ``kstd::shared_ptr<void>``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Drivers attach their state to a ``device`` using an untyped ``kstd::shared_ptr<void>`` via ``device::set_driver_data``.
+A driver retrieves its state via ``device::driver_data``.
+This allows the device tree to destroy driver data automatically when the device is released, without the device knowing the concrete driver type.
+
+Algorithm and Range Usage
+-------------------------
+
+Prefer ``std::ranges`` algorithms over manual loops
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When processing a range to search, filter, transform, or reduce, use the appropriate ``std::ranges`` algorithm or view pipeline instead of writing a raw ``for`` loop.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   auto already_published = std::ranges::any_of(m_entries, [&](auto const & entry) {
+     return entry.id() == id && entry.device().get() == device.get();
+   });
+
+   std::ranges::for_each(observers, [&](auto observer) { /* ... */ });
+
+   // Wrong — manual linear scan for a boolean result
+   for (auto const & entry : m_entries)
+   {
+     if (entry.id() == id && entry.device().get() == device.get())
+     {
+       return true;
+     }
+   }
+   return false;
+
+Acceptable uses of explicit loops include:
+
+- Accumulation or mutation that modifies state in-place and cannot be cleanly expressed with a ranges algorithm.
+- Low-level routines dealing with raw memory arithmetic (allocators, page mappers).
+- Iterator-pair loops in library internals (``kstd::vector``, ``kstd::basic_string``).
+
+Prefer view composition over intermediate containers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Build processing pipelines using ``std::views::filter``, ``std::views::transform``, ``std::views::reverse``, ``std::views::split``, and ``std::ranges::subrange`` rather than materialising intermediate vectors.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   auto descriptors = std::span{&__start_platform_drivers, &__stop_platform_drivers}
+                      | std::views::filter([](auto p) { return p != nullptr; });
+
+   auto modules_view = std::ranges::subrange(begin(), end())
+                       | std::views::filter(filter_modules)
+                       | std::views::transform(transform_module);
+
+Do not call the same function twice to avoid storing the result
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If an intermediate value is needed more than once, store it in a local variable.
+This applies especially to factory calls and heap allocations.
+
+.. code-block:: cpp
+
+   // Wrong — double invocation, two allocations, different objects
+   for (auto driver : descriptors)
+   {
+     kstd::println("registering driver '{}'", driver->make_instance()->name());
+     registry.add(driver->make_instance());
+   }
+
+   // Correct
+   for (auto driver : descriptors)
+   {
+     auto instance = driver->make_instance();
+     kstd::println("registering driver '{}'", instance->name());
+     registry.add(std::move(instance));
+   }
+
+Class and Struct Design
+-----------------------
+
+Prefer ``struct`` over ``class``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The entire codebase uses ``struct`` for all type definitions with explicit ``private:``
+sections where necessary.
+Do not introduce ``class``.
+
+Member ordering within a type
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Follow this ordering within a ``struct``:
+
+1. Nested types and type aliases.
+2. Static data members and static constexpr constants (e.g., ``static constexpr auto id = ...``).
+3. Constructors and destructor.
+4. Public member functions.
+5. ``protected:`` section with virtual hooks.
+6. ``private:`` section with helper functions, then data members.
+
+Data members are always in the ``private`` section and always prefixed with ``m_``.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   struct facet_registry
+   {
+     struct entry { /* ... */ };          // 1. nested type
+
+     facet_registry() = default;          // 3. constructor
+
+     auto static init() -> void;          // 4. public interface
+     auto static get() -> facet_registry &;
+     auto publish(...) -> kstd::result<void>;
+
+   private:
+     auto do_publish(...) -> kstd::result<void>;  // 6a. private helpers
+
+     mutable tracked_mutex m_lock{};      // 6b. data members, m_ prefix
+     kstd::vector<entry> m_entries;
+   };
+
+``explicit`` on single-argument constructors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mark every single-argument constructor ``explicit`` unless an implicit conversion is intentional and documented.
+A deliberate implicit constructor must be accompanied by a comment explaining the decision.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   explicit device(kstd::string const & name);
+   constexpr explicit facet_id(std::string_view name);
+
+   // Intentional implicit — documented at the declaration
+   //! This constructor allows implicit conversion from chunk<...> to page for
+   //! convenience. It is deliberately not explicit.
+   constexpr page(chunk other) : chunk{other} {}
+
+Declare deleted special members explicitly
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If a type is not copyable or not movable, declare the deleted special members explicitly rather than relying on implicit suppression.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   device(device const &) = delete;
+   auto operator=(device const &) -> device & = delete;
+
+Virtual destructors
+~~~~~~~~~~~~~~~~~~~
+
+Every base class with virtual member functions must have a ``virtual`` destructor, defaulted if not otherwise needed.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   virtual ~driver_descriptor() = default;
+   virtual ~facet_registry_observer() = default;
+
+Static Singletons
+-----------------
+
+Several subsystems expose a single global instance via an ``init()``/``get()`` pair.
+Follow this pattern:
+
+- Store the instance in an anonymous namespace as a ``constinit std::optional<T>``.
+- ``init()`` asserts the instance is not yet constructed, then constructs it using ``emplace()`` it.
+- ``get()`` asserts the instance exists and returns a reference to it.
+- Both functions panic on violation rather than returning an error code, because incorrect call order is a programming error, not a recoverable runtime condition.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   namespace
+   {
+     auto constinit instance = std::optional<device_registry>{};
+   }
+
+   auto device_registry::init() -> void
+   {
+     if (instance)
+     {
+       system::panic("[OS:DEV] Device registry has already been initialized.");
+     }
+     instance.emplace();
+   }
+
+   auto device_registry::get() -> device_registry &
+   {
+     if (!instance)
+     {
+       system::panic("[OS:DEV] Device registry has not been initialized.");
+     }
+     return *instance;
+   }
+
+Enumerations
+------------
+
+All enumerations use ``enum struct`` (scoped enums), never plain ``enum``.
+Specify the underlying type explicitly when the representation matters (e.g., for hardware register fields).
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   // Correct
+   enum struct state
+   {
+     uninitialized,
+     present,
+     bound,
+   };
+
+   // Wrong
+   enum state { uninitialized, present, bound };
+
+Naming
+------
+
+==================================  ============================= =====================================
+Symbol                              Convention                    Example
+==================================  ============================= =====================================
+Types (struct, enum)                ``lower_case``                ``device_registry``, ``facet_id``
+Functions and methods               ``lower_case``                ``add_child``, ``make_instance``
+Local variables                     ``lower_case``                ``entry``, ``block_index``
+Private data members                ``m_`` prefix, ``lower_case`` ``m_entries``, ``m_driver_data``
+Template type parameters            ``CamelCase``                 ``ValueType``, ``FacetType``
+Constants and constexpr variables   ``lower_case``                ``page_size``, ``direct_block_count``
+Type aliases                        ``lower_case``                ``value_type``, ``size_type``
+Namespaces                          ``lower_case``                ``kapi::devices``, ``kernel::vfs``
+==================================  ============================= =====================================
+
+
+Namespaces reflect directory structure: ``kernel::vfs``, ``kernel::filesystems::ext2``, ``arch::devices``, etc.
+
+``[[nodiscard]]``
+-----------------
+
+Mark any function ``[[nodiscard]]`` whose return value the caller should not silently discard.
+This includes in particular:
+
+- All functions returning ``kstd::result<T>``.
+- All query functions (getters, lookups) that return computed data.
+- Factory functions and builder utilities.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   [[nodiscard]] auto children() const -> kstd::vector<kstd::shared_ptr<device>>;
+   [[nodiscard]] auto resolve(kapi::capabilities::facet_id id, std::string_view name) -> void *;
+   [[nodiscard]] auto request_resource(resource_type type, std::size_t index = 0) const -> kstd::result<resource>;
+
+``constexpr`` and ``constinit``
+-------------------------------
+
+Mark functions ``constexpr`` whenever they can be evaluated at compile time or in constant expressions, even if they are also called at runtime.
+Mark module-scope variables ``constinit`` to guarantee zero-initialization before any dynamic initialization runs.
+
+Documentation
+-------------
+
+Every non-trivial public type, function, and data member must be documented with a Doxygen comment using the ``//!`` line style.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   //! A brief one-line description.
+   //!
+   //! Optional longer description providing context.
+   //!
+   //! @warning Any important warnings or preconditions.
+   //!
+   //! @param name Description of the parameter.
+   //! @return Description of the return value.
+   auto add_child(kstd::shared_ptr<device> child) -> void;
+
+Doxygen grouping (``@name``, ``@{``, ``@}``) may be used to organize the API surface into logical sections visible in generated documentation.
+
+Header Guards
+-------------
+
+All headers use traditional include guards, not ``#pragma once``.
+The guard name follows the pattern ``TEACHOS_<SUBPACKAGE>_<PATH>_HPP``, where each path component is uppercased and separators are replaced by ``_``.
+The only exception to this pattern are the bundled libraries, which follow the pattern ``<LIBRARYNAME>_<SUBPACKAGE>_<PATH>_HPP``.
+
+See the following code snippet for examples:
+
+.. code-block:: cpp
+
+   #ifndef TEACHOS_KAPI_DEVICES_BUS_HPP
+   #define TEACHOS_KAPI_DEVICES_BUS_HPP
+   // ...
+   #endif
+
+The closing ``#endif`` carries no comment with the guard name.