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-# 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.
-
----
-
-## 1. Language Standard and Vocabulary
-
-TeachOS targets **C++23** without compiler extensions (`CMAKE_CXX_EXTENSIONS NO`).
-Standard library features may be used freely where a hosted implementation is
-available (i.e., in `libs/`, `kapi/`, and `kernel/`), but the project ships its own
-standard library subset in `libs/kstd/`. **kstd types must be preferred over their
-`std::` equivalents** wherever a kstd equivalent exists.
-
-| Concept | Preferred | Avoid |
-|---|---|---|
-| Dynamic array | `kstd::vector<T>` | `std::vector<T>` |
-| String (owning) | `kstd::string` | `std::string` |
-| String view | `std::string_view` | (kstd has no alias; use `std::string_view` directly) |
-| 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>` |
-| Failable results | `kstd::result<T>` | exceptions, output parameters, `std::optional` for errors |
-| Printing | `kstd::println(...)` | `std::println(...)`, `printf` |
-
-`std::string_view`, `std::span`, `std::array`, `std::optional`, `std::byte`, and the
-`std::ranges` and `std::views` namespaces are used directly from the standard library
-because they have no kstd equivalents.
-
----
-
-## 2. Function Declarations — Trailing Return Types
-
-**All** functions and member functions use trailing return type syntax, including those
-returning `void`.
-
-```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.
-
----
-
-## 3. Parameter Passing Conventions
-
-The choice of passing convention encodes intent and must be consistent.
-
-### 3.1 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 applies to:
-- `std::string_view`
-- `std::span<T>`
-- `kstd::observer_ptr<T>`
-- `kstd::bytes`, `kstd::pages` and similar unit wrappers
-- `kapi::capabilities::facet_id`
-- `kapi::memory::page`, `kapi::memory::frame`, `kapi::memory::physical_address`, `kapi::memory::linear_address`
-
-```cpp
-// Correct
-auto resolve(kapi::capabilities::facet_id id, std::string_view name) -> void *;
-auto has(std::span<std::byte const> data) -> bool;
-
-// Wrong
-auto resolve(kapi::capabilities::facet_id const & id, std::string_view const & name) -> void *;
-```
-
-### 3.2 Large or non-trivial types — pass by `const &`
-
-For types that own heap memory or are non-trivially copyable, use `const &` when the
-callee does not take ownership.
-
-```cpp
-auto do_publish(kstd::string const & name) -> kstd::result<void>;
-auto add_child(kstd::string const & child_name) -> void;
-```
-
-### 3.3 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.
-
-```cpp
-// In the header
-auto add_child(kstd::shared_ptr<device> child) -> void;
-auto do_publish(kstd::shared_ptr<device> device, kstd::string name, ...) -> kstd::result<void>;
-
-// In the implementation
-auto bus::add_child(kstd::shared_ptr<device> child) -> void
-{
-  m_devices.push_back(std::move(child));  // ownership transferred here
-}
-```
-
-### 3.4 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.
-
-```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>;
-```
-
----
-
-## 4. Error Handling
-
-### 4.1 Recoverable errors — `kstd::result<T>`
-
-Functions that can fail in an expected, recoverable way must return `kstd::result<T>`
-(an alias for `std::expected<T, kstd::error_code>`). Use the `kstd::success()` and
-`kstd::failure()` helpers consistently.
-
-```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:
-
-```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.
-
-### 4.2 Unrecoverable errors — `kapi::system::panic`
-
-Violations of kernel invariants (e.g., a subsystem used before being initialized,
-OOM during boot) call `kapi::system::panic(...)`. `panic` is `[[noreturn]]`. It must
-not be used for recoverable errors.
-
-Log prefix convention: `[SUBSYSTEM:TAG] message`. Examples: `[OS:DEV]`,
-`[OS:VFS]`, `[ARCH:DRV]`.
-
-```cpp
-if (!instance)
-{
-  system::panic("[OS:DEV] Device registry has not been initialized.");
-}
-```
-
-### 4.3 No exceptions
-
-The kernel does not use C++ exceptions. Do not write `throw` or `try`/`catch` in
-kernel code.
-
----
-
-## 5. Ownership and Lifetime
-
-### 5.1 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>`.
-
-### 5.2 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.
-
-### 5.3 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.
-
----
-
-## 6. Algorithm and Range Usage
-
-### 6.1 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.
-
-```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`).
-
-### 6.2 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.
-
-```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);
-```
-
-### 6.3 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.
-
-```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));
-}
-```
-
----
-
-## 7. Class and Struct Design
-
-### 7.1 Prefer `struct` over `class`
-
-The entire codebase uses `struct` for all type definitions with explicit `private:`
-sections where necessary. Do not introduce `class`.
-
-### 7.2 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_`.
-
-```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;
-};
-```
-
-### 7.3 `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.
-
-```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} {}
-```
-
-### 7.4 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.
-
-```cpp
-device(device const &) = delete;
-auto operator=(device const &) -> device & = delete;
-```
-
-### 7.5 Virtual destructors
-
-Every base class with virtual member functions must have a `virtual` destructor,
-defaulted if not otherwise needed.
-
-```cpp
-virtual ~driver_descriptor() = default;
-virtual ~facet_registry_observer() = default;
-```
-
----
-
-## 8. 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 `emplace()`s 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.
-
-```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;
-}
-```
-
----
-
-## 9. 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).
-
-```cpp
-// Correct
-enum struct state
-{
-  uninitialized,
-  present,
-  bound,
-};
-
-// Wrong
-enum state { uninitialized, present, bound };
-```
-
----
-
-## 10. 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.
-
----
-
-## 11. `[[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.
-
-```cpp
-[[nodiscard]] auto resolve(kapi::capabilities::facet_id id, std::string_view name) -> void *;
-[[nodiscard]] auto children() const -> kstd::vector<kstd::shared_ptr<device>>;
-[[nodiscard]] auto request_resource(resource_type type, std::size_t index = 0) const -> kstd::result<resource>;
-```
-
----
-
-## 12. `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 — important in a kernel with no well-defined global
-initialization order.
-
----
-
-## 13. Documentation
-
-Every non-trivial public type, function, and data member must be documented with a
-Doxygen comment using the `//!` line style.
-
-```cpp
-//! A brief one-line description.
-//!
-//! Optional longer description providing context.
-//!
-//! @param name Description of the parameter.
-//! @return Description of the return value.
-//! @warning Any important warnings or preconditions.
-auto add_child(kstd::shared_ptr<device> child) -> void;
-```
-
-Doxygen grouping (`@addtogroup`, `@{`, `@}`) is used to organise the API surface into
-logical sections visible in generated documentation.
-
----
-
-## 14. 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 `_`.
-
-```cpp
-#ifndef TEACHOS_KAPI_DEVICES_BUS_HPP
-#define TEACHOS_KAPI_DEVICES_BUS_HPP
-// ...
-#endif
-```
-
-The closing `#endif` carries no comment with the guard name.