path: root/docs/code_style.md

# 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.