Coding Style Guide

Revision 6 2024-10-28

This guide provides a small set of code guidelines used in the SDK. This guide reflects the currently accepted style practices for the SDK and is subject to change.

The SDK was seeded from multiple different projects and contains contributions from many different companies and the SDK code therefore uses several different coding styles throughout the code base. Stylistically, code should attempt to conform to the dominant style of the code being modified, while also adhering to the guidelines below.

Language standard

Code in the SDK conforms to the following standards. Changes to the C++ standard happen relatively infrequently. Changes to the Python version are more frequent.

Language	Version
C++	C++17
Python	3.10

Product-specific software may elect to use later standards in their own work.

Coding Guidelines

Common

When in Rome

The most important convention and practice in the Matter SDK repo is “When in Rome...”, per the quote below.

[quote, St. Ambrose]

---

If you should be in Rome, live in the Roman manner; if you should be elsewhere, live as they do there.

---

Your extensions or fixes to existing code should match the prevailing style of the original code.

If you find the conventions so foreign or otherwise confusing, it may be best to let whoever owns the file make the necessary changes or seek the counsel of others in the group to find out what the right thing to do is. Never just start changing code wholesale for personal reasons without consulting others first.

Commenting Out or Disabling Code

Unused code shall not be disabled by commenting it out with C- or C++-style comments or with preprocessor #if 0 ... #endif semantics. Unused code should be removed.

Auto-formatters

We use the following auto-formatters on code:

Language	Formatter	Style File
C++	clang-format	.clang-format
Objective-C	clang-format	.clang-format
java	google-java-format	N/A
Python	pep8, isort, ruff	.restyled.yaml (command line), isort, ruff
YAML	prettier	None
JSON	prettier	None
markdown	prettier	None

All pull requests run formatting checks using these tools before merge is allowed. Generated code is not run through restyle.

C++

Use C++ cstdint for Plain Old Data Types

Standard, scalar data types defined in cstdint should be used for basic signed and unsigned integer types, especially when size and serialization to non-volatile storage or across a network is concerned.

Examples of these are: uint8_t, int8_t, etc.

Avoid top-level using namespace Statements in Headers

By doing this, you are effectively forcing every other module that includes the header to also be using the namespace. This causes namespace pollution and generally defeats the purposes of namespaces. Fully-qualified symbols or namespace blocks should be used instead.

Classes / objects not exposed in a header should be in an anonymous namespace

If a cpp class defines a class or instantiates a static object, it should be enclosed in an anonymous namespace.

namespace {
  // CPP internal defines go here
}  // namespace

Singleton use

The decision to use a singleton class should be considered with care. Do not default to using a singleton for ease of writing code.

If the class truly should be a singleton (ex. if it is controlling access to a hardware resource)

• The standard function name for accessing an SDK singleton is GetInstance().
• Singleton classes should delete copy and move constructors

Avoid Heap-based Resource Allocation and auto-resizing std containers

Heap-based resource allocation should be avoided in the core SDK for common code that may run on constrained embedded devices. This includes any container element in std that automatically re-sizes itself at runtime (ex. vector, string etc.) as these re-size operations are often large and can lead to memory exhaustion and fragmentation on embedded systems.

Heap-based allocation is allowed for controller code and is at the discretion of platform vendors for platform-specific code.

Alternatives

In either case, recommended resource allocation alternatives are:

• In-place allocation and initialization
• Pool-based allocators
• Platform-defined and -assigned allocators

CHIPMem.h provides support for platform defined allocators.

Pool.h is the Matter SDK pool allocator implementation.

Prefer CopySpanToMutableSpan over memcpy when using spans

See Span.h

Prefer std::optional to CHIP implementation in newer code

The Matter SDK Optional.h was implemented when the Matter SDK was C++14, but newer code can use std::optional, which offers some benefits over the Matter SDK implementation (ex. std::optional is trivially destructible if the underlying type is also trivially destructible)

Python

Type hints

Use type hints on function definitions for public APIs.

Docstrings

Docstrings should be included for all public APIs.

mypy

The current python code does not yet pass mypy checks, but we are working towards this goal. The more compliant new code is to mypy, the better.