name: pre-reviewer description: Automated pre-review tool to inspect C++ style, concurrency safety, test quality, and CL descriptions before submitting code for human review. metadata: version: "1.0.1"

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Pre-Reviewer Skill

This skill acts as a pre-review confidence check tool for WebRTC C++ contributions. Use this skill to evaluate code changes for common C++ anti-patterns, concurrency bugs, test hygiene issues, and CL description completeness.

1. C++ Style and Modernization Guardrails

Lambda Expressions

• No Capture Renaming: Do not use captures with initializers to introduce

new names. Capture variables with their original names.

• Correct: [on_certificate_ready = std::move(on_certificate_ready)]
• Incorrect: [callback = std::move(on_certificate_ready)]
• Reference:

Google C++ Style Guide - Lambda expressions

• Prefer Implicit Captures for Inline/Local Lambdas: If the lambda cannot

escape the current scope (e.g. executed inside BlockingCall), prefer auto-capture ([&]). Do not explicitly list captures as it reduces readability.

• Reference:

C++ Readability Advice - Lambda capture clauses

Pointer and Value Checks

• Explicit `nullptr` Comparisons: Always compare pointer types to nullptr

explicitly. Avoid implicit boolean conversion in conditions.

• Correct: if (tq_ != nullptr) / RTC_DCHECK(transport == nullptr);
• Incorrect: if (tq_) / RTC_DCHECK(!transport);
• Reference:

Abseil Tip #141: Test Container Emptiness with Boolean Functions

Structs vs. Classes

• Classes for API Invariants: Use a class if the object has invariants, has

wide visibility, or is expected to evolve. Use a struct only for passive data.

• Reference:

Google C++ Style Guide - Structs vs. Classes

• Encapsulation: Make classes' data members private unless they are

constants.

• Reference:

Google C++ Style Guide - Access Control

Virtual Functions

• No Default Arguments on Virtuals: Banned on virtual functions because

default arguments are bound statically at compile time, which leads to unexpected behavior when overriding.

• Reference:

Google C++ Style Guide - Default Arguments

Type Deduction (auto)

• Avoid Excessive `auto`: Explicitly spell out types unless the type is

overly verbose (e.g. templates) or using auto directly improves safety. Do not use it merely to avoid writing the type.

• Reference:

Google C++ Style Guide - Type deduction

Operator Style

• Pre-increment/decrement: Prefer the prefix form (++i, --i) unless

postfix semantics are required.

• Reference:

Google C++ Style Guide - Preincrement and Predecrement

Smart Pointer Initialization

• Assignment Syntax: Use assignment syntax when initializing directly with

smart pointers.

• Correct: std::unique_ptr<Foo> foo = std::make_unique<Foo>();
• Incorrect: std::unique_ptr<Foo> foo(std::make_unique<Foo>());
• Reference:

Abseil Tip #88: Use Assignment Syntax for Smart Pointer Initialization

Modern C++ Patterns (C++20 & Standard Library)

• Constinit: Use constinit directly instead of ABSL_CONST_INIT.
• Reference:

cppreference - constinit

• std::exchange: Use std::exchange to simplify reset-and-return logic.
• Reference:

cppreference - std::exchange

• Avoid std::make_pair / std::make_tuple: Prefer brace initialization or

class template argument deduction (CTAD) like std::pair{a, b} instead of std::make_pair(a, b).

• absl::flat_hash_map: Prefer as the default map type unless ordering is

required.

• Reference:

Abseil Container Guide

Type Casting and Conversions

• Prefer Braced Initialization for Non-Narrowing Conversions: Prefer braced

initialization (Type{value}) over static_cast<Type>(value) when converting constants or values where narrowing is not intended.

• Correct: if (temporal_index >= int{kMaxTemporalStreams})
• Incorrect: if (temporal_index >= static_cast<int>(kMaxTemporalStreams))
• Reference:

Google C++ Style Guide - Casting

Move Semantics

• Move First, Dereference After (for std::optional): For std::optional, it

is recommended to move first and dereference after (e.g. *std::move(opt)). Note that this is generally not recommended for std::unique_ptr.

• Reference:

Abseil Tip #181: Move First, Dereference After

• Reference:

Abseil Tip #224: Avoid value() accessors

• Avoid Redundant Return moves: Do not use std::move on return statements

when it prevents copy elision (NRVO/RVO). Moving a member variable out of a class when returning can still be useful.

• Avoid Brittle Move Loops: Avoid moving a member variable into an argument

of a method only to move it back inside the method. This looks like a "use after move" and makes code brittle.

• Example: Avoid RecreateEncoder(std::move(config_)) which internally

updates config_ back. Prefer separating it into a parameterless reset method or a config updater.

2. Concurrency, Threading, and Destructors

• Prevent Reentrancy in Destructors: When executing callbacks during

destruction, move or swap the callback list/queue into a local variable first, then execute. This prevents callbacks from accessing a partially destroyed class.

• No Sequence Checkers in Destructors: Remove or omit sequence/thread checks

(RTC_DCHECK_RUN_ON) inside destructors. In C++, it is assumed that no other threads can call into an object while it is being destroyed.

• Atomic Safety: Do not use relaxed memory order atomics when safety

invariants depend on the ordering of operations not protected by the same lock.

• Reference:

Abseil - Atomic Danger

3. Testing Best Practices

• Omit GMock Default Cardinality: Do not add .Times(1) to EXPECT_CALL as

it is the default.

• Omit Wildcard GMock Parameters: Omit parameter lists in EXPECT_CALL when

all parameters are wildcards (applicable to non-overloaded methods).

• Reference:

GoogleTest Mocking Reference - EXPECT_CALL

• Alias Testing Symbols with `using`: In test .cc files, add a

using-declaration such as using ::testing::IsTrue; near the top of the file, next to any existing ones, for each gMock matcher or other ::testing symbol. Do not spell out the ::testing:: prefix at each call site and do not use using namespace ::testing;.

• Correct: WaitUntil([&] { return done; }, IsTrue())
• Incorrect: WaitUntil([&] { return done; }, ::testing::IsTrue())
• Reference:

GoogleTest Cookbook

• Reference:

Google C++ Style Guide - Namespaces

• Prefer Container Matchers: Use EXPECT_THAT with matchers such as

SizeIs(), IsEmpty() and ElementsAre() instead of checking the result of size() or empty(). On failure, the matchers also print the contents of the container. For a container held in a std::optional, wrap the matcher in Optional(), e.g. EXPECT_THAT(values, Optional(SizeIs(2)));.

• Correct: EXPECT_THAT(values, SizeIs(2)); /

EXPECT_THAT(values, IsEmpty());

• Incorrect: EXPECT_EQ(values.size(), 2u); /

EXPECT_TRUE(values.empty());

• Reference:

GoogleTest Matchers Reference - Container Matchers

• Avoid Unrelated Assertions: Do not assert expectations on calls that are

not the focus of the test scenario. Use ON_CALL instead of EXPECT_CALL for setting default behavior without setting expectations.

• Reference:

GoogleTest Cookbook - Use ON_CALL

• Descriptive Test Names: Include the test scenario and expected outcome in

test names.

• Reference:

Testing on the Toilet: Writing Descriptive Test Names

• Do Not Crash in Tests: Use FAIL() << "message"; instead of triggering a

crash when verifying preconditions or expectations.

• Reference:

GoogleTest Reference: Success/Failure Assertions

• Avoid Double Negations: Use positive assertions for better readability.
• Reference:

Testing Blog: Improve Readability with Positive Assertions

4. Code Review and CL Process

• Meaningful CL Descriptions: The CL description must explain why the

change is made, not just what changed. List major goals, link issue tickets, and clarify the bigger architectural plan.

• Reference:

Google Engineering Practices: CL Descriptions

• Keep CLs Small: Split large changes into multiple CLs. Introduce interface

definitions in one CL, and migrate callers in subsequent CLs.

• Reference:

Google Engineering Practices: Small CLs

• Documentation Freshness: When adding new .md files, always include

freshness metadata to track ownership.

• Reference:

WebRTC Documentation Guidelines

5. Inclusive Language Guidelines

• Use Inclusive Terminology: Avoid gendered pronouns and non-inclusive terms

in code, comments, and documentation. Use precise, neutral technical alternatives.

• Reference:

Inclusive Chromium Code

• Reference:

Google Developer Documentation Style Guide - Word List