google-style-cpp

name: google-style-cpp description: > Apply Google's official C++ style guide when writing, reviewing, or formatting C++ code. Use this skill whenever the user asks to review C++ for style, asks about C++ naming conventions, header organisation, smart pointer usage, exception policy, template guidelines, or whether code follows Google C++ standards. Also triggers on: "is this idiomatic C++?", "clean up this C++ file", "how should I name this in C++?", "should I use exceptions in C++?", "how do I write C++ comments / Doxygen?", "should I use auto here?", "when should I use references vs pointers?", or any mention of clang-format, cpplint, or IWYU in a Google-style context. Note: Google's C++ guide is deliberately conservative — it restricts several modern C++ features (exceptions, RTTI, some template meta-programming) for reasons of build reproducibility and readability at scale.

Google C++ Style Guide

Google's C++ style guide is one of the most detailed in the industry and deliberately conservative about adopting new C++ features. The overarching philosophy: optimise for the reader, not the writer. Any construct that makes code harder to understand is suspect, regardless of how clever it is.

Key constraints that surprise people coming from other C++ styles:

• No exceptions (in most code; legacy constraint from Google's build system)
• No RTTI (dynamic_cast, typeid) in production code
• Prefer composition over inheritance; virtual dispatch only when necessary
• auto only when it genuinely aids clarity, not as a habit

Key Rules at a Glance

Headers (§1–2)

• Every .cc file has a corresponding .h — no inline implementation in headers except templates and small inlined functions.
• #pragma once is acceptable but Google's own code uses #ifndef header guards; the guard macro name is PATH_TO_FILE_H_.
• Headers include only what they use — no transitive includes as a crutch. Use forward declarations where possible to reduce compile-time coupling.
• .cc file must include its own .h first (ensures the header is self-contained).

Naming conventions

Classes

• Single responsibility — if it has more than one, split it.
• Data members are private by default; use accessors.
• Explicit constructors where appropriate; mark single-arg constructors explicit to prevent implicit conversion.
• Override = delete copy/move to prevent accidental copies when the type isn't copyable.
• Prefer composition over inheritance; use final on classes not meant to be subclassed.
• Virtual methods only on base classes that represent abstract interfaces.

Smart pointers & ownership

• std::unique_ptr<T> — sole ownership, default choice.
• std::shared_ptr<T> — shared ownership; use sparingly (hidden cost).
• Raw pointers — non-owning references only (borrowing). Never use new/delete in application code.
• std::make_unique<T>() / std::make_shared<T>() — always, never new.

No exceptions / No RTTI

// No throw/catch in new Google C++ code
// Signal errors via return codes or Status types:
absl::Status DoSomething();
absl::StatusOr<Result> Compute(Input in);

// No dynamic_cast or typeid — use virtual dispatch instead

Ownership of pointers in function signatures

// Pass by reference when caller retains ownership and value is required
void Process(const UserData& data);

// Pass by pointer when value is optional (may be nullptr)
void Process(const UserData* data);

// Pass by unique_ptr to transfer ownership
void Store(std::unique_ptr<UserData> data);

auto

• Use auto when the type is obvious from context or verbose to write:

  auto it = container.begin();           // Good — type clear
  auto result = ComputeTotal(items);     // Borderline — return type should be known
  auto x = 42;                           // Bad — use int
  

• Do not use auto for return types of non-trivial public functions.

Comments

// Single-line for brief explanations
// Multi-word OK: "This uses a sentinel value because..."

/*
 * Block comments for longer explanations.
 * Not preferred — use multiple // lines instead.
 */

// Function comments go in the header above the declaration:
// Returns the user with the given ID, or nullptr if not found.
// Thread-safe.
std::unique_ptr<User> FindUser(absl::string_view id) const;

Formatting

• Indentation: 2 spaces (consistent with other Google guides).
• Line length: 80 characters.
• Braces: Opening brace on same line (K&R). Always present.
• Namespace: Content indented 0 extra spaces; closing comment } // namespace foo.
• Run clang-format with Google style: clang-format -style=Google.

Mode: Reviewing C++ Code

When asked to review C++ code for Google style compliance:

1. Headers — header guards present? Only necessary includes? .cc includes its own .h first?
2. Naming — correct casing per the table? Trailing _ on class members? k prefix on constants?
3. Ownership — any raw new/delete? Smart pointer type matches ownership model?
4. Constructors — single-arg constructors marked explicit? Non-copyable types deleted?
5. Exceptions — any throw/catch? Should use absl::Status / error codes.
6. RTTI — any dynamic_cast or typeid?
7. auto — overused or in ambiguous places?
8. Inheritance — virtual methods in non-abstract base? Should prefer composition?
9. Line length — any lines > 80 chars?
10. Comments — public API documented in the header?

For each issue:

• Cite the section (e.g. "§Naming: Variables", "§Smart Pointers")
• Show the problematic snippet and the corrected version with rationale

Mode: Writing New C++ Code

When writing new C++ following Google style:

1. Create a .h / .cc pair; .cc includes its own .h first.
2. Use #ifndef header guards: MY_PROJECT_PATH_TO_FILE_H_.
3. 2-space indentation; 80-char line limit; K&R braces.
4. Name types PascalCase, variables snake_case, class members snake_case_, constants kPascalCase.
5. Mark single-arg constructors explicit; = delete copy/move for non-copyable types.
6. Use std::unique_ptr/std::make_unique for owned heap objects; raw pointers only for non-owning references.
7. Signal errors with return codes or absl::Status; no exceptions.
8. Write function documentation in the .h file, above the declaration.
9. Use auto only when type is verbose or obvious from context.
10. Run clang-format -style=Google and cpplint before considering code done.

When to Load the Full Guide

The C++ guide is very large (~9,000 lines after conversion). Load references/full_guide.md when:

• A specific rule isn't covered above (e.g. templates, lambdas, constexpr rules, Abseil usage)
• Doing a comprehensive file-level review
• Answering questions about: static, anonymous namespaces, [[nodiscard]], designated initialisers, ranges

Use references/TOC.md to navigate — search for the section heading directly rather than scrolling.

Sections of note:

• ## Header Files — includes, forward declarations, inline functions
• ## Scoping — namespaces, unnamed namespaces, static
• ## Classes — all class rules in one place
• ## Functions — parameters, return types, overloading, default args
• ## Naming — the complete naming table
• ## Comments — style and placement

Also see: google-style-common for cross-language naming philosophy.