cpp-coding-standards

name	cpp-coding-standards
description	C++ coding standards based on the C++ Core Guidelines (isocpp.github.io). Use when writing, reviewing, or refactoring C++ code to enforce modern, safe, and idiomatic practices.
source_path	skills/cpp-coding-standards/SKILL.md
origin	ECC

C++ Coding Standards (C++ Core Guidelines)

Comprehensive coding standards for modern C++ (C++17/20/23) derived from the C++ Core Guidelines. Enforces type safety, resource safety, immutability, and clarity.

When to Use

• Writing new C++ code (classes, functions, templates)
• Reviewing or refactoring existing C++ code
• Making architectural decisions in C++ projects
• Enforcing consistent style across a C++ codebase
• Choosing between language features (e.g., enum vs enum class, raw pointer vs smart pointer)

When NOT to Use

• Non-C++ projects
• Legacy C codebases that cannot adopt modern C++ features
• Embedded/bare-metal contexts where specific guidelines conflict with hardware constraints (adapt selectively)

Cross-Cutting Principles

These themes recur across the entire guidelines and form the foundation:

1. RAII everywhere (P.8, R.1, E.6, CP.20): Bind resource lifetime to object lifetime
2. Immutability by default (P.10, Con.1-5, ES.25): Start with const/constexpr; mutability is the exception
3. Type safety (P.4, I.4, ES.46-49, Enum.3): Use the type system to prevent errors at compile time
4. Express intent (P.3, F.1, NL.1-2, T.10): Names, types, and concepts should communicate purpose
5. Minimize complexity (F.2-3, ES.5, Per.4-5): Simple code is correct code
6. Value semantics over pointer semantics (C.10, R.3-5, F.20, CP.31): Prefer returning by value and scoped objects

Philosophy & Interfaces (P., I.)

Key Rules

Rule	Summary
P.1	Express ideas directly in code
P.3	Express intent
P.4	Ideally, a program should be statically type safe
P.5	Prefer compile-time checking to run-time checking
P.8	Don't leak any resources
P.10	Prefer immutable data to mutable data
I.1	Make interfaces explicit
I.2	Avoid non-const global variables
I.4	Make interfaces precisely and strongly typed
I.11	Never transfer ownership by a raw pointer or reference
I.23	Keep the number of function arguments low

DO

// P.10 + I.4: Immutable, strongly typed interface
struct Temperature {
    double kelvin;
};

Temperature boil(const Temperature& water);

DON'T

// Weak interface: unclear ownership, unclear units
double boil(double* temp);

// Non-const global variable
int g_counter = 0;  // I.2 violation

Functions (F.*)

Key Rules

Rule	Summary
F.1	Package meaningful operations as carefully named functions
F.2	A function should perform a single logical operation
F.3	Keep functions short and simple
F.4	If a function might be evaluated at compile time, declare it constexpr
F.6	If your function must not throw, declare it noexcept
F.8	Prefer pure functions
F.16	For "in" parameters, pass cheaply-copied types by value and others by const&
F.20	For "out" values, prefer return values to output parameters
F.21	To return multiple "out" values, prefer returning a struct
F.43	Never return a pointer or reference to a local object

Parameter Passing

// F.16: Cheap types by value, others by const&
void print(int x);                           // cheap: by value
void analyze(const std::string& data);       // expensive: by const&
void transform(std::string s);               // sink: by value (will move)

// F.20 + F.21: Return values, not output parameters
struct ParseResult {
    std::string token;
    int position;
};

ParseResult parse(std::string_view input);   // GOOD: return struct

// BAD: output parameters
void parse(std::string_view input,
           std::string& token, int& pos);    // avoid this

Pure Functions and constexpr

// F.4 + F.8: Pure, constexpr where possible
constexpr int factorial(int n) noexcept {
    return (n <= 1) ? 1 : n * factorial(n - 1);
}

static_assert(factorial(5) == 120);

Anti-Patterns

• Returning T&& from functions (F.45)
• Using va_arg / C-style variadics (F.55)
• Capturing by reference in lambdas passed to other threads (F.53)
• Returning const T which inhibits move semantics (F.49)

Classes & Class Hierarchies (C.*)

Key Rules

Rule	Summary
C.2	Use class if invariant exists; struct if data members vary independently
C.9	Minimize exposure of members
C.20	If you can avoid defining default operations, do (Rule of Zero)
C.21	If you define or =delete any copy/move/destructor, handle them all (Rule of Five)
C.35	Base class destructor: public virtual or protected non-virtual
C.41	A constructor should create a fully initialized object
C.46	Declare single-argument constructors explicit
C.67	A polymorphic class should suppress public copy/move
C.128	Virtual functions: specify exactly one of virtual, override, or final

Rule of Zero

// C.20: Let the compiler generate special members
struct Employee {
    std::string name;
    std::string department;
    int id;
    // No destructor, copy/move constructors, or assignment operators needed
};

Rule of Five

// C.21: If you must manage a resource, define all five
class Buffer {
public:
    explicit Buffer(std::size_t size)
        : data_(std::make_unique<char[]>(size)), size_(size) {}

    ~Buffer() = default;

    Buffer(const Buffer& other)
        : data_(std::make_unique<char[]>(other.size_)), size_(other.size_) {
        std::copy_n(other.data_.get(), size_, data_.get());
    }

    Buffer& operator=(const Buffer& other) {
        if (this != &other) {
            auto new_data = std::make_unique<char[]>(other.size_);
            std::copy_n(other.data_.get(), other.size_, new_data.get());
            data_ = std::move(new_data);
            size_ = other.size_;
        }
        return *this;
    }

    Buffer(Buffer&&) noexcept = default;
    Buffer& operator=(Buffer&&) noexcept = default;

private:
    std::unique_ptr<char[]> data_;
    std::size_t size_;
};

Class Hierarchy

// C.35 + C.128: Virtual destructor, use override
class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;  // C.121: pure interface
};

class Circle : public Shape {
public:
    explicit Circle(double r) : radius_(r) {}
    double area() const override { return 3.14159 * radius_ * radius_; }

private:
    double radius_;
};

Anti-Patterns

• Calling virtual functions in constructors/destructors (C.82)
• Using memset/memcpy on non-trivial types (C.90)
• Providing different default arguments for virtual function and overrider (C.140)
• Making data members const or references, which suppresses move/copy (C.12)

Resource Management (R.*)

Key Rules

Rule	Summary
R.1	Manage resources automatically using RAII
R.3	A raw pointer (T*) is non-owning
R.5	Prefer scoped objects; don't heap-allocate unnecessarily
R.10	Avoid malloc()/free()
R.11	Avoid calling new and delete explicitly
R.20	Use unique_ptr or shared_ptr to represent ownership
R.21	Prefer unique_ptr over shared_ptr unless sharing ownership
R.22	Use make_shared() to make shared_ptrs

Smart Pointer Usage

// R.11 + R.20 + R.21: RAII with smart pointers
auto widget = std::make_unique<Widget>("config");  // unique ownership
auto cache  = std::make_shared<Cache>(1024);        // shared ownership

// R.3: Raw pointer = non-owning observer
void render(const Widget* w) {  // does NOT own w
    if (w) w->draw();
}

render(widget.get());

RAII Pattern

// R.1: Resource acquisition is initialization
class FileHandle {
public:
    explicit FileHandle(const std::string& path)
        : handle_(std::fopen(path.c_str(), "r")) {
        if (!handle_) throw std::runtime_error("Failed to open: " + path);
    }

    ~FileHandle() {
        if (handle_) std::fclose(handle_);
    }

    FileHandle(const FileHandle&) = delete;
    FileHandle& operator=(const FileHandle&) = delete;
    FileHandle(FileHandle&& other) noexcept
        : handle_(std::exchange(other.handle_, nullptr)) {}
    FileHandle& operator=(FileHandle&& other) noexcept {
        if (this != &other) {
            if (handle_) std::fclose(handle_);
            handle_ = std::exchange(other.handle_, nullptr);
        }
        return *this;
    }

private:
    std::FILE* handle_;
};

Anti-Patterns

• Naked new/delete (R.11)
• malloc()/free() in C++ code (R.10)
• Multiple resource allocations in a single expression (R.13 -- exception safety hazard)
• shared_ptr where unique_ptr suffices (R.21)

Expressions & Statements (ES.*)

Key Rules

Rule	Summary
ES.5	Keep scopes small
ES.20	Always initialize an object
ES.23	Prefer {} initializer syntax
ES.25	Declare objects const or constexpr unless modification is intended
ES.28	Use lambdas for complex initialization of const variables
ES.45	Avoid magic constants; use symbolic constants
ES.46	Avoid narrowing/lossy arithmetic conversions
ES.47	Use nullptr rather than 0 or NULL
ES.48	Avoid casts
ES.50	Don't cast away const

Initialization

// ES.20 + ES.23 + ES.25: Always initialize, prefer {}, default to const
const int max_retries{3};
const std::string name{"widget"};
const std::vector<int> primes{2, 3, 5, 7, 11};

// ES.28: Lambda for complex const initialization
const auto config = [&] {
    Config c;
    c.timeout = std::chrono::seconds{30};
    c.retries = max_retries;
    c.verbose = debug_mode;
    return c;
}();

Anti-Patterns

• Uninitialized variables (ES.20)
• Using 0 or NULL as pointer (ES.47 -- use nullptr)
• C-style casts (ES.48 -- use static_cast, const_cast, etc.)
• Casting away const (ES.50)
• Magic numbers without named constants (ES.45)
• Mixing signed and unsigned arithmetic (ES.100)
• Reusing names in nested scopes (ES.12)

Error Handling (E.*)

Key Rules

Rule	Summary
E.1	Develop an error-handling strategy early in a design
E.2	Throw an exception to signal that a function can't perform its assigned task
E.6	Use RAII to prevent leaks
E.12	Use noexcept when throwing is impossible or unacceptable
E.14	Use purpose-designed user-defined types as exceptions
E.15	Throw by value, catch by reference
E.16	Destructors, deallocation, and swap must never fail
E.17	Don't try to catch every exception in every function

Exception Hierarchy

// E.14 + E.15: Custom exception types, throw by value, catch by reference
class AppError : public std::runtime_error {
public:
    using std::runtime_error::runtime_error;
};

class NetworkError : public AppError {
public:
    NetworkError(const std::string& msg, int code)
        : AppError(msg), status_code(code) {}
    int status_code;
};

void fetch_data(const std::string& url) {
    // E.2: Throw to signal failure
    throw NetworkError("connection refused", 503);
}

void run() {
    try {
        fetch_data("https://api.example.com");
    } catch (const NetworkError& e) {
        log_error(e.what(), e.status_code);
    } catch (const AppError& e) {
        log_error(e.what());
    }
    // E.17: Don't catch everything here -- let unexpected errors propagate
}

Anti-Patterns

• Throwing built-in types like int or string literals (E.14)
• Catching by value (slicing risk) (E.15)
• Empty catch blocks that silently swallow errors
• Using exceptions for flow control (E.3)
• Error handling based on global state like errno (E.28)

Constants & Immutability (Con.*)

All Rules

Rule	Summary
Con.1	By default, make objects immutable
Con.2	By default, make member functions const
Con.3	By default, pass pointers and references to const
Con.4	Use const for values that don't change after construction
Con.5	Use constexpr for values computable at compile time

// Con.1 through Con.5: Immutability by default
class Sensor {
public:
    explicit Sensor(std::string id) : id_(std::move(id)) {}

    // Con.2: const member functions by default
    const std::string& id() const { return id_; }
    double last_reading() const { return reading_; }

    // Only non-const when mutation is required
    void record(double value) { reading_ = value; }

private:
    const std::string id_;  // Con.4: never changes after construction
    double reading_{0.0};
};

// Con.3: Pass by const reference
void display(const Sensor& s) {
    std::cout << s.id() << ": " << s.last_reading() << '\n';
}

// Con.5: Compile-time constants
constexpr double PI = 3.14159265358979;
constexpr int MAX_SENSORS = 256;

Concurrency & Parallelism (CP.*)

Key Rules

Rule	Summary
CP.2	Avoid data races
CP.3	Minimize explicit sharing of writable data
CP.4	Think in terms of tasks, rather than threads
CP.8	Don't use volatile for synchronization
CP.20	Use RAII, never plain lock()/unlock()
CP.21	Use std::scoped_lock to acquire multiple mutexes
CP.22	Never call unknown code while holding a lock
CP.42	Don't wait without a condition
CP.44	Remember to name your lock_guards and unique_locks
CP.100	Don't use lock-free programming unless you absolutely have to

Safe Locking

// CP.20 + CP.44: RAII locks, always named
class ThreadSafeQueue {
public:
    void push(int value) {
        std::lock_guard<std::mutex> lock(mutex_);  // CP.44: named!
        queue_.push(value);
        cv_.notify_one();
    }

    int pop() {
        std::unique_lock<std::mutex> lock(mutex_);
        // CP.42: Always wait with a condition
        cv_.wait(lock, [this] { return !queue_.empty(); });
        const int value = queue_.front();
        queue_.pop();
        return value;
    }

private:
    std::mutex mutex_;             // CP.50: mutex with its data
    std::condition_variable cv_;
    std::queue<int> queue_;
};

Multiple Mutexes

// CP.21: std::scoped_lock for multiple mutexes (deadlock-free)
void transfer(Account& from, Account& to, double amount) {
    std::scoped_lock lock(from.mutex_, to.mutex_);
    from.balance_ -= amount;
    to.balance_ += amount;
}

Anti-Patterns

• volatile for synchronization (CP.8 -- it's for hardware I/O only)
• Detaching threads (CP.26 -- lifetime management becomes nearly impossible)
• Unnamed lock guards: std::lock_guard<std::mutex>(m); destroys immediately (CP.44)
• Holding locks while calling callbacks (CP.22 -- deadlock risk)
• Lock-free programming without deep expertise (CP.100)

Templates & Generic Programming (T.*)

Key Rules

Rule	Summary
T.1	Use templates to raise the level of abstraction
T.2	Use templates to express algorithms for many argument types
T.10	Specify concepts for all template arguments
T.11	Use standard concepts whenever possible
T.13	Prefer shorthand notation for simple concepts
T.43	Prefer using over typedef
T.120	Use template metaprogramming only when you really need to
T.144	Don't specialize function templates (overload instead)

Concepts (C++20)

#include <concepts>

// T.10 + T.11: Constrain templates with standard concepts
template<std::integral T>
T gcd(T a, T b) {
    while (b != 0) {
        a = std::exchange(b, a % b);
    }
    return a;
}

// T.13: Shorthand concept syntax
void sort(std::ranges::random_access_range auto& range) {
    std::ranges::sort(range);
}

// Custom concept for domain-specific constraints
template<typename T>
concept Serializable = requires(const T& t) {
    { t.serialize() } -> std::convertible_to<std::string>;
};

template<Serializable T>
void save(const T& obj, const std::string& path);

Anti-Patterns

• Unconstrained templates in visible namespaces (T.47)
• Specializing function templates instead of overloading (T.144)
• Template metaprogramming where constexpr suffices (T.120)
• typedef instead of using (T.43)

Standard Library (SL.*)

Key Rules

Rule	Summary
SL.1	Use libraries wherever possible
SL.2	Prefer the standard library to other libraries
SL.con.1	Prefer std::array or std::vector over C arrays
SL.con.2	Prefer std::vector by default
SL.str.1	Use std::string to own character sequences
SL.str.2	Use std::string_view to refer to character sequences
SL.io.50	Avoid endl (use '\n' -- endl forces a flush)

// SL.con.1 + SL.con.2: Prefer vector/array over C arrays
const std::array<int, 4> fixed_data{1, 2, 3, 4};
std::vector<std::string> dynamic_data;

// SL.str.1 + SL.str.2: string owns, string_view observes
std::string build_greeting(std::string_view name) {
    return "Hello, " + std::string(name) + "!";
}

// SL.io.50: Use '\n' not endl
std::cout << "result: " << value << '\n';

Enumerations (Enum.*)

Key Rules

Rule	Summary
Enum.1	Prefer enumerations over macros
Enum.3	Prefer enum class over plain enum
Enum.5	Don't use ALL_CAPS for enumerators
Enum.6	Avoid unnamed enumerations

// Enum.3 + Enum.5: Scoped enum, no ALL_CAPS
enum class Color { red, green, blue };
enum class LogLevel { debug, info, warning, error };

// BAD: plain enum leaks names, ALL_CAPS clashes with macros
enum { RED, GREEN, BLUE };           // Enum.3 + Enum.5 + Enum.6 violation
#define MAX_SIZE 100                  // Enum.1 violation -- use constexpr

Source Files & Naming (SF., NL.)

Key Rules

Rule	Summary
SF.1	Use .cpp for code files and .h for interface files
SF.7	Don't write using namespace at global scope in a header
SF.8	Use #include guards for all .h files
SF.11	Header files should be self-contained
NL.5	Avoid encoding type information in names (no Hungarian notation)
NL.8	Use a consistent naming style
NL.9	Use ALL_CAPS for macro names only
NL.10	Prefer underscore_style names

Header Guard

// SF.8: Include guard (or #pragma once)
#ifndef PROJECT_MODULE_WIDGET_H
#define PROJECT_MODULE_WIDGET_H

// SF.11: Self-contained -- include everything this header needs
#include <string>
#include <vector>

namespace project::module {

class Widget {
public:
    explicit Widget(std::string name);
    const std::string& name() const;

private:
    std::string name_;
};

}  // namespace project::module

#endif  // PROJECT_MODULE_WIDGET_H

Naming Conventions

// NL.8 + NL.10: Consistent underscore_style
namespace my_project {

constexpr int max_buffer_size = 4096;  // NL.9: not ALL_CAPS (it's not a macro)

class tcp_connection {                 // underscore_style class
public:
    void send_message(std::string_view msg);
    bool is_connected() const;

private:
    std::string host_;                 // trailing underscore for members
    int port_;
};

}  // namespace my_project

Anti-Patterns

• using namespace std; in a header at global scope (SF.7)
• Headers that depend on inclusion order (SF.10, SF.11)
• Hungarian notation like strName, iCount (NL.5)
• ALL_CAPS for anything other than macros (NL.9)

Performance (Per.*)

Key Rules

Rule	Summary
Per.1	Don't optimize without reason
Per.2	Don't optimize prematurely
Per.6	Don't make claims about performance without measurements
Per.7	Design to enable optimization
Per.10	Rely on the static type system
Per.11	Move computation from run time to compile time
Per.19	Access memory predictably

Guidelines

// Per.11: Compile-time computation where possible
constexpr auto lookup_table = [] {
    std::array<int, 256> table{};
    for (int i = 0; i < 256; ++i) {
        table[i] = i * i;
    }
    return table;
}();

// Per.19: Prefer contiguous data for cache-friendliness
std::vector<Point> points;           // GOOD: contiguous
std::vector<std::unique_ptr<Point>> indirect_points; // BAD: pointer chasing

Anti-Patterns

• Optimizing without profiling data (Per.1, Per.6)
• Choosing "clever" low-level code over clear abstractions (Per.4, Per.5)
• Ignoring data layout and cache behavior (Per.19)

Quick Reference Checklist

Before marking C++ work complete:

• No raw new/delete -- use smart pointers or RAII (R.11)
• Objects initialized at declaration (ES.20)
• Variables are const/constexpr by default (Con.1, ES.25)
• Member functions are const where possible (Con.2)
• enum class instead of plain enum (Enum.3)
• nullptr instead of 0/NULL (ES.47)
• No narrowing conversions (ES.46)
• No C-style casts (ES.48)
• Single-argument constructors are explicit (C.46)
• Rule of Zero or Rule of Five applied (C.20, C.21)
• Base class destructors are public virtual or protected non-virtual (C.35)
• Templates are constrained with concepts (T.10)
• No using namespace in headers at global scope (SF.7)
• Headers have include guards and are self-contained (SF.8, SF.11)
• Locks use RAII (scoped_lock/lock_guard) (CP.20)
• Exceptions are custom types, thrown by value, caught by reference (E.14, E.15)
• '\n' instead of std::endl (SL.io.50)
• No magic numbers (ES.45)

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