C++11 use in Chromium

This document lives at src/styleguide/c++/c++11.html in a Chromium checkout.

This document summarizes the features of C++11 (both in the language itself and in enhancements to the Standard Library) and describes which features are allowed in Chromium and contains pointers to more detailed information. The Guide applies to Chromium and its subprojects. Subprojects can choose to be more restrictive if they need to compile on more toolchains than Chromium.

You can propose to make a feature available or to ban a feature by sending an email to cxx@chromium.org. Ideally include a short blurb on what the feature is, and why you think it should or should not be allowed. Ideally, the list will arrive at some consensus and the wiki page will be updated to mention that consensus. If there's no consensus, src/styleguide/c++/OWNERS get to decide -- for divisive features, we expect the decision to be to not use the feature yet and possibly discuss it again a few months later, when we have more experience with the language.

Table of Contents

  1. Allowed Features
    1. Language
    2. Library
  2. Banned Features
    1. Language
    2. Library
  3. To Be Discussed
    1. Language
    2. Library

C++11 Allowed Features

The following features are currently allowed.

Feature Snippet Description Documentation Link Notes and Discussion Thread
Aliases using new_alias = typename Allow parameterized typedefs Type alias (using syntax) Use instead of typedef, unless the header needs to be compatible with C. Discussion thread
Angle Bracket Parsing in Templates >> for > >, <:: for < :: More intuitive parsing of template parameters C++ Templates Angle Brackets Pitfall Recommended to increase readability. Approved without discussion.
Arrays std::array A fixed-size replacement for built-in arrays, with STL support std::array Useful in performance-critical situations, with small, fixed-size arrays. In most cases, consider std::vector instead. std::vector is cheaper to std::move and is more widely used. Discussion thread.
Automatic Types auto Automatic type deduction auto specifier Google Style Guide. Discussion thread. Another discussion thread.
Constant Expressions constexpr Compile-time constant expressions constexpr specifier Prefer to const for variables where possible. Use cautiously on functions. Don't go out of the way to convert existing code. Google Style Guide
Declared Type Accessor decltype(expression) Provides a means to determine the type of an expression at compile-time, useful most often in templates. decltype specifier Usage should be rare. Discussion thread
Default Function Creation Function(arguments) = default; Instructs the compiler to generate a default version of the indicated function What's the point in defaulting functions in C++11? Discussion thread
Default Function Template Arguments template <typename T = type>
type Function(T var) {}
Allow function templates, like classes, to have default arguments Default Template Arguments for Function Templates Discussion thread
Delegated Constructors Class() : Class(0) {}
Class(type var) : Class(var, 0) {}
Allow overloaded constructors to use common initialization code Introduction to the C++11 feature: delegating constructors Discussion thread
Enumerated Type Classes and Enum Bases enum class classname
enum class classname : base-type
enum enumname : base-type
Provide enums as full classes, with no implicit conversion to booleans or integers. Provide an explicit underlying type for enum classes and regular enums. enum-class Enum classes are still enums and follow enum naming rules (which means SHOUTY_CASE in the Chromium Style Guide). Discussion thread
Explicit Conversion Operators explicit operator type() { ... } Allows conversion operators that cannot be implicitly invoked explicit specifier Prefer to the "safe bool" idiom. Discussion thread
Final Specifier final Indicates that a class or function is final and cannot be overridden final Language Reference Recommended for new code. Discussion thread
Function Suppression Function(arguments) = delete; Suppresses the implementation of a function, especially a synthetic function such as a copy constructor TODO: documentation link Discussion thread
Lambda Expressions [captures](params) -> ret { body } Anonymous functions Lambda functions Do not bind or store lambdas; use base::Bind and base::Callback instead, because they offer protection against a large class of object lifetime mistakes. Don't use default captures ([=], [&]Google Style Guide). Lambdas are typically useful as a parameter to methods or functions that will use them immediately, such as those in <algorithm>. Discussion thread
Local Types as Template Arguments void func() {
  class Pred {
    bool operator()(const T&) { ... }
  std::remove_if(vec.begin(), vec.end(), Pred());
Allows local and unnamed types as template arguments Local types, types without linkage and unnamed types as template arguments Usage should be rare. Approved without discussion.
Non-Static Class Member Initializers class C {
  type var = value;
  C()  // copy-initializes var
Allows non-static class members to be initialized at their definitions (outside constructors) Non-static data members Discussion thread
Null Pointer Constant nullptr Declares a type-safe null pointer nullptr Prefer over NULL or 0. Discussion thread. Google Style Guide. std::nullptr_t can be used too.
Override Specifier override Indicates that a class or function overrides a base implementation override Language Reference Recommended for new code. Discussion thread
Range-Based For Loops for (type var : range) Facilitates a more concise syntax for iterating over the elements of a container (or a range of iterators) in a for loop Range-based for loop As a rule of thumb, use for (const auto& ...), for (auto& ...), or for (concrete type ...). For pointers, use for (auto* ...) to make clear that the copy of the loop variable is intended, and only a pointer is copied. Discussion thread
Rvalue References T(T&& t) and T& operator=(T&& t)

template <typename T>
void Function(T&& t) { ... }
Reference that only binds to a temporary object Rvalue references As per the Google Style Guide: Only use these to define move constructors and move assignment operators, and for perfect forwarding. Most classes should not be copyable, even if movable. Continue to use DISALLOW_COPY_AND_ASSIGN (or DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND if needed) in most cases. Discussion thread. Another discussion thread.
Standard Integers Typedefs within <stdint.h> and <inttypes> Provides fixed-size integers independent of platforms <stdint.h> (cstdint) Already in common use in the codebase. Approved without discussion.
Static Assertions static_assert(bool, string) Tests compile-time conditions Static Assertion Discussion thread
Trailing Return Types auto function declaration -> return_type Allows trailing function return value syntax Declaring functions Use only where it considerably improves readability. Discussion thread. Another discussion thread.
Uniform Initialization Syntax type name {[value ..., value]}; Allows any object of primitive, aggregate or class type to be initialized using brace syntax Uniform initialization syntax and semantics See the Chromium C++ Dos And Don'ts guidance on when to use this. Discussion thread
Variadic Macros #define MACRO(...) Impl(args, __VA_ARGS__) Allows macros that accept a variable number of arguments Are Variadic macros nonstandard? Usage should be rare. Discussion thread
Variadic Templates template <typename ... arg> Allows templates that accept a variable number of arguments Parameter pack Usage should be rare. Use instead of .pump files. Discussion thread

C++11 Allowed Library Features

The following library features are currently allowed.

Feature or Library Snippet Description Documentation Link Notes
Access to underlying std::vector data v.data() Returns a pointer to a std::vector's underlying data, accounting for empty vectors. std::vector::data Discussion thread
Algorithms All C++11 features in <algorithm>:
all_of, any_of, none_of
copy_if, copy_n
move, move_backward (see note)
is_partitioned, partition_copy, partition_point
is_sorted, is_sorted_until
is_heap, is_heap_until
minmax, minmax_element
Safe and performant implementations of common algorithms <algorithm> Discussion thread
Note that <algorithm> contains a range-based move method. This is allowed, but because people may confuse it with the single-arg std::move, there is often a way to write code without it that is more readable. Discussion thread
Begin and End Non-Member Functions std::begin(), std::end() Allows use of free functions on any container, including fixed-size arrays std::begin, std::end Useful for fixed-size arrays. Note that non-member cbegin() and cend() are not available until C++14. Discussion thread
Conditional Type Selection std::enable_if, std::conditional Enables compile-time conditional type selection std::enable_if, conditional Usage should be rare. Discussion thread
Constant Iterator Methods on Containers E.g. std::vector::cbegin(), std::vector::cend() Allows more widespread use of const_iterator std::vector::cbegin, std::vector::cend Applies to all containers, not just vector. Consider using const_iterator over iterator where possible for the same reason as using const variables and functions where possible; see Effective Modern C++ item 13 for motivation. Discussion thread
Containers containing movable types vector<scoped_ptr> Enables containers that contain move-only types like scoped_ptr TODO Prefer over ScopedVector.
Declared Type As Value std::declval<class>() Converts a type to a reference of the type to allow use of members of the type without constructing it in templates. std::declval Usage should be rare. Discussion thread
Emplacement methods for containers emplace(), emplace_back(), emplace_front(), emplace_hint() Constructs elements directly within a container without a copy or a move. Less verbose than push_back() due to not naming the type being constructed. E.g. std::vector::emplace_back std::map::emplace() is not yet available on all libstdc++ versions we support. When using emplacement for performance reasons, your type should probably be movable (since e.g. a vector of it might be resized); given a movable type, then, consider whether you really need to avoid the move done by push_back(). For readability concerns, treat like auto; sometimes the brevity over push_back() is a win, sometimes a loss. Discussion thread
Forwarding references std::forward() Perfectly forwards arguments (including rvalues) std::forward Allowed, though usage should be rare (primarily for forwarding constructor arguments, or in carefully reviewed library code). Discussion thread
Initializer Lists std::initializer_list<T> Allows containers to be initialized with aggregate elements std::initializer_list Be cautious adding new constructors which take these, as they can hide non-initializer_list constructors in undesirable ways; see Effective Modern C++ Item 7 for more.
Math functions All C++11 features in <cmath>, e.g.:
float_t, double_t
fmax, fmin, trunc, round
isinf, isnan
Useful for math-related code <cmath> Discussion thread
Move Iterator Adaptor std::make_move_iterator() Wraps an iterator so that it moves objects instead of copying them. std::make_move_iterator Useful to move objects between containers that contain move-only types like scoped_ptr. Discussion thread
Move Semantics std::move() Facilitates efficient move operations std::move Discussion thread
String Direct Reference Functions std::string::front(), std::string::back() Returns a reference to the front or back of a string std::basic_string::front, std::basic_string::back Discussion thread
Type Traits All C++11 features in <type_traits> except for aligned storage (see separate item), e.g.:
is_floating_point, is_rvalue_reference, is_scalar
is_const, is_pod, is_unsigned
is_default_constructible, is_move_constructible, is_copy_assignable
enable_if, conditional, result_of
Allows compile-time inspection of the properties of types <type_traits> Note that not all type traits are available on all platforms (e.g. std::underlying_type doesn't work in libstdc++4.6). Use judiciously. Discussion thread
Tuples All C++11 features in <tuple>, e.g. std::tie and std::tuple. A fixed-size ordered collection of values of mixed types <tuple> Prefer std::tuple over base::Tuple.
Unordered Associative Containers std::unordered_set, std::unordered_map, std::unordered_multiset, std::unordered_multimap Allows efficient containers of key/value pairs std::unordered_map, std::unordered_set Per the Google Style Guide, specify custom hashers instead of specializing std::hash for custom types. Discussion thread.
Unique Pointers std::unique_ptr<type> A smart pointer with sole ownership of the owned object. std::unique_ptr Use in all newly written code. Google Style Guide. Discussion thread. scoped_ptr is a typedef for std::unique_ptr and is going away shortly.

C++11 Blacklist (Disallowed and Banned Features)

This section lists features that are not allowed to be used yet.

C++11 Banned Features

This section lists C++11 features that are not allowed in the Chromium codebase.

Feature or Library Snippet Description Documentation Link Notes
Alignment Features alignas specifier, alignof operator Object alignment alignas, alignof Reevaluate now that MSVS2015 is available. Discussion thread
Function Local Variable __func__ Provides a local variable of the name of the enclosing function for logging purposes The __func__ Predeclared Identifier is Coming to C++ Reevaluate now that MSVS2015 is available. Discussion thread
Inherited Constructors class Derived : Base {
  using Base::Base;
Allow derived classes to inherit constructors from base classes Using-declaration Reevaluate now that MSVS2015 is available. Discussion thread
long long Type long long var= value; An integer of at least 64 bits Fundamental types Use a stdint.h type if you need a 64bit number. Discussion thread
Raw String Literals string var=R"(raw_string)"; Allows a string to be encoded without any escape sequences, easing parsing in regex expressions, for example string literal Causes incorrect line numbers in MSVS2013 and gcc. Reevaluate once that works. Discussion thread
UTF-16 and UTF-32 Support (16-Bit and 32-Bit Character Types) char16_t and char32_t Provides character types for handling 16-bit and 32-bit code units (useful for encoding UTF-16 and UTF-32 string data) Fundamental types Reevaluate now that MSVS2015 is available. Non-UTF-8 text is banned by the Google Style Guide. However, may be useful for consuming non-ASCII data. Discussion thread
UTF-8, UTF-16, UTF-32 String Literals u8"string", u"string", U"string" Enforces UTF-8, UTF-16, UTF-32 encoding on all string literals string literal Reevaluate now that MSVS2015 is available. Discussion thread
Ref-qualified Member Functions class T {
  void f() & {}
  void f() && {}
t.f();  // first
T().f();  // second
std::move(t).f();  // second
Allows class member functions to only bind to |this| as an rvalue or lvalue. Member functions Banned in the Google Style Guide. Banned in Chromium except by explicit approval from styleguide/c++/OWNERS. Discussion Thread

C++11 Banned Library Features

This section lists C++11 library features that are not allowed in the Chromium codebase.

Feature Snippet Description Documentation Link Notes
Chrono Library <chrono> Provides a standard date and time library Date and time utilities Duplicated Time APIs in base/. Keep using the base/ classes.
Regex Library <regex> Provides a standard regular expressions library Regular expressions library We already have too many regular expression libraries in Chromium. Use re2 when in doubt.
Thread Library <thread> support, including
<future>, <mutex>, <condition_variable>
Provides a standard mulitthreading library using std::thread and associates Thread support library C++11 has all kinds of classes for threads, mutexes, etc. Since we already have good code for this in base/, we should keep using the base classes, at least at first. base::Thread is tightly coupled to MessageLoop which would make it hard to replace. We should investigate using standard mutexes, or unique_lock, etc. to replace our locking/synchronization classes.
Atomics std::atomic and others in <atomic> Fine-grained atomic types and operations <atomic> Use in tcmalloc has caused performance regressions. Banned until we understand this better. Discussion Thread
Shared Pointers std::shared_ptr Allows shared ownership of a pointer through reference counts std::shared_ptr Needs a lot more evaluation for Chromium, and there isn't enough of a push for this feature. Google Style Guide. Discussion Thread.

C++11 Features To Be Discussed

The following C++ language features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections. Note that not all of these features work in all our compilers yet.

Feature Snippet Description Documentation Link Notes
Attributes [[attribute_name]] Attaches properties to declarations that specific compiler implementations may use. C++11 generalized attributes
Exception Features noexcept, exception_ptr, current_exception(), rethrow_exception, nested_exception Enhancements to exception throwing and handling std::exception Exceptions are banned by the Google Style Guide. Discussion thread
Inline Namespaces inline Allows better versioning of namespaces Namespaces Unclear how it will work with components
Union Class Members union name {class var} Allows class type members Union declarations
User-Defined Literals type var = literal_value_type Allows user-defined literal expressions User-defined literals

C++11 Standard Library Features To Be Discussed

The following C++ library features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections. Note that not all of these features work in all our compilers yet.

Feature Snippet Description Documentation Link Style Guide Usage
Address Retrieval std::addressof() Obtains the address of an object even with overloaded operator& std::addressof Usage should be rare as operator overloading is rare and & should suffice in most cases. May be preferable over & for performing object identity checks.
Aligned Storage std::aligned_storage<Size, Align>::type
std::alignment_of<T>, std::aligned_union<Size, ...Types> std::max_align_t
Declare uninitialized storage having a specified alignment, or determine alignments. std::aligned_storage std::aligned_storage and std::aligned_union are disallowed in google3 over concerns about compatibility with internal cross-compiling toolchains.
Allocator Traits std::allocator_traits Provides an interface for accessing custom allocators std::allocator_traits Usage should be rare.
Bind Operations std::bind(function, args, ...) Declares a function object bound to certain arguments TODO: documentation link
C Floating-Point Environment <cfenv>, <fenv.h> Provides floating point status flags and control modes for C-compatible code Standard library header <cfenv> Banned in Google Style Guide due to concerns about compiler support.
Complex Inverse Trigonometric and Hyperbolic Functions Functions within <complex> Adds inverse trigonomentric and hyperbolic non-member functions to the <complex> library. std::complex
Container Compaction Functions std::vector::shrink_to_fit(), std::deque::shrink_to_fit(), std::string::shrink_to_fit() Requests the removal of unused space in the container std::vector::shrink_to_fit, std::deque::shrink_to_fit, std::basic_string::shrink_to_fit
Date/Time String Formatting Specifiers std::strftime() Converts date and time information into a formatted string using new specifiers std::strftime
Function Return Type Deduction std::result_of<Functor(ArgTypes...)> Extracts the return type from the type signature of a function call invocation at compile-time. std::result_of Why does std::result_of take an (unrelated) function type as a type argument?
Function Objects std::function Wraps a standard polymorphic function TODO: documentation link
Forward Lists std::forward_list Provides an efficient singly linked list std::forward_list
Gamma Natural Log std::lgamma() Computes the natural log of the gamma of a floating point value std::lgamma
Garbage Collection Features std::{un}declare_reachable(), std::{un}declare_no_pointers() Enables garbage collection implementations std::declare_reachable, std::declare_no_pointers
Iterator Operators std::next(), std::prev() Copies an iterator and increments or decrements the copy by some value std::next, std::prev
Pointer Traits Class Template std::pointer_traits Provides a standard way to access properties of pointers and pointer-like types std::pointer_traits Useful for determining the element type pointed at by a (possibly smart) pointer.
Random Number Generators Functions within <random> Random number generation algorithms and utilities Pseudo-random number generation
Ratio Template Class std::ratio<numerator, denominator> Provides compile-time rational numbers std::ratio Banned in Google Style Guide due to concerns that they are tied to a more template-heavy interface style.
Reference Wrapper Classes std::reference_wrapper and std::ref(), std::cref() Allows you to wrap a reference within a standard object (and use those within containers) Reference Wrappers
Soft Program Exits std::at_quick_exit(), std::quick_exit() Allows registration of functions to be called upon exit, allowing cleaner program exit than abort() or exit std:quick_exit
String to Number Functions std::stoi(), std::stol(), std::stoul(), std::stoll, std::stoull(), std::stof(), std::stod(), std::stold(), std::to_string() Converts strings to numbers std::stoi, std::stol, std::stoll, std::stoul, std::stoull, std::stof, std::stod, std::stold
System Errors <system_error> Provides a standard system error library std::system_error
Type-Generic Math Functions Functions within <ctgmath> Provides a means to call real or complex functions based on the type of arguments Standard library header <ctgmath>
Type Info Enhancements std::type_index, std::type_info::hash_code() Allows type information (most often within containers) that can be copied, assigned, or hashed std::type_index, std::type_info::hash_code std::type_index is a thin wrapper for std::type_info, allowing you to use it directly within both associative and unordered containers
Variadic Copy Macro va_copy(va_list dest, va_list src) Makes a copy of the variadic function arguments
Weak Pointers std::weak_ptr Allows a weak reference to a std::shared_ptr std::weak_ptr Ownership and Smart Pointers
Wide String Support std::wstring_convert, std::wbuffer_convert
std::codecvt_utf8, std::codecvt_utf16, std::codecvt_utf8_utf16
Converts between string encodings std::wstring_convert, std::wbuffer_convert, std::codecvt_utf8, std::codecvt_utf16, std::codecvt_utf8_utf16 Non-UTF-8 text is banned by the Google Style Guide. However, may be useful for consuming non-ASCII data.