C++11 use in Chromium

This document lives at src/styleguide/c++/c++11.html in a Chromium checkout and is part of the more general Chromium C++ style guide.

This summarizes the new and updated features in C++11 (for both the language itself and the Standard Library) from the perspective of what's allowed in Chromium. When applicable, it contains pointers to more detailed information. This Guide applies to Chromium and its subprojects, though subprojects can choose to be more restrictive if necessary for toolchain support.

You can propose changing the status of a feature by sending an email to cxx@chromium.org. Include a short blurb on what the feature is and why you think it should or should not be allowed, along with links to any relevant previous discussion. If the list arrives at some consensus, send a codereview to change this file accordingly, linking to your discussion thread.

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
__func__ Local Variable __func__ Provides a local variable containing the name of the enclosing function __func__ Use instead of the non-standard __FUNCTION__. 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. Discussion thread
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. enumeration declaration 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 specifier 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 Deleted functions Discussion thread
Inherited Constructors class Derived : Base {
  using Base::Base;
};
Allows derived classes to inherit constructors from base classes Inheriting constructors Discussion thread
Lambda Expressions [captures](params) -> ret { body } Anonymous functions Lambda functions Lambdas are typically useful as a parameter to methods or functions that will use them immediately, such as those in <algorithm>. Be careful with default captures ([=], [&]), and do not bind or store any capturing lambdas outside the lifetime of the stack frame they are defined in; use base::Callback for this instead, as it helps prevent object lifetime mistakes. (Captureless lambdas can be used with base::Bind as they do not have the same risks.) Google Style Guide. Discussion thread. Another discussion thread (about captureless lambdas and base::Bind).
Local Types as Template Arguments void func() {
  class Pred {
   public:
    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.
Noexcept Specifier void f() noexcept Specifies that a function will not throw exceptions noexcept specifier Chromium compiles without exception support, but there are still cases where explicitly marking a function as noexcept may be necessary to compile, or for performance reasons. Usage should be rare. Discussion thread
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, the pointer literal Prefer over NULL or 0. Google Style Guide. Discussion thread
Override Specifier override Indicates that a class or function overrides a base implementation override specifier 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 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 in most cases. Google Style Guide. Discussion thread. Another discussion thread
Standard Integers Typedefs within <stdint.h> and <inttypes> Provides fixed-size integers independent of platforms Standard library header <cstdint> Approved without discussion. Required by the Google Style Guide.
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
Type Aliases ("using" instead of "typedef") using new_alias = typename Allows parameterized typedefs Type alias, alias template Use instead of typedef, unless the header needs to be compatible with C. 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
Union Class Members union name {class var} Allows class type members Union declaration Usage should be rare. 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 and Discussion Thread
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
find_if_not
copy_if, copy_n
move, move_backward (see note)
shuffle
is_partitioned, partition_copy, partition_point
is_sorted, is_sorted_until
is_heap, is_heap_until
minmax, minmax_element
is_permutation
Safe and performant implementations of common algorithms Standard library header <algorithm> 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. Another 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. Discussion thread
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. Discussion thread. Another discussion thread
Math functions All C++11 features in <cmath>, e.g.:
INFINITY, NAN, FP_NAN
float_t, double_t
fmax, fmin, trunc, round
isinf, isnan
Useful for math-related code Standard library header <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 std::unique_ptr. Discussion thread
Move Semantics std::move() Facilitates efficient move operations std::move Discussion thread
Null Pointer Type std::nullptr_t Allows referencing the type of nullptr, e.g. in templates std::nullptr_t Discussion thread
Random Number Generators <random> Random number generation algorithms and utilities Pseudo-random number generation Because the standard does not define precisely how the xxx_distribution objects generate output, the same object may produce different output for the same seed across platforms, or even across different versions of the STL. Do not use these objects in any scenario where behavioral consistency is required. 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 Beware of passing these as macro arguments, which can trigger a lexer bug on older GCC versions. 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.:
integral_constant
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 Standard library header <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 Standard library header <tuple> libstdc++ 4.6 has a bug where std::get can return an lvalue-reference when it should return an rvalue-reference. base::get can be used to avoid this. Discussion thread. Another discussion thread
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 Specify custom hashers instead of specializing std::hash for custom types. Google Style Guide. Discussion thread
Unique Pointers std::unique_ptr<type> A smart pointer with sole ownership of the owned object. std::unique_ptr Google Style Guide. Discussion thread

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 and Discussion Thread
Inline Namespaces inline namespace foo { ... } Allows better versioning of namespaces Inline namespaces Banned in the Google Style Guide. Unclear how it will work with components.
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
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. const-, volatile-, and ref-qualified member functions Banned in the Google Style Guide. May only be used in Chromium with explicit approval from styleguide/c++/OWNERS. Discussion Thread
User-Defined Literals type var = literal_value_type Allows user-defined literal expressions User-defined literals Banned in the Google Style Guide.

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 and Discussion Thread
Atomics <atomic> Fine-grained atomic types and operations Atomic operations library Use in tcmalloc has caused performance regressions. Banned until we understand this better. Discussion Thread
Bind Operations std::bind(function, args, ...) Declares a function object bound to certain arguments std::bind Use base::Bind instead. Compared to std::bind, base::Bind helps prevent lifetime issues by preventing binding of capturing lambdas and by forcing callers to declare raw pointers as Unretained. Discussion thread
C Floating-Point Environment <cfenv>, <fenv.h> Provides floating point status flags and control modes for C-compatible code Standard library header <cfenv> Banned by the Google Style Guide due to concerns about compiler support.
Date and time utilities <chrono> A standard date and time library Date and time utilities Overlaps with Time APIs in base/. Keep using the base/ classes.
Exceptions <exception> Enhancements to exception throwing and handling Standard library header <exception> Exceptions are banned by the Google Style Guide and disabled in Chromium compiles. Note that the noexcept specifier is explicitly allowed above. Discussion thread
Function Objects std::function Wraps a standard polymorphic function std::function Use base::Callback instead. Compared to std::function, base::Callback directly supports Chromium's refcounting classes and weak pointers and deals with additional thread safety concerns. Discussion thread
Ratio Template Class std::ratio<numerator, denominator> Provides compile-time rational numbers std::ratio Banned by the Google Style Guide due to concerns that this is tied to a more template-heavy interface style.
Regular Expressions <regex> A standard regular expressions library Regular expressions library Overlaps with many regular expression libraries in Chromium. When in doubt, use re2.
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
Thread Library <thread> and related headers, including
<future>, <mutex>, <condition_variable>
Provides a standard multithreading library using std::thread and associates Thread support library Overlaps with many classes in base/. Keep using the base/ classes for now. 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.

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.

Feature Snippet Description Documentation Link Notes and Discussion Thread
Alignment Features alignas specifier, alignof operator Object alignment alignas specifier, alignof operator Reevaluate now that MSVS2015 is available. Discussion thread
Attributes [[attribute_name]] Attaches properties to declarations that specific compiler implementations may use. C++11 generalized attributes
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
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

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.

Feature Snippet Description Documentation Link Notes
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.
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?
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.
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) std::reference_wrapper
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-Number Conversion Functions std::stoi(), std::stol(), std::stoul(), std::stoll, std::stoull(), std::stof(), std::stod(), std::stold(), std::to_string() Converts strings to/from numbers std::stoi, std::stol, std::stoll, std::stoul, std::stoull, std::stof, std::stod, std::stold, std::to_string May be useful to replace dmg_fp.
System Errors <system_error> Provides a standard system error library Standard library header <system_error>
Type-Generic Math Functions <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 va_copy
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.