This document is part of the more general Chromium C++ style guide. It summarizes the supported state of new and updated language and library features in recent C++ standards and the Abseil library. This guide applies to both Chromium and its subprojects, though subprojects can choose to be more restrictive if necessary for toolchain support.
The C++ language has in recent years received an updated standard every three years (C++11, C++14, etc.). For various reasons, Chromium does not immediately allow new features on the publication of such a standard. Instead, once Chromium supports the toolchain to a certain extent (e.g., build support is ready), a standard is declared "initially supported", with new language/library features banned pending discussion but not yet allowed.
You can propose changing the status of a feature by sending an email to [email protected]. 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.
If an item remains on the TBD list two years after initial support is added, style arbiters should explicitly move it to an appropriate allowlist or blocklist, allowing it if there are no obvious reasons to ban.
The current status of existing standards and Abseil features is:
- C++11: Default allowed; see banned features below
- C++14: Default allowed
- C++17: Initially supported December 23, 2021; see allowed/banned/TBD features below
- C++20: Initially supported November 13, 2023; see allowed/banned/TBD features below
- C++23: Not yet officially standardized
- Abseil: Default allowed; see banned/TBD features below. The following
dates represent the start of the two-year TBD periods for certain parts of
Abseil:
- absl::AnyInvocable: Initially added to third_party June 20, 2022
- Log library: Initially added to third_party Aug 31, 2022
- CRC32C library: Initially added to third_party Dec 5, 2022
- Nullability annotation: Initially added to third_party Jun 21, 2023
- Overload: Initially added to third_party Sep 27, 2023
- NoDestructor: Initially added to third_party Nov 15, 2023
[TOC]
The following C++11 language features are not allowed in the Chromium codebase.
inline namespace foo { ... }Description: Allows better versioning of namespaces.
Documentation: Inline namespaces
Notes: *** promo Banned in the Google Style Guide. Unclear how it will work with components.
long long var = value;Description: An integer of at least 64 bits.
Documentation: Fundamental types
Notes:
*** promo
Use a <stdint.h> type if you need a 64-bit number.
Discussion thread
DistanceType var = 12_km;Description: Allows user-defined literal expressions.
Documentation: User-defined literals
Notes: *** promo Banned in the Google Style Guide.
The following C++11 library features are not allowed in the Chromium codebase.
#include <cctype>
#include <cwctype>
#include <ctype.h>
#include <wctype.h>Description: Provides utilities for ASCII characters.
Documentation:
Standard library header <cctype>,
Standard library header <cwctype>
Notes:
*** promo
Banned due to dependence on the C locale as well as UB when arguments don't fit
in an unsigned char/wchar_t. Use similarly-named replacements in
third_party/abseil-cpp/absl/strings/ascii.h
instead.
#include <cfenv>
#include <fenv.h>Description: Provides floating point status flags and control modes for C-compatible code.
Documentation:
Standard library header <cfenv>
Notes: *** promo Banned by the Google Style Guide due to concerns about compiler support.
#include <chrono>Description: A standard date and time library.
Documentation: Date and time utilities
Notes:
*** promo
Overlaps with base/time. Keep using the base/time classes.
#include <exception>Description: Exception throwing and handling.
Documentation:
Standard library header <exception>
Notes:
*** promo
Exceptions are banned by the
Google Style Guide
and disabled in Chromium compiles. However, the noexcept specifier is
explicitly allowed.
std::mt19937 generator;Description: Methods of generating random numbers.
Documentation: Pseudo-random number generation
Notes:
*** promo
Do not use any random number engines or generators from <random>. Instead, use
base::RandomBitGenerator. (You may use the distributions from <random>.)
#include <ratio>Description: Provides compile-time rational numbers.
Documentation:
std::ratio
Notes: *** promo Banned by the Google Style Guide due to concerns that this is tied to a more template-heavy interface style.
#include <regex>Description: A standard regular expressions library.
Documentation: Regular expressions library
Notes:
*** promo
Overlaps with many regular expression libraries in Chromium. When in doubt, use
third_party/re2.
auto x = std::bind(function, args, ...);Description: Declares a function object bound to certain arguments.
Documentation:
std::bind
Notes:
*** promo
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.
std::function x = [] { return 10; };
std::function y = std::bind(foo, args);Description: Wraps a standard polymorphic function.
Documentation:
std::function
Notes:
*** promo
Use base::{Once,Repeating}Callback or base::FunctionRef instead. Compared
to std::function, base::{Once,Repeating}Callback directly supports
Chromium's refcounting classes and weak pointers and deals with additional
thread safety concerns.
std::shared_ptr<int> x = std::make_shared<int>(10);Description: Allows shared ownership of a pointer through reference counts.
Documentation:
std::shared_ptr
Notes:
*** promo
Unlike base::RefCounted, uses extrinsic rather than intrinsic reference
counting. Could plausibly be used in Chromium, but would require significant
migration.
Google Style Guide, Discussion Thread
int x = std::stoi("10");Description: Converts strings to/from numbers.
Documentation:
std::stoi, std::stol, std::stoll,
std::stoul, std::stoull,
std::stof, std::stod, std::stold,
std::to_string
Notes:
*** promo
The string-to-number conversions rely on exceptions to communicate failure,
while the number-to-string conversions have performance concerns and depend on
the locale. Use base/strings/string_number_conversions.h instead.
std::weak_ptr<int> x = my_shared_x;Description: Allows a weak reference to a std::shared_ptr.
Documentation:
std::weak_ptr
Notes:
*** promo
Banned because std::shared_ptr is banned. Use base::WeakPtr instead.
#include <barrier> // C++20
#include <condition_variable>
#include <future>
#include <latch> // C++20
#include <mutex>
#include <semaphore> // C++20
#include <stop_token> // C++20
#include <thread>Description: Provides a standard multithreading library using std::thread
and associates
Documentation: Thread support library
Notes:
*** promo
Overlaps with many classes in base/synchronization. base::Thread is tightly
coupled to base::MessageLoop which would make it hard to replace. We should
investigate using standard mutexes, or unique_lock, etc. to replace our
locking/synchronization classes.
The following C++17 language features are allowed in the Chromium codebase.
template <typename T>
struct MyContainer {
MyContainer(T val) : val{val} {}
// ...
};
MyContainer c1(1); // Type deduced to be `int`.Description: Automatic template argument deduction much like how it's done for functions, but now including class constructors.
Documentation: Class template argument deduction
Notes: *** promo Usage is governed by the Google Style Guide.
if constexpr (cond) { ...Description: Write code that is instantiated depending on a compile-time condition.
Documentation:
if statement
Notes: *** promo Discussion thread
auto identity = [](int n) constexpr { return n; };
static_assert(identity(123) == 123);Description: Compile-time lambdas using constexpr.
Documentation: Lambda expressions
Notes: *** promo Discussion thread
template <auto... seq>
struct my_integer_sequence {
// ...
};
auto seq = my_integer_sequence<0, 1, 2>(); // Type deduced to be `int`.Description: Following the deduction rules of auto, while respecting the
non-type template parameter list of allowable types, template arguments can be
deduced from the types of its arguments.
Documentation: Template parameters
Notes: *** promo Discussion thread
case 1:
DoSomething();
[[fallthrough]];
case 2:
break;Description:
The [[fallthrough]] attribute can be used in switch statements to indicate
when intentionally falling through to the next case.
Documentation:
C++ attribute: fallthrough
Notes: *** promo Discussion thread
template <typename... Args>
auto sum(Args... args) {
return (... + args);
}Description: A fold expression performs a fold of a template parameter pack over a binary operator.
Documentation: Fold expression
Notes: *** promo Discussion thread
struct S {
static constexpr int kZero = 0; // constexpr implies inline here.
};
inline constexpr int kOne = 1; // Explicit inline needed here.Description: The inline specifier can be applied to variables as well as
to functions. A variable declared inline has the same semantics as a function
declared inline. It can also be used to declare and define a static member
variable, such that it does not need to be initialized in the source file.
Documentation:
inline specifier
Notes: *** promo Inline variables in anonymous namespaces in header files will still get one copy per translation unit, so they must be outside of an anonymous namespace to be effective.
Mutable inline variables and taking the address of inline variables are banned since these will break the component build.
#if __has_include(<optional>) ...Description: Checks whether a file is available for inclusion, i.e. the file exists.
Documentation: Source file inclusion
Notes: *** promo Discussion thread
const auto l = [*this] { return member_; }Description: *this captures the current object by copy, while this
continues to capture by reference.
Documentation: Lambda capture
Notes: *** promo Discussion thread
struct [[maybe_unused]] MyUnusedThing;
[[maybe_unused]] int x;Description:
The [[maybe_unused]] attribute can be used to indicate that individual
variables, functions, or fields of a class/struct/enum can be left unused.
Documentation:
C++ attribute: maybe_unused
Notes: *** promo Discussion thread
namespace A::B::C { ...Description: Using the namespace resolution operator to create nested namespace definitions.
Documentation: Namespaces
Notes: *** promo Discussion thread
struct [[nodiscard]] ErrorOrValue;
[[nodiscard]] bool DoSomething();Description:
The [[nodiscard]] attribute can be used to indicate that
- the return value of a function should not be ignored
- values of annotated classes/structs/enums returned from functions should not be ignored
Documentation:
C++ attribute: nodiscard
Notes:
*** promo
This replaces the previous WARN_UNUSED_RESULT macro, which was a wrapper
around the compiler-specific __attribute__((warn_unused_result)).
if (int a = Func(); a < 3) { ...
switch (int a = Func(); a) { ...Description: New versions of the if and switch statements which simplify common code patterns and help users keep scopes tight.
Documentation:
if statement,
switch statement
Notes: *** promo Discussion thread
const auto [x, y] = FuncReturningStdPair();Description: Allows writing auto [x, y, z] = expr; where the type of
expr is a tuple-like object, whose elements are bound to the variables x,
y, and z (which this construct declares). Tuple-like objects include
std::tuple, std::pair, std::array, and aggregate structures.
Documentation: Structured binding declaration Explanation of structured binding types
Notes:
*** promo
This feature forces omitting type names. Its use should follow
the guidance around auto in Google C++ Style guide.
[[using CC: opt(1), debug]] // same as [[CC:opt(1), CC::debug]]Description: Specifies a common namespace for a list of attributes.
Documentation: Attribute specifier sequence
Notes:
*** promo
See similar attribute macros in base/compiler_specific.h.
The following C++17 language features are allowed in the Chromium codebase.
double dist = std::hypot(1.0, 2.5, 3.7);Description: Computes the distance from the origin in 3D space.
Documentation:
std::hypot
Notes: *** promo Discussion thread
auto it = std::search(haystack.begin(), haystack.end(),
std::boyer_moore_searcher(needle.begin(), needle.end()));Description: Alternate string searching algorithms.
Documentation: Searchers
Notes: *** promo Discussion thread
static_assert(std::apply(std::plus<>(), std::make_tuple(1, 2)) == 3);Description: Invokes a Callable object with a tuple of arguments.
Documentation:
std::apply
Notes: *** promo Discussion thread
auto&& const_ref = std::as_const(mutable_obj);Description: Forms reference to const T.
Documentation:
std::as_const
Notes: *** promo Discussion thread
template <typename T>
struct is_lock_free_impl
: std::integral_constant<bool, std::atomic<T>::is_always_lock_free> {};Description: True when the given atomic type is always lock-free.
Documentation:
std::atomic<T>::is_always_lock_free
Notes: *** promo Discussion thread
int x = std::clamp(inp, 0, 100);Description: Clamps a value between a minimum and a maximum.
Documentation:
std::clamp
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...) { ...Description: Performs logical operations on type traits.
Documentation:
std::conjunction,
std::disjunction,
std::negation
Notes: *** promo Discussion thread
std::exclusive_scan(data.begin(), data.end(), output.begin());Description: Like std::inclusive_scan but omits the current element from
the written output at each step; that is, results are "one value behind" those
of std::inclusive_scan.
Documentation:
std::exclusive_scan
Notes: *** promo Discussion thread
static_assert(std::gcd(12, 18) == 6);Description: Computes the greatest common divisor of its arguments.
Documentation:
std::gcd
Notes: *** promo Discussion thread
std::has_unique_object_representations_v<foo>Description: Checks wither the given type is trivially copyable and any two objects with the same value have the same object representation.
Documentation:
std::has_unique_object_representations
Notes: *** promo Discussion thread
std::optional<std::complex<double>> opt{std::in_place, 0, 1};Description: std::in_place is a disambiguation tag for std::optional to
indicate that the object should be constructed in-place.
Documentation:
std::in_place
Notes:
*** promo
Allowed now that std::optional is allowed.
Migration bug and
discussion thread
std::inclusive_scan(data.begin(), data.end(), output.begin());Description: Like std::accumulate but writes the result at each step into
the output range.
Documentation:
std::inclusive_scan
Notes: *** promo Discussion thread
static_assert(std::invoke(std::plus<>(), 1, 2) == 3);Description: Invokes a callable object with parameters. A callable object is
e.g. a function, function pointer, functor (that is, an object that provides
operator()), lambda, etc.
Documentation:
std::invoke
Notes: *** promo Migration bug
if constexpr(std::is_aggregate_v<T>) { ...Description: Checks wither the given type is an aggregate type.
Documentation:
std::is_aggregate
Notes: *** promo Discussion thread
std::is_invocable_v<Fn, 1, "Hello">Description: Checks whether a function may be invoked with the given
argument types. The _r variant also evaluates whether the result is
convertible to a given type.
Documentation:
std::is_invocable
Notes: *** promo Discussion thread
std::is_swappable<T>
std::is_swappable_with_v<T, U>Description: Checks whether classes may be swapped.
Documentation:
std::is_swappable
Notes: *** promo Discussion thread
struct Y { int z; };
alignas(Y) std::byte s[sizeof(Y)];
Y* q = new(&s) Y{2};
const int h = std::launder(reinterpret_cast<Y*>(&s))->z;Description: When used to wrap a pointer, makes it valid to access the resulting object in cases it otherwise wouldn't have been, in a very limited set of circumstances.
Documentation:
std::launder
Notes: *** promo Discussion thread
static_assert(std::lcm(12, 18) == 36);Description: Computes the least common multiple of its arguments.
Documentation:
std::lcm
Notes: *** promo Discussion thread
// Calls Foo(int, double):
auto foo = std::make_from_tuple<Foo>(std::make_tuple(1, 3.5));Description: Constructs an object from a tuple of arguments.
Documentation:
std::make_from_tuple
Notes: *** promo Discussion thread
std::map<...>::extract
std::map<...>::merge
std::set<...>::extract
std::set<...>::mergeDescription: Moving nodes and merging containers without the overhead of expensive copies, moves, or heap allocations/deallocations.
Documentation:
std::map::extract,
std::map::merge
Notes: *** promo Discussion thread
std::map<std::string, std::string> m;
m.insert_or_assign("c", "cherry");
m.insert_or_assign("c", "clementine");Description: Like operator[], but returns more information and does not
require default-constructibility of the mapped type.
Documentation:
std::map::insert_or_assign
Notes: *** promo Discussion thread
std::map<std::string, std::string> m;
m.try_emplace("c", 10, 'c');
m.try_emplace("c", "Won't be inserted");Description: Like emplace, but does not move from rvalue arguments if the
insertion does not happen.
Documentation:
std::map::try_emplace,
Notes: *** promo Discussion thread
auto nonwhite = std::find_if(str.begin(), str.end(), std::not_fn(IsWhitespace));Description: Creates a forwarding call wrapper that returns the negation of the callable object it holds.
Documentation:
std::not_fn
Notes: *** promo Migration bug
std::optional<std::string> s;Description: The class template std::optional manages an optional
contained value, i.e. a value that may or may not be present.
Documentation:
std::optional
Notes: *** promo Migration bug and discussion thread
char buffer[260];
memcpy(std::data(buffer), source_str.data(), std::size(buffer));
if (!std::empty(container)) { ... }Description: Non-member versions of what are often member functions on STL containers. Primarily useful when:
- using
std::size()as a replacement for the oldarraysize()macro. - writing code that needs to generically operate across things like
std::vectorandstd::list(which providesize(),empty(), anddata() member functions),std::arrayandstd::initialize_list` (which only provide a subset of the aforementioned member functions), and regular arrays (which have no member functions at all).
Documentation:
std::size,
std::empty,
std::data
Notes: *** promo Discussion thread
Prefer range-based for loops over std::size(): range-based for loops work even
for regular arrays.
std::string_view str = "foo";
std::u16string_view str16 = u"bar";Description: A non-owning reference to a string. Useful for providing an abstraction on top of strings (e.g. for parsing).
Documentation:
std::basic_string_view
Notes: *** promo Migration bug and discussion thread
bool b = std::is_same_v<int, std::int32_t>;Description: Syntactic sugar to provide convenient access to ::value
members by simply adding _v.
Documentation: Type support
Notes: *** promo Discussion thread
std::destroy(ptr, ptr + 8);
std::destroy_at(ptr);
std::destroy_n(ptr, 8);
std::uninitialized_move(src.begin(), src.end(), dest.begin());
std::uninitialized_value_construct(std::begin(storage), std::end(storage));Description: Replaces direct constructor and destructor calls when manually managing memory.
Documentation:
std::destroy,
std::destroy_at,
std::destroy_n,
std::uninitialized_move,
std::uninitialized_value_construct
Notes: *** promo Discussion thread
The following C++17 library features are not allowed in the Chromium codebase.
int* p2 = static_cast<int*>(std::aligned_alloc(1024, 1024));Description: Allocates uninitialized storage with the specified alignment.
Documentation:
std::aligned_alloc
Notes:
*** promo
Will be allowed soon; for now, use
base::AlignedAlloc.
std::any x = 5;Description: A type-safe container for single values of any type.
Documentation:
std::any
Notes: *** promo Discussion thread
Banned since workaround for lack of RTTI isn't compatible with the component
build (Bug). Also see absl::any.
#include <filesystem>Description: A standard way to manipulate files, directories, and paths in a filesystem.
Documentation: Filesystem library
Notes: *** promo Banned by the Google Style Guide.
struct SharedData {
ReadOnlyFrequentlyUsed data;
alignas(std::hardware_destructive_interference_size) std::atomic<size_t> counter;
};Description: The std::hardware_destructive_interference_size constant is
useful to avoid false sharing (destructive interference) between variables that
would otherwise occupy the same cacheline. In contrast,
std::hardware_constructive_interference_size is helpful to promote true
sharing (constructive interference), e.g. to support better locality for
non-contended data.
Documentation:
std::hardware_destructive_interference_size,
std::hardware_constructive_interference_size
Notes: *** promo Banned for now since these are not supported yet. Allow once supported. Discussion thread
std::variant<int, float> v{std::in_place_type<int>, 1.4};Description: std::in_place_type and std::in_place_index are
disambiguation tags for std::variant and std::any to indicate that the
object should be constructed in-place.
Documentation:
std::in_place_type
Notes:
*** promo
Banned for now because std::variant and std::any are banned. Because
absl::variant is used instead, and it requires absl::in_place_type, use
absl::in_place_type for non-Abseil Chromium
code. See the
discussion thread.
int count = std::uncaught_exceptions();Description: Determines whether there are live exception objects.
Documentation:
std::uncaught_exceptions
Notes: *** promo Banned because exceptions are banned.
std::variant<int, double> v = 12;Description: The class template std::variant represents a type-safe
union. An instance of std::variant at any given time holds a value of one of
its alternative types (it's also possible for it to be valueless).
Documentation:
std::variant
Notes:
*** promo
Will be allowed soon; for now, use absl::variant.
std::map<std::weak_ptr<T>, U, std::owner_less<>>Description: Function object providing mixed-type owner-based ordering of shared and weak pointers, regardless of the type of the pointee.
Documentation:
std::owner_less
Notes:
*** promo
Banned since std::shared_ptr and std::weak_ptr are banned.
auto weak_ptr = weak_from_this();Description: Returns a std::weak_ptr<T> that tracks ownership of *this
by all existing std::shared_ptrs that refer to *this.
Documentation:
std::enable_shared_from_this<T>::weak_from_this
Notes:
*** promo
Banned since std::shared_ptr and std::weak_ptr are banned.
The following C++17 language features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
char x = u8'x'; // C++17
char8_t x = u8'x'; // C++20Description: A character literal that begins with u8 is a character
literal of type char (C++17) or char8_t (C++20). The value of a UTF-8
character literal is equal to its ISO 10646 code point value.
Documentation: Character literal
Notes: *** promo None
The following C++17 library features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
std::assoc_laguerre()
std::assoc_legendre()
std::beta()
std::comp_ellint_1()
std::comp_ellint_2()
std::comp_ellint_3()
std::cyl_bessel_i()
std::cyl_bessel_j()
std::cyl_bessel_k()
std::cyl_neumann()
std::ellint_1()
std::ellint_2()
std::ellint_3()
std::expint()
std::hermite()
std::legendre()
std::laguerre()
std::riemann_zeta()
std::sph_bessel()
std::sph_legendre()
std::sph_neumann()Description: A variety of mathematical functions.
Documentation: Mathematical special functions
Notes: *** promo May not be supported in libc++, according to the library features table
auto it = std::find(std::execution::par, std::begin(vec), std::end(vec), 2);Description: Many of the STL algorithms, such as the copy, find and
sort methods, now support the parallel execution policies: seq, par, and
par_unseq which translate to "sequentially", "parallel" and
"parallel unsequenced".
Notes: *** promo May not be supported in libc++, according to the library features table
std::byte b = 0xFF;
int i = std::to_integer<int>(b); // 0xFFDescription: A standard way of representing data as a byte. std::byte is
neither a character type nor an arithmetic type, and the only operator overloads
available are bitwise operations.
Documentation:
std::byte
Notes: *** promo No current consensus; see discussion thread.
#include <memory_resource>Description: Manages memory allocations using runtime polymorphism.
Documentation:
std::pmr::memory_resource,
std::pmr::polymorphic_allocator
Notes: *** promo May not be supported in libc++, according to the library features table
std::reduce(std::execution::par, v.cbegin(), v.cend());Description: Like std::accumulate except the elements of the range may be
grouped and rearranged in arbitrary order.
Documentation: std::reduce
Notes:
*** promo
Makes the most sense in conjunction with std::execution::par.
std::timespec ts;
std::timespec_get(&ts, TIME_UTC);Description: Gets the current calendar time in the given time base.
Documentation:
std::timespec_get
Notes: *** promo None
std::from_chars(str.data(), str.data() + str.size(), result);
std::to_chars(str.data(), str.data() + str.size(), 42);Description: Locale-independent, non-allocating, non-throwing functions to convert values from/to character strings, designed for use in high-throughput contexts.
Documentation:
std::from_chars
std::to_chars,
Notes: *** promo None
The following C++20 language features are allowed in the Chromium codebase.
// template <typename T>
// void f1(T x);
void f1(auto x);
// template <C T> // `C` is a concept
// void f2(T x);
void f2(C auto x);
// template <typename T, C U> // `C` is a concept
// void f3(T x, U y);
template <typename T>
void f3(T x, C auto y);
// template<typename... Ts>
// void f4(Ts... xs);
void f4(auto... xs);Description: Function params of type auto become syntactic sugar for
declaring a template type for each such parameter.
Documentation: Abbreviated function template
Notes: *** promo Migration bug
consteval int sqr(int n) { return n * n; }
constexpr int kHundred = sqr(10); // OK
constexpr int quad(int n) { return sqr(sqr(n)); } // ERROR, might be runtimeDescription: Specified that a function may only be used in a compile-time context.
Documentation:
consteval specifier
Notes: *** promo None
// `Hashable` is a concept satisfied by any type `T` for which the expression
// `std::hash<T>{}(a)` compiles and produces a value convertible to `size_t`.
template<typename T>
concept Hashable = requires(T a)
{
{ std::hash<T>{}(a) } -> std::convertible_to<size_t>;
};
template <Hashable T> // Only instantiable for `T`s that satisfy `Hashable`.
void f(T) { ... }Description: Allows bundling sets of requirements together as named concepts, then enforcing them on template arguments.
Documentation: Constraints and concepts
Notes: *** promo Migration bug
struct S : public T {
bool operator==(const S&) const = default; // Compares `T` bases, then `x`,
// then `y`, short-circuiting.
int x;
bool y;
};Description: Requests that the compiler generate the implementation of any
comparison operator, including <=>. Defaulting <=> and not declaring ==
implicitly defaults ==, which together are sufficient to allow any comparison
as long as callers do not need to take the address of any non-declared operator.
Documentation: Default comparisons
Notes: *** promo Migration bug
struct S { int x = 1; int y = 2; }
S s{ .y = 3 }; // OK, s.x == 1, s.y == 3Description: Allows explicit initialization of subsets of aggregate members at construction.
Documentation: Designated initializers
Notes: *** promo None
#if __has_cpp_attribute(assume) // Toolchain supports C++23 `[[assume]]`.
...
#endifDescription: Checks whether the toolchain supports a particular standard attribute.
Documentation: Feature testing
Notes: *** promo None
constinit int x = 3;
void foo() {
++x;
}Description: Ensures that a variable can be compile-time initialized. This
is like a milder form of constexpr that does not force variables to be const
or have constant destruction.
Documentation:
constinit specifier
Notes: *** promo Migration bug
struct S {
uint32_t x : 27 = 2;
};Description: Allows specifying the default initial value of a bit-field member, as can already be done for other member types.
Documentation: Bit-field
Notes: *** promo None
template <typename... Args>
void foo(Args... args) {
const auto l = [...n = args] { (x(n), ...); };
}Description: Allows initializing a capture with a pack expansion.
Documentation: Lambda capture
Notes: *** promo None
#if !defined(__cpp_modules) || (__cpp_modules < 201907L)
... // Toolchain does not support modules
#endifDescription: Provides a standardized way to test the toolchain's implementation of a particular language feature.
Documentation: Feature testing
Notes: *** promo None
T foo();
...
for (auto& x : foo().items()) { ... } // UAF before C++23!
for (T thing = foo(); auto& x : thing.items()) { ... } // OKDescription: Like C++17's selection statements with initializer. Particularly useful before C++23, since temporaries inside range-expressions are not lifetime-extended until the end of the loop before C++23.
Documentation:
Range-based for loop
Notes: *** promo Migration bug
// `ordering` is an instance of `std::strong_odering` or `std::partial_ordering`
// that describes how `a` and `b` are related.
const auto ordering = a <=> b;
if (ordering < 0) { ... } // `a` < `b`
else if (ordering > 0) { ... } // `a` > `b`
else { ... } // `a` == `b`Description: Compares two objects in a fashion similar to strcmp. Perhaps
most useful when defined as an overload in a class, in which case it can replace
definitions of other inequalities. See also "Default comparisons".
Documentation: Three-way comparison
Notes: *** promo Migration bug
The following C++20 library features are allowed in the Chromium codebase.
#include <bit>Description: Provides various byte- and bit-twiddling functions, e.g. counting leading zeros.
Documentation:
Standard library header <bit>
Notes: *** promo Migration bug
#include <compare>Description: Concepts and classes used to implement three-way comparison
("spaceship", <=>) support.
Documentation:
Standard library header <compare>
Notes: *** promo None
#include <concepts>Description: Various useful concepts, many of which replace pre-concept
machinery in <type_traits>.
Documentation:
Standard library header <concepts>
Notes: *** promo None
#if !defined(__cpp_lib_atomic_value_initialization) || \
(__cpp_lib_atomic_value_initialization < 201911L)
... // `std::atomic` is not value-initialized by default.
#endifDescription: Provides a standardized way to test the toolchain's implementation of a particular library feature.
Documentation: Feature testing
Notes: *** promo None
#include <numbers>Description: Provides compile-time constants for many common mathematical values, e.g. pi and e.
Documentation: Mathematical constants
Notes: *** promo Migration bug
void f(int* p) {
int* aligned = std::assume_aligned<256>(p);
...Description: Informs the compiler that a pointer points to an address aligned to at least some particular power of 2.
Documentation:
std::assume_aligned
Notes: *** promo Migration bug
std::vector<int> numbers = ...;
std::erase_if(numbers, [](int x) { return x % 2 == 0; });Description: Erases from a container by value comparison or predicate,
avoiding the need to use the erase(remove(... paradigm.
Documentation:
std::erase, std::erase_if (std::vector)
Notes: *** promo Migration bug
template <typename T>
static constexpr bool kBoundedArray = std::is_bounded_array_v<T>;Description: Checks if a type is an array type with a known or unknown bound.
Documentation:
std::is_bounded_array,
std::is_unbounded_array
Notes: *** promo None
double val = std::lerp(start, end, t);Description: Linearly interpolates (or extrapolates) between two values.
Documentation:
std::lerp
Notes: *** promo Migration bug
auto obj = std::make_obj_using_allocator<Obj>(alloc, ...);Description: Constructs an object using uses-allocator construction.
Documentation:
std::make_obj_using_allocator
Notes: *** promo None
auto ptr = std::make_unique_for_overwrite<int>(); // `*ptr` is uninitializedDescription: Like calling std::unique_ptr<T>(new T) instead of the more
typical std::unique_ptr<T>(new T(...)).
Documentation:
std::make_unique, std::make_unique_for_overwrite
Notes: *** promo None
int center = std::midpoint(top, bottom);Description: Finds the midpoint between its two arguments, avoiding any possible overflow. For integral inputs, rounds towards the first argument.
Documentation:
std::midpoint
Notes: *** promo Migration bug
template <typename T,
typename = std::enable_if_t<std::is_same_v<std::remove_cvref_t<T>,
int>>>
void foo(T t);Description: Provides a way to remove const, volatile, and reference qualifiers from a type.
Documentation:
std::remove_cvref
Notes: *** promo Migration bug
str.replace(it, it + std::ssize(substr), 1, 'x');Description: Returns the size of an object as a signed type.
Documentation:
std::size, std::ssize
Notes: *** promo Migration bug
const std::string str = "Foo bar";
const bool is_true = str.ends_with("bar");Description: Tests whether a string starts or ends with a particular character or string.
Documentation:
std::basic_string<CharT,Traits,Allocator>::starts_with,
std::basic_string<CharT,Traits,Allocator>::ends_with
Notes: *** promo Migration bug
std::vector<int> numbers;
int* i = std::to_address(numbers.begin());Description: Converts a pointer-like object to a pointer, even if the
pointer does not refer to a constructed object (in which case an expression like
&*p is UB).
Documentation:
std::to_address
Notes: *** promo Migration bug
The following C++20 language features are not allowed in the Chromium codebase.
export module helloworld; // module declaration
import <iostream>; // import declaration
export void hello() { // export declaration
std::cout << "Hello world!\n";
}Description: Modules provide an alternative to many uses of headers which allows for faster compilation, better tooling support, and reduction of problems like "include what you use".
Documentation: Modules
Notes: *** promo Not yet sufficiently supported in Clang and GN. Re-evaluate when support improves.
struct Empty {};
struct X {
int i;
[[no_unique_address]] Empty e;
};Description: Allows a data member to be overlapped with other members.
Documentation:
C++ attribute: no_unique_address
Notes:
*** promo
Has no effect on Windows, for compatibility with Microsoft's ABI. Use
NO_UNIQUE_ADDRESS from base/compiler_specific.h instead. Do not use (either
form) on members of unions due to
potential memory safety problems.
The following C++20 library features are not allowed in the Chromium codebase.
struct S { int a; int b; };
S not_atomic;
std::atomic_ref<S> is_atomic(not_atomic);Description: Allows atomic access to objects that might not themselves be atomic types. While any atomic_ref to an object exists, the object must be accessed exclusively through atomic_ref instances.
Documentation:
std::atomic_ref
Notes: *** promo Banned due to being unimplemented in libc++.
Migration bug (once this is allowed)
int minus(int a, int b);
auto fifty_minus_x = std::bind_front(minus, 50);
int forty = fifty_minus_x(10);Description: An updated version of std::bind with fewer gotchas, similar
to absl::bind_front.
Documentation:
std::bind_front, std::bind_back
Notes:
*** promo
Overlaps with base::Bind.
float quake_rsqrt(float number) {
long i = std::bit_cast<long>(number);
i = 0x5f3759df - (i >> 1); // wtf?
float y = std::bit_cast<float>(i);
return y * (1.5f - (0.5f * number * y * y));
}Description: Returns an value constructed with the same bits as an value of a different type.
Documentation:
std::bit_cast
Notes:
*** promo
The std:: version of bit_cast allows casting of pointer and reference types,
which is both useless in that it doesn't avoid UB, and dangerous in that it
allows arbitrary casting away of modifiers like const. Instead of using
bit_cast on pointers, use standard C++ casts. For use on values, use
base::bit_cast which does not allow this unwanted usage.
std::u8string_view strv = u8"zß水🍌";
std::mbstate_t state;
char out[MB_LEN_MAX] = {0};
for (char8_t c : strv) {
size_t rc = std::c8rtomb(out, c, &state);
...Description: Converts a code point between UTF-8 and a multibyte character encoded using the current C locale.
Documentation:
std::c8rtomb,
std::mbrtoc8
Notes: *** promo Chromium functionality should not vary with the C locale.
#include <syncstream>Description: Facilities for multithreaded access to streams.
Documentation:
Standard library header <syncstream>
Notes: *** promo Banned due to being unimplemented per the libc++ C++20 status page. Reevaluate usefulness once implemented.
The following C++20 language features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
struct B {
int a;
int&& r;
} b2(1, 1); // Warning: dangling referenceDescription: Allows initialization of aggregates using parentheses, not just braces.
Documentation: Aggregate initialization, Direct initialization
Notes: *** promo There are subtle but important differences between brace- and paren-init of aggregates. The parenthesis style appears to have more pitfalls (allowing narrowing conversions, not extending lifetimes of temporaries bound to references).
char8_t c = u8'x';Description: A single UTF-8 code unit. Similar to unsigned char, but
considered a distinct type.
Documentation: Fundamental types
Notes:
*** promo
char8_t* is not interconvertible with char* and many UTF-8 APIs take
char*.
co_return 1;Description: Allows writing functions that logically block while physically returning control to a caller. This enables writing some kinds of async code in simple, straight-line ways without storing state in members or binding callbacks.
Documentation: Coroutines
Notes: *** promo Requires significant support code and planning around API and migration.
if (n > 0) [[likely]] {
return 1;
}Description: Tells the optimizer that a particular codepath is more or less likely than an alternative.
Documentation:
C++ attribute: likely, unlikely
Notes:
*** promo
Will be allowed soon; for now, use [UN]LIKELY.
The following C++20 library features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
#include <coroutine>Description: Header which defines various core coroutine types.
Documentation: Coroutine support
Notes: *** promo See notes on "Coroutines" above.
std::cout << std::format("Hello {}!\n", "world");Description: Utilities for producing formatted strings.
Documentation: Formatting library
Notes:
*** promo
Has both pros and cons compared to absl::StrFormat (which we don't yet use).
Migration would be nontrivial.
constexpr int arr[] = {6, 2, 8, 4, 4, 2};
constexpr auto plus_one = std::views::transform([](int n){ return n + 1; });
static_assert(std::ranges::equal(arr | plus_one, {7, 3, 9, 5, 5, 3}));Description: Generalizes algorithms using range views, which are lightweight objects that represent iterable sequences. Provides facilities for eager and lazy operations on ranges, along with composition into pipelines.
Documentation: Ranges library
Notes:
*** promo
Significant concerns expressed internally. We should consider whether there are
clearly-safe pieces to allow (e.g. to replace base/ranges/algorithm.h) and
engage with the internal library team.
#include <source_location>Description: Provides a class that can hold source code details such as filenames, function names, and line numbers.
Documentation:
Standard library header <source_location>
Notes:
*** promo
Seems to regress code size vs. base::Location.
#include <span>Description: Utilities for non-owning views over a sequence of objects.
Notes:
*** promo
Use base::span for now.
std::u8string str = u8"Foo";Description: A string whose character type is char8_t, intended to hold
UTF-8-encoded text.
Documentation:
std::basic_string
Notes:
*** promo
See notes on char8_t above.
The following Abseil library features are not allowed in the Chromium codebase.
absl::any a = int{5};
EXPECT_THAT(absl::any_cast<int>(&a), Pointee(5));
EXPECT_EQ(absl::any_cast<size_t>(&a), nullptr);Description: Early adaptation of C++17 std::any.
Documentation: std::any
Notes:
*** promo
Banned since workaround for lack of RTTI isn't compatible with the component
build (Bug). Also see std::any.
absl::bind_frontDescription: Binds the first N arguments of an invocable object and stores them by value.
Documentation:
Notes:
*** promo
Banned due to overlap with base::Bind. Use base::Bind instead.
ABSL_FLAG(bool, logs, false, "print logs to stderr");
app --logs=true;Description: Allows programmatic access to flag values passed on the command-line to binaries.
Documentation: Flags Library
Notes:
*** promo
Banned since workaround for lack of RTTI isn't compatible with the component
build. (Bug) Use base::CommandLine instead.
auto it = absl::c_find(container, value);Description: Container-based versions of algorithmic functions within C++ standard library.
Documentation: container.h
Notes:
*** promo
Banned due to overlap with base/ranges/algorithm.h. Use the base/ranges/
facilities instead.
absl::FixedArray<MyObj> objs_;Description: A fixed size array like std::array, but with size determined
at runtime instead of compile time.
Documentation: fixed_array.h
Notes:
*** promo
Direct construction is banned due to the risk of UB with uninitialized
trivially-default-constructible types. Instead use base/types/fixed_array.h,
which is a light-weight wrapper that deletes the problematic constructor.
absl::FunctionRefDescription: Type for holding a non-owning reference to an object of any invocable type.
Documentation: function_ref.h
Notes: *** promo
absl::FunctionRefis banned due to allowing implicit conversions between function signatures in potentially surprising ways. For example, a callable with the signatureint()will bind toabsl::FunctionRef<void()>: the return value from the callable will be silently discarded.- In Chromium, use
base::FunctionRefinstead. - Unlike
base::OnceCallbackandbase::RepeatingCallback,base::FunctionRefsupports capturing lambdas. - Useful when passing an invocable object to a function that synchronously calls
the invocable object, e.g.
ForEachFrame(base::FunctionRef<void(Frame&)>). This can often result in clearer code than code that is templated to accept lambdas, e.g. withtemplate <typename Invocable> void ForEachFrame(Invocable invocable), it is much less obvious what arguments will be passed toinvocable. - For now,
base::OnceCallbackandbase::RepeatingCallbackintentionally disallow conversions tobase::FunctionRef, under the theory that the callback should be a capturing lambda instead. Attempting to use this conversion will trigger astatic_assertrequesting additional feedback for use cases where this conversion would be valuable. - Important:
base::FunctionRefmust not outlive the function call. Likebase::StringPiece,base::FunctionRefis a non-owning reference. Using abase::FunctionRefas a return value or class field is dangerous and likely to result in lifetime bugs. - Discussion thread
absl::BitGen bitgen;
size_t index = absl::Uniform(bitgen, 0u, elems.size());Description: Functions and utilities for generating pseudorandom data.
Documentation: Random library
Notes:
*** promo
Banned because most uses of random values in Chromium should be using a
cryptographically secure generator. Use base/rand_util.h instead.
absl::SpanDescription: Early adaptation of C++20 std::span.
Documentation: Using absl::Span
Notes:
*** promo
Banned due to being less std::-compliant than base::span. Keep using
base::span.
absl::StatusOr<T>Description: An object that is either a usable value, or an error Status explaining why such a value is not present.
Documentation: statusor.h
Notes:
*** promo
Banned due to overlap with base::expected. Use base::expected instead.
absl::StrFormatDescription: A typesafe replacement for the family of printf() string formatting routines.
Documentation: String Formatting
Notes:
*** promo
Banned for now due to overlap with base::StringPrintf(). See
migration bug.
absl::string_viewDescription: Early adaptation of C++17 std::string_view.
Documentation: absl::string_view
Notes:
*** promo
Originally banned due to only working with 8-bit characters. Now it is
unnecessary because, in Chromium, it is the same type as std::string_view.
Please use std::string_view instead.
absl::StrSplit
absl::StrJoin
absl::StrCat
absl::StrAppend
absl::Substitute
absl::StrContainsDescription: Classes and utility functions for manipulating and comparing strings.
Documentation: String Utilities
Notes:
*** promo
Banned for now due to overlap with base/strings. We
should re-evalute
when we've
migrated from
base::StringPiece to std::string_view.
absl::MutexDescription: Primitives for managing tasks across different threads.
Documentation: Synchronization
Notes:
*** promo
Banned due to overlap with base/synchronization/. We would love
more testing on
whether there are compelling reasons to prefer base, absl, or std
synchronization primitives; for now, use base/synchronization/.
absl::Duration
absl::Time
absl::TimeZone
absl::CivilDayDescription: Abstractions for holding time values, both in terms of absolute time and civil time.
Documentation: Time
Notes:
*** promo
Banned due to overlap with base/time/. Use base/time/ instead.
The following Abseil library features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
absl::AnyInvocableDescription: An equivalent of the C++23 std::move_only_function.
Documentation:
Notes:
*** promo
Overlaps with base::RepeatingCallback, base::OnceCallback.
absl::flat_hash_map
absl::flat_hash_set
absl::node_hash_map
absl::node_hash_set
absl::btree_map
absl::btree_set
absl::btree_multimap
absl::btree_multisetDescription: Alternatives to STL containers designed to be more efficient in the general case.
Documentation:
Notes:
*** promo
Supplements base/containers/.
absl::InlinedVector is explicitly allowed, see the discussion thread.
Description: API for computing CRC32C values as checksums for arbitrary sequences of bytes provided as a string buffer.
Documentation: crc32.h
Notes: *** promo Overlaps with //third_party/crc32c.
LOG(INFO) << message;
CHECK(condition);
absl::AddLogSink(&custom_sink_to_capture_absl_logs);Description: Macros and related classes to perform debug loggings
Notes:
*** promo
Overlaps and uses same macros names as base/logging.h.
// Global or namespace scope.
ABSL_CONST_INIT absl::NoDestructor<MyRegistry> reg{"foo", "bar", 8008};
// Function scope.
const std::string& MyString() {
static const absl::NoDestructor<std::string> x("foo");
return *x;
}Description: absl::NoDestructor<T> is a wrapper around an object of
type T that behaves as an object of type T but never calls T's destructor.
Documentation: no_destructor.h
Notes:
*** promo
Overlaps with base::NoDestructor.
void PaySalary(absl::NotNull<Employee *> employee) {
pay(*employee); // OK to dereference
}Description: Annotations to more clearly specify contracts
Documentation: nullability.h
Notes: *** promo These nullability annotations are primarily a human readable signal about the intended contract of the pointer. They are not types and do not currently provide any correctness guarantees.
std::variant<int, std::string, double> v(int{1});
assert(std::visit(absl::Overload(
[](int) -> absl::string_view { return "int"; },
[](const std::string&) -> absl::string_view {
return "string";
},
[](double) -> absl::string_view { return "double"; }),
v) == "int");Description: Returns a functor that provides overloads based on the functors passed to it
Documentation: overload.h
Notes:
*** promo
Overlaps with base::Overloaded from base/functional/overloaded.h.