Modern C++ use in Chromium

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 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: Default allowed; see banned 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

Banned features and third-party code

Third-party libraries may generally use banned features internally, although features with poor compiler support or poor security properties may make the library unsuitable to use with Chromium.

Chromium code that calls functions exported from a third-party library may use banned library types that are required by the interface, as long as:

  • The disallowed type is used only at the interface, and converted to and from an equivalent allowed type as soon as practical on the Chromium side.
  • The feature is not banned due to security issues or lack of compiler support. If it is, discuss with to find a workaround.

C++11 Banned Language Features

The following C++11 language features are not allowed in the Chromium codebase.

Inline Namespaces [banned]

inline namespace foo { ... }

Description: Allows better versioning of namespaces.

Documentation: Inline namespaces


Banned in the Google Style Guide. Unclear how it will work with components.

long long Type [banned]

long long var = value;

Description: An integer of at least 64 bits.

Documentation: Fundamental types


Use a <stdint.h> type if you need a 64-bit number.

Discussion thread

User-Defined Literals [banned]

DistanceType var = 12_km;

Description: Allows user-defined literal expressions.

Documentation: User-defined literals


Banned in the Google Style Guide.

C++11 Banned Library Features

The following C++11 library features are not allowed in the Chromium codebase.

<cctype>, <ctype.h>, <cwctype>, <wctype.h> [banned]

#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>


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.

<cfenv>, <fenv.h> [banned]

#include <cfenv>
#include <fenv.h>

Description: Provides floating point status flags and control modes for C-compatible code.

Documentation: Standard library header <cfenv>


Banned by the Google Style Guide due to concerns about compiler support.

<chrono> [banned]

#include <chrono>

Description: A standard date and time library.

Documentation: Date and time utilities


Overlaps with base/time.

<exception> [banned]

#include <exception>

Description: Exception throwing and handling.

Documentation: Standard library header <exception>


Exceptions are banned by the Google Style Guide and disabled in Chromium compiles. However, the noexcept specifier is explicitly allowed.

Discussion thread

Engines And Generators From <random> [banned]

std::mt19937 generator;

Description: Methods of generating random numbers.

Documentation: Pseudo-random number generation


Do not use any random number engines or generators from <random>. Instead, use base::RandomBitGenerator. (You may use the distributions from <random>.)

Discussion thread

<ratio> [banned]

#include <ratio>

Description: Provides compile-time rational numbers.

Documentation: std::ratio


Banned by the Google Style Guide due to concerns that this is tied to a more template-heavy interface style.

<regex> [banned]

#include <regex>

Description: A standard regular expressions library.

Documentation: Regular expressions library


Overlaps with many regular expression libraries in Chromium. When in doubt, use third_party/re2.

std::bind [banned]

auto x = std::bind(function, args, ...);

Description: Declares a function object bound to certain arguments.

Documentation: 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

std::function [banned]

std::function x = [] { return 10; };
std::function y = std::bind(foo, args);

Description: Wraps a standard polymorphic function.

Documentation: std::function


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.

Discussion thread

std::shared_ptr [banned]

std::shared_ptr<int> x = std::make_shared<int>(10);

Description: Allows shared ownership of a pointer through reference counts.

Documentation: std::shared_ptr


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

std::{sto{i,l,ul,ll,ull,f,d,ld},to_string} [banned]

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


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 [banned]

std::weak_ptr<int> x = my_shared_x;

Description: Allows a weak reference to a std::shared_ptr.

Documentation: std::weak_ptr


Banned because std::shared_ptr is banned. Use base::WeakPtr instead.

Thread Support Library [banned]

#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


Overlaps with base/synchronization. base::Thread is tightly coupled to base::MessageLoop which would make it hard to replace. We should investigate using standard mutexes, or std::unique_lock, etc. to replace our locking/synchronization classes.

C++17 Banned Language Features

The following C++17 language features are not allowed in the Chromium codebase.

UTF-8 character literals [banned]

char x = u8'x';     // C++17
char8_t x = u8'x';  // C++20

Description: 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


Banned because char8_t is banned. Use an unprefixed character or string literal; it should be encoded in the binary as UTF-8 on all supported platforms.

C++17 Banned Library Features

The following C++17 library features are not allowed in the Chromium codebase.

Mathematical special functions [banned]


Description: A variety of mathematical functions.

Documentation: Mathematical special functions


Banned due to lack of libc++ support.

Parallel algorithms [banned]

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”.

Documentation: std::execution::sequenced_policy, std::execution::parallel_policy, std::execution::parallel_unsequenced_policy, std::execution::unsequenced_policy


Banned because libc++ support is incomplete and the interaction of its threading implementation with Chrome‘s is unclear. Prefer to explicitly parallelize long-running algorithms using Chrome’s threading APIs, so the same scheduler controls, shutdown policies, tracing, etc. apply as in any other multithreaded code.

std::aligned_alloc [banned]

int* p2 = static_cast<int*>(std::aligned_alloc(1024, 1024));

Description: Allocates uninitialized storage with the specified alignment.

Documentation: std::aligned_alloc


Will be allowed soon; for now, use base::AlignedAlloc.

std::any [banned]

std::any x = 5;

Description: A type-safe container for single values of any type.

Documentation: std::any


Banned since workaround for lack of RTTI isn't compatible with the component build. See also absl::any.

Discussion thread

std::byte [banned]

std::byte b = 0xFF;
int i = std::to_integer<int>(b);  // 0xFF

Description: The contents of a single memory unit. std::byte has the same size and aliasing rules as unsigned char, but does not semantically represent a character or arithmetic value, and does not expose operators other than bitwise ops.

Documentation: std::byte


Banned due to low marginal utility in practice, high conversion costs, and programmer confusion about “byte” vs. “octet”. Use uint8_t for the common case of “8-bit unsigned value”, and char for the atypical case of code that works with memory without regard to its contents' values or semantics (e.g allocator implementations).

Discussion thread

std::filesystem [banned]

#include <filesystem>

Description: A standard way to manipulate files, directories, and paths in a filesystem.

Documentation: Filesystem library


Banned by the Google Style Guide.

std::{from,to}_chars [banned]

std::from_chars(, + str.size(), result);
std::to_chars(, + 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,


Overlaps with utilities in base/strings/string_number_conversions.h, which are easier to use correctly.

std::hardware_{con,de}structive_interference_size [banned]

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


Banned for now since these are not supported yet. Allow once supported.

Discussion thread

std::in_place{_type,_index}[_t] [banned]

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


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.

Discussion thread

std::{pmr::memory_resource,polymorphic_allocator} [banned]

#include <memory_resource>

Description: Manages memory allocations using runtime polymorphism.

Documentation: std::pmr::memory_resource, std::pmr::polymorphic_allocator


Banned because Chromium does not customize allocators (PartitionAlloc is used globally).

std::timespec_get [banned]

std::timespec ts;
std::timespec_get(&ts, TIME_UTC);

Description: Gets the current calendar time in the given time base.

Documentation: std::timespec_get


Banned due to unclear, implementation-defined behavior. On POSIX, use base::TimeDelta::ToTimeSpec(); this could be supported on other platforms if desirable.

std::uncaught_exceptions [banned]

int count = std::uncaught_exceptions();

Description: Determines whether there are live exception objects.

Documentation: std::uncaught_exceptions


Banned because exceptions are banned.

std::variant [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


Will be allowed soon; for now, use absl::variant.

Transparent std::owner_less [banned]

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


Banned since std::shared_ptr and std::weak_ptr are banned.

weak_from_this [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


Banned since std::shared_ptr and std::weak_ptr are banned.

C++20 Allowed Language Features

The following C++20 language features are allowed in the Chromium codebase.

Abbreviated function templates [allowed]

// 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


consteval [allowed]

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 runtime

Description: Specified that a function may only be used in a compile-time context.

Documentation: consteval specifier



Constraints and concepts [allowed]

// `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


Default comparisons [allowed]

class S : public T {
  // Non-member equality operator with access to private members.
  // Compares `T` bases, then `x`, then `y`, short-circuiting when
  // it finds inequality.
  friend bool operator==(const S&, const S&) = default;

  // Non-member ordering operator with access to private members.
  // Compares `T` bases, then `x`, then `y`, short-circuiting when
  // it finds an ordering difference.
  friend auto operator<=>(const S&, const S&) = default;

  int x;
  bool y;

Description: Requests that the compiler generate the implementation of any comparison operator, including <=>. Prefer non-member comparison operators. When defaulting <=>, also explicitly default ==. Together these 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


Unlike constructors/destructors, our compiler extensions do not require these to be written out-of-line in the .cc file. Feel free to write = default directly in the header, as this is much simpler to write.

Designated initializers [allowed]

struct S { int x = 1; int y = 2; }
S s{ .y = 3 };  // OK, s.x == 1, s.y == 3

Description: Allows explicit initialization of subsets of aggregate members at construction.

Documentation: Designated initializers



__has_cpp_attribute [allowed]

#if __has_cpp_attribute(assume)  // Toolchain supports C++23 `[[assume]]`.

Description: Checks whether the toolchain supports a particular standard attribute.

Documentation: Feature testing



constinit [allowed]

constinit int x = 3;
void foo() {

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


Initializers for bit-field members [allowed]

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



Lambda captures with initializers that are pack expansions [allowed]

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



Language feature-test macros [allowed]

#if !defined(__cpp_modules) || (__cpp_modules < 201907L)
...  // Toolchain does not support modules

Description: Provides a standardized way to test the toolchain's implementation of a particular language feature.

Documentation: Feature testing



Range-for statements with initializer [allowed]

T foo();
for (auto& x : foo().items()) { ... }                   // UAF before C++23!
for (T thing = foo(); auto& x : thing.items()) { ... }  // OK

Description: 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


Three-way comparison (“spaceship”) operator [allowed]

// `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


C++20 Allowed Library Features

The following C++20 library features are allowed in the Chromium codebase.

<bit> [allowed]

#include <bit>

Description: Provides various byte- and bit-twiddling functions, e.g. counting leading zeros.

Documentation: Standard library header <bit>


<compare> [allowed]

#include <compare>

Description: Concepts and classes used to implement three-way comparison (“spaceship”, <=>) support.

Documentation: Standard library header <compare>



<concepts> [allowed]

#include <concepts>

Description: Various useful concepts, many of which replace pre-concept machinery in <type_traits>.

Documentation: Standard library header <concepts>



Library feature-test macros and <version> [allowed]

#if !defined(__cpp_lib_atomic_value_initialization) || \
    (__cpp_lib_atomic_value_initialization < 201911L)
...  // `std::atomic` is not value-initialized by default.

Description: Provides a standardized way to test the toolchain's implementation of a particular library feature.

Documentation: Feature testing



<numbers> [allowed]

#include <numbers>

Description: Provides compile-time constants for many common mathematical values, e.g. pi and e.

Documentation: Mathematical constants


std::assume_aligned [allowed]

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



std::erase[_if] for containers [allowed]

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)


std::is_[un]bounded_array [allowed]

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



std::lerp [allowed]

double val = std::lerp(start, end, t);

Description: Linearly interpolates (or extrapolates) between two values.

Documentation: std::lerp


std::make_obj_using_allocator etc. [allowed]

auto obj = std::make_obj_using_allocator<Obj>(alloc, ...);

Description: Constructs an object using uses-allocator construction.

Documentation: std::make_obj_using_allocator



std::make_unique_for_overwrite [allowed]

auto ptr = std::make_unique_for_overwrite<int>();  // `*ptr` is uninitialized

Description: 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



std::midpoint [allowed]

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


std::remove_cvref[_t] [allowed]

template <typename T,
          typename = std::enable_if_t<std::is_same_v<std::remove_cvref_t<T>,
void foo(T t);

Description: Provides a way to remove const, volatile, and reference qualifiers from a type.

Documentation: std::remove_cvref



std::ssize [allowed]

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


std::string::(starts,ends)_with [allowed]

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


C++20 Banned Language Features

The following C++20 language features are not allowed in the Chromium codebase.

char8_t [banned]

char8_t c = u8'x';

Description: A single UTF-8 code unit. Similar to unsigned char, but considered a distinct type.

Documentation: Fundamental types


Use char and unprefixed character literals. Non-UTF-8 encodings are rare enough in Chromium that the value of distinguishing them at the type level is low, and char8_t* is not interconvertible with char* (what ~all Chromium, STL, and platform-specific APIs use), so using u8 prefixes would obligate us to insert casts everywhere. If you want to declare at a type level that a block of data is string-like and not an arbitrary binary blob, prefer std::string[_view] over char*.

Modules [banned]

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


Not yet sufficiently supported in Clang and GN. Re-evaluate when support improves.

[[no_unique_address]] [banned]

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


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.

C++20 Banned Library Features

The following C++20 library features are not allowed in the Chromium codebase.

std::atomic_ref [banned]

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


Banned due to being unimplemented in libc++.

Migration bug (once this is allowed)

std::bind_front [banned]

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


Overlaps with base::Bind.

std::bit_cast [banned]

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


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::{c8rtomb,mbrtoc8} [banned]

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


Chromium functionality should not vary with the C locale.

std::to_address [banned]

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


Banned because it is not guaranteed to be SFINAE-compatible. Use base::to_address, which does guarantee this.

<syncstream> [banned]

#include <syncstream>

Description: Facilities for multithreaded access to streams.

Documentation: Standard library header <syncstream>


Banned due to being unimplemented per the libc++ C++20 status page. Reevaluate usefulness once implemented.

C++20 TBD Language Features

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.

Aggregate initialization using parentheses [tbd]

struct B {
  int a;
  int&& r;
} b2(1, 1);  // Warning: dangling reference

Description: Allows initialization of aggregates using parentheses, not just braces.

Documentation: Aggregate initialization, Direct initialization


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).

Coroutines [tbd]

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


Requires significant support code and planning around API and migration.

Prototyping bug

[[likely]], [[unlikely]] [tbd]

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


Will be allowed soon; for now, use [UN]LIKELY.

C++20 TBD Library Features

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.

<coroutine> [tbd]

#include <coroutine>

Description: Header which defines various core coroutine types.

Documentation: Coroutine support


See notes on “Coroutines” above.

<format> [tbd]

std::cout << std::format("Hello {}!\n", "world");

Description: Utilities for producing formatted strings.

Documentation: Formatting library


Has both pros and cons compared to absl::StrFormat (which we don't yet use). Migration would be nontrivial.

<ranges> [tbd]

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


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.

<source_location> [tbd]

#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>


Seems to regress code size vs. base::Location.

<span> [tbd]

#include <span>

Description: Utilities for non-owning views over a sequence of objects.



Use base::span for now.

Migration bug

std::u8string [tbd]

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


See notes on char8_t above.

Abseil Banned Library Features

The following Abseil library features are not allowed in the Chromium codebase.

Any [banned]

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


Banned since workaround for lack of RTTI isn't compatible with the component build. See also std::any.

bind_front [banned]


Description: Binds the first N arguments of an invocable object and stores them by value.



Overlaps with base::Bind.

Command line flags [banned]

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


Banned since workaround for lack of RTTI isn't compatible with the component build. Use base::CommandLine instead.

Container utilities [banned]

auto it = absl::c_find(container, value);

Description: Container-based versions of algorithmic functions within C++ standard library.

Documentation: container.h


Overlaps with base/ranges/algorithm.h.

FixedArray [banned]

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


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.

FunctionRef [banned]


Description: Type for holding a non-owning reference to an object of any invocable type.

Documentation: function_ref.h


  • absl::FunctionRef is banned due to allowing implicit conversions between function signatures in potentially surprising ways. For example, a callable with the signature int() will bind to absl::FunctionRef<void()>: the return value from the callable will be silently discarded.
  • In Chromium, use base::FunctionRef instead.
  • Unlike base::OnceCallback and base::RepeatingCallback, base::FunctionRef supports 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. with template <typename Invocable> void ForEachFrame(Invocable invocable), it is much less obvious what arguments will be passed to invocable.
  • For now, base::OnceCallback and base::RepeatingCallback intentionally disallow conversions to base::FunctionRef, under the theory that the callback should be a capturing lambda instead. Attempting to use this conversion will trigger a static_assert requesting additional feedback for use cases where this conversion would be valuable.
  • Important: base::FunctionRef must not outlive the function call. Like base::StringPiece, base::FunctionRef is a non-owning reference. Using a base::FunctionRef as a return value or class field is dangerous and likely to result in lifetime bugs.

Discussion thread

Random [banned]

absl::BitGen bitgen;
size_t index = absl::Uniform(bitgen, 0u, elems.size());

Description: Functions and utilities for generating pseudorandom data.

Documentation: Random library


Banned because most uses of random values in Chromium should be using a cryptographically secure generator. Use base/rand_util.h instead.

Span [banned]


Description: Early adaptation of C++20 std::span.

Documentation: Using absl::Span


Banned due to being less std::-compliant than base::span. Keep using base::span.

StatusOr [banned]


Description: An object that is either a usable value, or an error Status explaining why such a value is not present.

Documentation: statusor.h


Overlaps with base::expected.

String Formatting [banned]


Description: A typesafe replacement for the family of printf() string formatting routines.

Documentation: String Formatting


Overlaps with base::StringPrintf(). See migration bug.

string_view [banned]


Description: Early adaptation of C++17 std::string_view.

Documentation: absl::string_view


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.

Strings Library [banned]


Description: Classes and utility functions for manipulating and comparing strings.

Documentation: String Utilities


Overlaps with base/strings. We should re-evalute when we've migrated from base::StringPiece to std::string_view.

Synchronization [banned]


Description: Primitives for managing tasks across different threads.

Documentation: Synchronization


Overlaps 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/.

Time library [banned]


Description: Abstractions for holding time values, both in terms of absolute time and civil time.

Documentation: Time


Overlaps with base/time/.

Abseil TBD Features

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.

AnyInvocable [tbd]


Description: An equivalent of the C++23 std::move_only_function.



Overlaps with base::RepeatingCallback, base::OnceCallback.

Containers [tbd]


Description: Alternatives to STL containers designed to be more efficient in the general case.



Supplements base/containers/.

absl::InlinedVector is explicitly allowed, see the discussion thread.

CRC32C library [tbd]

Description: API for computing CRC32C values as checksums for arbitrary sequences of bytes provided as a string buffer.

Documentation: crc32.h


Overlaps with third_party/crc32c.

Log macros and related classes [tbd]

LOG(INFO) << message;

Description: Macros and related classes to perform debug loggings

Documentation: log.h check.h


Overlaps with base/logging.h.

NoDestructor [tbd]

// 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


Overlaps with base::NoDestructor.

Nullability annotations [tbd]

void PaySalary(absl::NotNull<Employee *> employee) {
  pay(*employee);  // OK to dereference

Description: Annotations to more clearly specify contracts

Documentation: nullability.h


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.

Overload [tbd]

std::variant<int, std::string, double> v(int{1});
                       [](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


Overlaps with base::Overloaded.