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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 [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; see banned features below
  • C++17: Initially supported December 23, 2021; see allowed/banned/TBD features below
  • C++20: Not yet supported in Chromium
  • C++23: Not yet standardized
  • Abseil: Default allowed; see banned/TBD features below
    • absl::AnyInvocable: Initially supported June 20, 2022
    • Log library: Initially supported Aug 31, 2022
    • CRC32C library: Initially supported Dec 5, 2022

[TOC]

C++11 Banned Language Features {#core-blocklist-11}

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

Notes: *** promo 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

Notes: *** promo Use a stdint.h type if you need a 64-bit number. Discussion thread

User-Defined Literals [banned]

type var = literal_value_type;

Description: Allows user-defined literal expressions.

Documentation: User-defined literals

Notes: *** promo Banned in the Google Style Guide.

thread_local Storage Class [banned]

thread_local int foo = 1;

Description: Puts variables into thread local storage.

Documentation: Storage duration

Notes: *** promo Some surprising effects on Mac (discussion, fork). Use base::SequenceLocalStorageSlot for sequence support, and base::ThreadLocal/base::ThreadLocalStorage otherwise.

C++11 Banned Library Features {#library-blocklist-11}

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

Bind Operations [banned]

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. Discussion thread

C Floating-Point Environment [banned]

#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.

Date and time utilities [banned]

#include <chrono>

Description: A standard date and time library

Documentation: Date and time utilities

Notes: *** promo Overlaps with Time APIs in base/. Keep using the base/ classes.

Exceptions [banned]

#include <exception>

Description: Enhancements to 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. Discussion thread

Function Objects [banned]

std::function

Description: Wraps a standard polymorphic function

Documentation: std::function

Notes: *** promo Use base::{Once,Repeating}Callback 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

Random Number Engines [banned]

*** aside The random number engines defined in <random> (see separate item for random number distributions), e.g.: linear_congruential_engine, mersenne_twister_engine, minstd_rand0, mt19937, ranlinux48, random_device

Description: Random number generation algorithms and utilities.

Documentation: Pseudo-random number generation

Notes: *** promo Do not use any random number engines from <random>. Instead, use base::RandomBitGenerator. Discussion thread

Ratio Template Class [banned]

std::ratio<numerator, denominator>

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.

Regular Expressions [banned]

#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 re2.

Shared Pointers [banned]

std::shared_ptr

Description: Allows shared ownership of a pointer through reference counts

Documentation: std::shared_ptr

Notes: *** promo Needs a lot more evaluation for Chromium, and there isn't enough of a push for this feature.

String-Number Conversion Functions [banned]

std::stoi()
std::stol()
std::stoul()
std::stoll
std::stoull()
std::stof()
std::stod()
std::stold()
std::to_string()

Description: Converts strings to/from numbers

Documentation:

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 the routines in base/strings/string_number_conversions.h instead.

Thread Library [banned]

*** aside <thread> and related headers, including <future>, <mutex>, <condition_variable>

Description: Provides a standard multithreading library using std::thread and associates

Documentation: Thread support library

Notes: *** promo 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.

Weak Pointers [banned]

std::weak_ptr

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.

C++14 Banned Library Features {#library-blocklist-14}

The following C++14 library features are not allowed in the Chromium codebase.

std::chrono literals [banned]

using namespace std::chrono_literals;
auto timeout = 30s;

Description: Allows std::chrono types to be more easily constructed.

Documentation: std::literals::chrono_literals::operator""s

Notes: *** promo Banned because <chrono> is banned.

C++17 Allowed Language Features {#core-allowlist-17}

The following C++17 language features are allowed in the Chromium codebase.

Nested namespaces [allowed]

namespace A::B::C { ...

Description: Using the namespace resolution operator to create nested namespace definitions.

Documentation: Namespaces

Notes: *** promo Discussion thread

Template argument deduction for class templates [allowed]

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.

Fold expressions [allowed]

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

Selection statements with initializer [allowed]

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 @cxx discussion thread

fallthrough attribute [allowed]

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 See discussion thread.

See migration task.

constexpr if [allowed]

if constexpr (cond) { ...

Description: Write code that is instantiated depending on a compile-time condition.

Documentation: if statement

Notes: *** promo See discussion thread.

nodiscard attribute [allowed]

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)).

Discussion thread

maybe_unused attribute [allowed]

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 See discussion thread.

Structured bindings [allowed]

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 In C++17, structured bindings don't work with lambda captures. C++20 will allow capturing structured bindings by value.

This feature forces omitting type names. Its use should follow the guidance around auto in Google C++ Style guide.

See discussion thread.

Inline variables [allowed]

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.

See discussion thread.

C++17 Allowed Library Features {#library-allowlist-17}

The following C++17 language features are allowed in the Chromium codebase.

Allocation functions with explicit alignment [allowed]

class alignas(32) Vec3d {
  double x, y, z;
};
auto p_vec = new Vec3d[10];  // 32-byte aligned in C++17, maybe not previously

Description: Performs heap allocation of objects whose alignment requirements exceed __STDCPP_DEFAULT_NEW_ALIGNMENT__.

Documentation: operator new

Notes: *** promo None

Type trait variable templates [allowed]

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::map::try_emplace [allowed]

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

std::map::insert_or_assign [allowed]

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::apply [allowed]

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

std::as_const [allowed]

auto&& const_ref = std::as_const(mutable_obj);

Description: Forms reference to const T.

Documentation: std::as_const

Notes: *** promo Discussion thread

Non-member std::size/std::empty/std::data [allowed]

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 old arraysize() macro.
  • writing code that needs to generically operate across things like std::vector and std::list (which provide size(), empty(), and data() 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::is_invocable [allowed]

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::conjunction/std::disjunction/std::negation [allowed]

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::hardware_{constructive|destructive}_interference_size [allowed]

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 Discussion thread

C++17 Banned Library Features {#library-blocklist-17}

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

std::any [banned]

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.

std::filesystem [banned]

#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.

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::weak_from_this

Notes: *** promo Banned since std::shared_ptr and std::weak_ptr are banned.

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

Notes: *** promo Banned since std::shared_ptr and std::weak_ptr are banned.

Array support for std::shared_ptr [banned]

std::shared_ptr<int[]> p(new int[10]{0,1,2,3,4,5,6,7,8,9});
std::cout << p[3];  // "3"

Description: Supports memory management of arrays via std::shared_ptr.

Documentation: std::shared_ptr

Notes: *** promo Banned since std::shared_ptr is banned.

std::uncaught_exceptions [banned]

int count = std::uncaught_exceptions();

Description: Determines whether there are live exception objects.

Documentation: std::uncaught_exceptions

Notes: *** promo Banned because exceptions are banned.

Rounding functions for duration and time_point [banned]

std::chrono::ceil<std::chrono::seconds>(dur);
std::chrono::ceil<std::chrono::seconds>(time_pt);
std::chrono::floor<std::chrono::seconds>(dur);
std::chrono::floor<std::chrono::seconds>(time_pt);
std::chrono::round<std::chrono::seconds>(dur);
std::chrono::round<std::chrono::seconds>(time_pt);

Description: Converts durations and time_points by rounding.

Documentation: std::chrono::duration, std::chrono::time_point

Notes: *** promo Banned since std::chrono is 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

Notes: *** promo Banned for now because it does not provide safety guarantees in the case of misuse. The Chromium C++ team is investigating the possibility of hardening the C++ library so that the standard version can be used. In the meanwhile, use absl::variant instead.

std::optional [banned]

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. A common use case for optional is the return value of a function that may fail.

Documentation: std::optional

Notes: *** promo Banned for now because it does not provide safety guarantees in the case of misuse. The Chromium C++ team is investigating the possibility of hardening the C++ library so that the standard version can be used. In the meanwhile, use absl::optional instead.

std::in_place/in_place_type/in_place_index/in_place_t/in_place_type_t/in_place_index_t [banned]

std::optional<std::complex<double>> opt{std::in_place, 0, 1};
std::variant<int, float> v{std::in_place_type<int>, 1.4};

Description: The std::in_place are disambiguation tags for std::optional, std::variant, and std::any to indicate that the object should be constructed in-place.

Documentation: std::in_place

Notes: *** promo Banned for now because std::optional, std::variant, and std::any are all banned for now. Because absl::optional and absl::variant are used instead, and they require absl::in_place, use absl::in_place for non-Abseil Chromium code. See the discussion thread.

std::clamp [banned]

int x = std::clamp(inp, 0, 100);

Description: Clamps a value between a minimum and a maximum.

Documentation: std::clamp

Notes: *** promo Banned for now because it does not provide safety guarantees in the case of misuse. The Chromium C++ team is investigating the possibility of hardening the C++ library so that the standard version can be used. In the meanwhile, use base::clamp instead.

C++17 TBD Language Features {#core-review-17}

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.

Declaring non-type template parameters with auto [tbd]

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 None

constexpr lambda [tbd]

auto identity = [](int n) constexpr { return n; };
static_assert(identity(123) == 123);

Description: Compile-time lambdas using constexpr.

Documentation: Lambda expressions

Notes: *** promo None

Lambda capture this by value [tbd]

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 None

UTF-8 character literals [tbd]

char x = u8'x';

Description: A character literal that begins with u8 is a character literal of type char. The value of a UTF-8 character literal is equal to its ISO 10646 code point value.

Documentation: Character literal

Notes: *** promo C++20 changes the type to char8_t, causing migration hazards for code using this.

using declaration for attributes [tbd]

[[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.

__has_include [tbd]

#if __has_include(<optional>) ...

Description: Checks whether a file is available for inclusion, i.e. the file exists.

Documentation: Source file inclusion

Notes: *** promo None

C++17 TBD Library Features {#library-review-17}

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::string_view [tbd]

std::string_view str = "foo";

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 See also absl::string_view and base::StringPiece.

std::invoke [tbd]

static_assert(std::invoke(std::plus<>(), 1, 2) == 3);

Description: Invokes a Callable object with parameters. An example of a Callable object is base::Callback where an object can be called similarly to a regular function.

Documentation: std::invoke

Notes: *** promo See also base::invoke.

std::byte [tbd]

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

Description: 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 None

Splicing for maps and sets [tbd]

std::map<...>::extract
std::map<...>::merge
std::set<...>::extract
std::set<...>::merge

Description: 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 None

Parallel algorithms [tbd]

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: execution_policy_tag_t

Notes: *** promo None

std::make_from_tuple [tbd]

// 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 See also absl::make_from_tuple.

Searchers [tbd]

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 None

std::not_fn [tbd]

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 See also base::not_fn.

Uninitialized memory algorithms [tbd]

std::destroy_at(ptr);
std::destroy(ptr, ptr + 8);
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_at, std::destroy, std::destroy_n, std::uninitialized_move, std::uninitialized_value_construct

Notes: *** promo None

std::pmr::memory_resource and std::polymorphic_allocator [tbd]

#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::aligned_alloc [tbd]

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

Description: Allocates uninitialized storage with the specified alignment.

Documentation: std::aligned_alloc

Notes: *** promo None

std::is_swappable [tbd]

std::is_swappable<T>
std::is_swappable_with_v<T, U>

Description: Checks whether classes may be swapped.

Documentation: std::is_swappable

Notes: *** promo None

std::is_aggregate [tbd]

if constexpr(std::is_aggregate_v<T>) { ...

Description: Checks wither the given type is an aggregate type.

Documentation: std::is_aggregate

Notes: *** promo None

std::has_unique_object_representations [tbd]

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 None

std::reduce [tbd]

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::inclusive_scan [tbd]

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 None

std::exclusive_scan [tbd]

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 None

std::gcd [tbd]

static_assert(std::gcd(12, 18) == 6);

Description: Computes the greatest common divisor of its arguments.

Documentation: std::gcd

Notes: *** promo None

std::lcm [tbd]

static_assert(std::lcm(12, 18) == 36);

Description: Computes the least common multiple of its arguments.

Documentation: std::lcm

Notes: *** promo None

Mathematical special functions [tbd]

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

3D std::hypot [tbd]

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 None

std::launder [tbd]

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 None

std::to_chars/std::from_chars [tbd]

std::to_chars(str.data(), str.data() + str.size(), 42);
std::from_chars(str.data(), str.data() + str.size(), result);

Description: Locale-independent, non-allocating, non-throwing functions to convert values to/from character strings, designed for use in high-throughput contexts.

Documentation: std::to_chars, std::from_chars

Notes: *** promo None

std::atomic::is_always_lock_free [tbd]

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::is_always_lock_free

Notes: *** promo None

std::scoped_lock [tbd]

std::scoped_lock lock(e1.m, e2.m);

Description: Provides an RAII-style mechanism for owning one or more mutexes for the duration of a scoped block.

Documentation: std::scoped_lock

Notes: *** promo See also base::AutoLock.

std::timespec_get [tbd]

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

Abseil Banned Library Features {#absl-blocklist}

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

Notes: *** promo Banned since workaround for lack of RTTI isn't compatible with the component build (Bug). Also see std::any.

bind_front [banned]

absl::bind_front

Description: 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.

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

Notes: *** promo Banned since workaround for lack of RTTI isn't compatible with the component build. (Bug) 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

Notes: *** promo Banned due to overlap with base/ranges/algorithm.h. Use the base/ranges/ facilities instead.

FunctionRef [banned]

absl::FunctionRef

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

Documentation: function_ref.h

Notes: *** promo

  • 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

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

Span [banned]

absl::Span

Description: 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.

StatusOr [banned]

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.

String Formatting [banned]

absl::StrFormat

Description: 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.

string_view [banned]

absl::string_view

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

Documentation: absl::string_view

Notes: *** promo Banned due to only working with 8-bit characters. Keep using base::StringPiece from base/strings/.

Strings Library [banned]

absl::StrSplit
absl::StrJoin
absl::StrCat
absl::StrAppend
absl::Substitute
absl::StrContains

Description: 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.

Synchronization [banned]

absl::Mutex

Description: 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/.

Time library [banned]

absl::Duration
absl::Time
absl::TimeZone
absl::CivilDay

Description: 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.

Abseil TBD Features {#absl-review}

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]

absl::AnyInvocable

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

Documentation:

Notes: *** promo Overlaps with base::RepeatingCallback, base::OnceCallback.

Containers [tbd]

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_multiset
absl::InlinedVector
absl::FixedArray

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

Documentation:

Notes: *** promo Supplements base/containers/.

CRC32C library [tbd]

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 macros and related classes [tbd]

LOG(INFO) << message;
CHECK(condition);
absl::AddLogSink(&custom_sink_to_capture_absl_logs);

Description: Macros and related classes to perform debug loggings

Documentation: log.h check.h

Notes: *** promo Overlaps and uses same macros names as base/logging.h.