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: Default allowed; see banned features below
- C++20: Initially supported November 13, 2023; see allowed/banned/TBD features below
- C++23: Initially supported January 2026; see allowed/banned/TBD features below
- C++26: Not yet supported
- 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::linked_hash_set & map: Initially added to third_party Dec 30, 2025
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 [email protected] 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
Notes:
long long Type [banned]
long long var = value;
Description: An integer of at least 64 bits.
Documentation: Fundamental types
Notes:
User-Defined Literals [banned]
DistanceType var = 12_km;
Description: Allows user-defined literal expressions.
Documentation: User-defined literals
Notes:
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>
Notes:
<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>
Notes:
<chrono> [banned]
#include <chrono>
Description: A standard date and time library.
Documentation: Date and time utilities
Notes:
Overlaps with base/time.
<exception> [banned]
#include <exception>
Description: Exception throwing and handling.
Documentation: Standard library header <exception>
Notes:
Engines And Generators From <random> [banned]
std::mt19937 generator;
Description: Methods of generating random numbers.
Documentation: Pseudo-random number generation
Notes:
<ratio> [banned]
#include <ratio>
Description: Provides compile-time rational numbers.
Documentation: std::ratio
Notes:
<regex> [banned]
#include <regex>
Description: A standard regular expressions library.
Documentation: Regular expressions library
Notes:
Overlaps with many regular expression libraries in Chromium. When in doubt, use third_party/re2.
std::aligned_{storage,union} [banned]
std::aligned_storage<sizeof(T), alignof<T>>::type buf;
Description: Creates aligned, uninitialized storage to later hold one or more objects.
Documentation: std::aligned_storage
Notes:
Deprecated in C++23. Generally, use alignas(T) char buf[sizeof(T)];. Aligned unions can be handled similarly, using the max alignment and size of the union members, either passed via a pack or computed inline.
std::bind [banned]
auto x = std::bind(function, args, ...);
Description: Declares a function object bound to certain arguments.
Documentation: std::bind
Notes:
std::function [banned]
std::function x = [] { return 10; }; std::function y = std::bind(foo, args);
Description: Wraps a standard polymorphic function.
Documentation: std::function
Notes:
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
Notes:
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
Notes:
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
Notes:
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
Notes:
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
Notes:
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]
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:
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”.
Notes:
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
Notes:
std::any [banned]
std::any x = 5;
Description: A type-safe container for single values of any type.
Documentation: std::any
Notes:
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
Notes:
std::filesystem [banned]
#include <filesystem>
Description: A standard way to manipulate files, directories, and paths in a filesystem.
Documentation: Filesystem library
Notes:
std::{from,to}_chars [banned]
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:
Overlaps with utilities in base/strings/string_number_conversions.h, which are easier to use correctly.
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
Notes:
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
Notes:
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
Notes:
Banned because exceptions 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:
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
Notes:
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
Notes:
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
Notes:
None
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
Notes:
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
Notes:
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
Notes:
None
__has_cpp_attribute [allowed]
#if __has_cpp_attribute(assume) // Toolchain supports C++23 `[[assume]]`. ... #endif
Description: Checks whether the toolchain supports a particular standard attribute.
Documentation: Feature testing
Notes:
None
constinit [allowed]
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:
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
Notes:
None
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
Notes:
None
Language feature-test macros [allowed]
#if !defined(__cpp_modules) || (__cpp_modules < 201907L) ... // Toolchain does not support modules #endif
Description: Provides a standardized way to test the toolchain's implementation of a particular language feature.
Documentation: Feature testing
Notes:
None
[[likely]], [[unlikely]] [allowed]
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:
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
Notes:
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
Notes:
using enum declarations [allowed]
enum class E { kA = 1 }; void f() { using enum E; auto a = kA; }
Description: Introduces enumerator element names into the current scope.
Documentation: using enum declaration
Notes:
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>
Notes:
<compare> [allowed]
#include <compare>
Description: Concepts and classes used to implement three-way comparison (“spaceship”, <=>) support.
Documentation: Standard library header <compare>
Notes:
None
<concepts> [allowed]
#include <concepts>
Description: Various useful concepts, many of which replace pre-concept machinery in <type_traits>.
Documentation: Standard library header <concepts>
Notes:
None
Range algorithms [allowed]
constexpr int kArr[] = {2, 4, 6, 8, 10, 12}; constexpr auto is_even = [] (auto x) { return x % 2 == 0; }; static_assert(std::ranges::all_of(kArr, is_even));
Description: Provides versions of most algorithms that accept either an iterator-sentinel pair or a single range argument.
Documentation: Ranges algorithms
Notes:
Range access, range primitives, dangling iterator handling, and range concepts [allowed]
// Range access: constexpr int kArr[] = {2, 4, 6, 8, 10, 12}; static_assert(std::ranges::size(kArr) == 6); // Range primitives: static_assert( std::same_as<std::ranges::iterator_t<decltype(kArr)>, const int*>); // Range concepts: static_assert(std::ranges::contiguous_range<decltype(kArr)>);
Description: Various helper functions and types for working with ranges.
Documentation: Ranges library
Notes:
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. #endif
Description: Provides a standardized way to test the toolchain's implementation of a particular library feature.
Documentation: Feature testing
Notes:
None
<numbers> [allowed]
#include <numbers>
Description: Provides compile-time constants for many common mathematical values, e.g. pi and e.
Documentation: Mathematical constants
Notes:
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
Notes:
None
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)
Notes:
std::hardware_{con,de}structive_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:
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
Notes:
None
std::lerp [allowed]
double val = std::lerp(start, end, t);
Description: Linearly interpolates (or extrapolates) between two values.
Documentation: std::lerp
Notes:
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
Notes:
None
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
Notes:
None
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
Notes:
std::ranges::subrange [allowed]
void transform(const std::multimap<int, char>& map, int key) { auto [first, last] = map.equal_range(key); for (const auto& [_, value] : std::ranges::subrange(first, last)) { ...
Description: Creates a view from an iterator and a sentinel. Useful for treating non-contiguous storage (e.g. a std::map) as a range.
Documentation: std::ranges::subrange
Notes:
std::remove_cvref[_t] [allowed]
template <typename T> requires (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:
None
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
Notes:
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
Notes:
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
Notes:
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
Notes:
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
Notes:
C++20 Banned Library Features
The following C++20 library features are not allowed in the Chromium codebase.
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
Notes:
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
Notes:
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
Notes:
Chromium functionality should not vary with the C locale.
Range factories and range adaptors [banned]
// Prints 1, 2, 3, 4, 5, 6. for (auto i : std::ranges::iota_view(1, 7)) { std::cout << i << '\n'; } constexpr int kArr[] = {6, 2, 8, 4, 4, 2}; constexpr auto plus_one = std::views::transform([](int n){ return n + 1; }); static_assert(std::ranges::equal(kArr | plus_one, {7, 3, 9, 5, 5, 3}));
Description: Lightweight objects that represent iterable sequences. Provides facilities for lazy operations on ranges, along with composition into pipelines.
Documentation: Ranges library
Notes:
std::ranges::view_interface [banned]
class MyView : public std::ranges::view_interface<MyView> { ... };
Description: CRTP base class for implementing custom view objects.
Documentation: std::ranges::view_interface
Notes:
<span> [banned]
#include <span>
Description: Utilities for non-owning views over a sequence of objects.
Notes:
Superseded by base::span, which has a richer functionality set.
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
Notes:
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>
Notes:
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
Notes:
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
Notes:
Requires significant support code and planning around API and migration.
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
Notes:
See notes on “Coroutines” above.
<format> [tbd]
std::cout << std::format("Hello {}!\n", "world");
Description: Utilities for producing formatted strings.
Documentation: Formatting library
Notes:
Has both pros and cons compared to absl::StrFormat (which we don't yet use). Migration would be nontrivial.
<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>
Notes:
Seems to regress code size vs. base::Location.
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
Notes:
See notes on char8_t above.
C++23 Allowed Language Features
The following C++23 language features are allowed in the Chromium codebase.
#elifdef, #elifndef [allowed]
#ifdef FOO ... #elifdef BAR // New. ... #elifndef BAZ // New. ... #endif
Description: New conditional inclusion preprocessor directives.
Documentation: Conditional inclusion
Notes:
C++23 Allowed Library Features
The following C++23 library features are allowed in the Chromium codebase.
std::basic_string::contains [allowed]
if (str.contains("foo")) ...
Description: More concise substring check.
Documentation: std::basic_string::contains
Notes:
std::byteswap [allowed]
auto x = std::byteswap(y);
Description: Reverses the bytes of an integer.
Documentation: std::byteswap
Notes:
std::to_underlying [allowed]
auto x = std::to_underlying(enum_val);
Description: Converts an enumeration to its underlying type.
Documentation: std::to_underlying
Notes:
C++23 TBD Language Features
The following C++23 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.
Explicit object parameter [tbd]
struct S { void f(this S& self); };
Description: Allows explicit specification of the object parameter (deducing this) in member functions.
Documentation: Explicit object parameter
Notes:
None
Multidimensional subscript operator [tbd]
struct S { int operator[](int i, int j); };
Description: Allows multiple arguments in the subscript operator.
Documentation: Operator overloading
Notes:
None
auto(x), auto{x} [tbd]
void f(const int& i) { auto copy = auto(i); }
Description: Prvalue copy (decay-copy).
Documentation: Functional cast
Notes:
None
[[assume]] [tbd]
[[assume(n > 0)]];
Description: Provides a hint to the optimizer.
Documentation: dcl.attr.assume
Notes:
None
#warning [tbd]
#warning "This is a warning"
Description: Standardized preprocessor warning directive.
Documentation: #warning
Notes:
This was standardized in C++23, but was already supported by clang. From thakis@: #warning doesn't honor -Werror, and we want things to either be errors, or silent.
Literal suffix for size_t [tbd]
auto s = 10uz;
Description: Literal suffix z or uz for std::size_t.
Documentation: Integer literal
Notes:
None
Named character escapes [tbd]
auto c = "\N{LATIN CAPITAL LETTER A}";
Description: Universal character names using \N{...}.
Documentation: Escape sequences
Notes:
None
C++23 TBD Library Features
The following C++23 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.
Constructing containers with std::from_range [tbd]
std::set<int> a_very_long_container_name = {1, 2, 3}; std::vector<int> old_way( a_very_long_container_name.begin(), a_very_long_container_name.end()); std::vector<int> new_way(std::from_range, a_very_long_container_name);
Description: More concise conversion from one container type to another.
Documentation: std::from_range
Notes:
See also std::ranges::to which offers something similar.
Monadic operations for std::optional [tbd]
opt.and_then(f).transform(g).or_else(h);
Description: and_then, transform, or_else member functions.
Documentation: std::optional
Notes:
None
std::expected [tbd]
std::expected<int, std::string> e;
Description: A vocabulary type that contains an expected value or an error.
Documentation: std::expected
Notes:
Overlaps with base::expected.
std::flat_map, std::flat_multimap, std::flat_set, std::flat_multiset [tbd]
std::flat_map<int, std::string> map;
Description: Container adaptors that provide the functionality of associative containers using sorted vectors.
Documentation: std::flat_map std::flat_multimap std::flat_set std::flat_multiset
Notes:
Overlaps with base::flat_map and base::flat_set.
std::out_ptr, std::inout_ptr [tbd]
std::unique_ptr<T> p; void GetT(T** out); GetT(std::out_ptr(p));
Description: Smart pointer adapters for functions that take raw pointers as out-parameters.
Documentation: std::out_ptr, std::inout_ptr
Notes:
None
std::mdspan [tbd]
std::mdspan m(ptr, 10, 10);
Description: Multidimensional array view.
Documentation: std::mdspan
Notes:
We ban std::span in favor of base::span, which has better safety guarantees. If we want to support this, maybe we should implement base::mdspan.
std::ranges::to [tbd]
std::set<int> s = {1, 2, 3}; auto u = std::ranges::to<std::vector>(s); auto v = s | std::ranges::to<std::vector>();
Description: Converts a range to a container.
Documentation: std::ranges::to
Notes:
We should ban the 2nd case in the snippet (use as an adaptor), but might want to allow the 1st case (simple container conversion). Note there's also std::from_range for use cases like the 1st one.
Range Formatting [tbd]
LOG(INFO) << std::format("Values: {}", my_vector);
Description: Extends <format> to support printing containers and ranges.
Documentation: <format>
Notes:
As of 01/2026, <format> is still a TBD feature from C++20.
std::print [tbd]
std::print("Hello {}", "world");
Description: Formatted output.
Documentation: Print functions
Notes:
Overlaps with LOG() and friends.
std::generator [tbd]
std::generator<int> gen() { co_yield 1; }
Description: Coroutine generator.
Documentation: std::generator
Notes:
As of 01/2026, coroutine support in Chromium is still TBD.
std::stacktrace [tbd]
auto trace = std::stacktrace::current();
Description: Captures a stack trace.
Documentation: std::stacktrace
Notes:
Overlaps with base::debug::StackTrace.
std::move_only_function [tbd]
std::move_only_function<void()> f = [x = std::make_unique<int>(1)] {};
Description: Function wrapper for move-only objects.
Documentation: std::move_only_function
Notes:
Overlaps with base::OnceCallback.
std::unreachable [tbd]
std::unreachable();
Description: Indicates a codepath that is unreachable and invokes undefined behavior if executed.
Documentation: std::unreachable
Notes:
Standard version of __builtin_unreachable(). Unlike NOTREACHED(), which cleanly aborts, this hints the compiler to optimize assuming this point is unreachable, with no bounds on how the program behaves if the annotation was wrong. This and __builtin_unreachable() should only be used in rare circumstances.
std::spanstream [tbd]
std::spanstream s(buffer);
Description: Input/output stream using a span as buffer.
Documentation: std::spanstream
Notes:
None
Fixed width floating-point types [tbd]
#include <stdfloat> int main() { std::float64_t f = 0.1f64; }
Description: Similar to int32_t and friends but for floats.
Notes:
None
std::start_lifetime_as [tbd]
void* storage = std::malloc(sizeof(T)); T* p = std::start_lifetime_as<T>(storage);
Description: Explicitly starts the lifetime of an object of type T in the given storage.
Documentation: std::start_lifetime_as
Notes:
None
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
Notes:
AnyInvocable [banned]
absl::AnyInvocable
Description: An equivalent of the C++23 std::move_only_function.
Documentation:
Notes:
Banned due to overlap with base::RepeatingCallback, base::OnceCallback.
Attributes [banned]
T* data() ABSL_ATTRIBUTE_LIFETIME_BOUND { return data_; } ABSL_ATTRIBUTE_NO_TAIL_CALL ReturnType Loop(); struct S { bool b; int32_t i; } ABSL_ATTRIBUTE_PACKED;
Description: Cross-platform macros to expose compiler-specific functionality.
Documentation: attributes.h
Notes:
btree_* containers [banned]
absl::btree_map absl::btree_set absl::btree_multimap absl::btree_multiset
Description: Alternatives to the tree-based standard library containers designed to be more efficient in the general case.
Documentation: Containers
Notes:
In theory these should be superior alternatives that could replace most uses of std::map and company. In practice they have been found to introduce a substantial code size increase. Until this problem can be resolved the use of these containers is banned. Use the standard library containers instead.
bind_front [banned]
absl::bind_front
Description: Binds the first N arguments of an invocable object and stores them by value.
Documentation:
Notes:
Banned due to overlap 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
Notes:
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:
Superseded by algorithms in std::ranges::.
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
Notes:
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]
absl::FunctionRef
Description: Type for holding a non-owning reference to an object of any invocable type.
Documentation: function_ref.h
Notes:
Log macros and related classes [banned]
LOG(INFO) << message; CHECK(condition); absl::AddLogSink(&custom_sink_to_capture_absl_logs);
Description: Macros and related classes to perform debug loggings
Notes:
Banned due to overlap with base/logging.h. We‘d like to drop Chromium’s version and replace with the Abseil one, but no one has looked into how to migrate and what impacts (e.g. build time) we‘d incur. If you’d like to do this work, please contact cxx@.
NoDestructor [banned]
// 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:
Nullability annotations [banned]
void PaySalary(Employee* absl_nonnull employee) { pay(*employee); // OK to dereference }
Description: Annotations to more clearly specify contracts
Documentation: nullability.h
Notes:
Banned due to no feasible path to codebase-wide use and little mechanism for enforcement.
Optional [banned]
absl::optional<int> Func(bool b) { return b ? absl::make_optional(1) : absl::nullopt; }
Description: Early adaptation of C++17 std::optional.
Documentation: std::optional
Notes:
Superseded by std::optional. Use std::optional instead.
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:
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:
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:
Overlaps with base::expected.
string_view [banned]
absl::string_view
Description: Early adaptation of C++17 std::string_view.
Documentation: absl::string_view
Notes:
Originally banned due to only working with 8-bit characters. Now it is unnecessary because it is the same type as std::string_view. Please use std::string_view instead.
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:
Synchronization [banned]
absl::Mutex
Description: Primitives for managing tasks across different threads.
Documentation: Synchronization
Notes:
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:
Overlaps with base/time/.
Variant [banned]
absl::bad_variant_access; absl::get; absl::get_if; absl::holds_alternative; absl::monostate; absl::variant; absl::variant_alternative; absl::variant_alternative_t; absl::variant_npos; absl::variant_size; absl::variant_size_v; absl::visit;
Description: A backport of C++17's std::variant type-safe union and related utilities.
Notes:
These are just aliases to the std counterparts these days. Use std instead.
Utility library [banned]
absl::apply; absl::exchange; absl::forward; absl::in_place; absl::in_place_index; absl::in_place_index_t; absl::in_place_t; absl::in_place_type; absl::in_place_type_t; absl::index_sequence; absl::index_sequence_for; absl::integer_sequence; absl::make_from_tuple; absl::make_index_sequence; absl::make_integer_sequence; absl::move;
Description: Backports of various C++17 template utilities.
Notes:
These are just aliases to the std counterparts these days. Use std instead.
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.
absl::linked_hash_set, absl::linked_hash_map [tbd]
absl::linked_hash_set<int> m; m.insert(2); m.insert(1); m.insert(3); EXPECT_THAT(m, ElementsAre(2, 1, 3));
Description: A simple insertion-ordered set or map. It provides O(1) amortized insertions and lookups, as well as iteration in the insertion order.
Documentation: