styleguide/c++/c++11.md

# Modern C++ use in Chromium

_This document is part of the more general
[Chromium C++ style guide](https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md).
It summarizes the supported state of new and updated language and library
features in recent C++ standards and the [Abseil](https://abseil.io/about/)
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, C++17). For various reasons, Chromium does not immediately
allow new features on the publication of such a standard. Instead, once
toolchain support is sufficient, a standard is declared "initially supported",
with new language/library features banned pending discussion.

You can propose changing the status of a feature by sending an email to
[cxx@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/cxx).
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:** _Not yet supported in Chromium, unlikely before mid-2021;
    [tracking bug](https://crbug.com/752720)_
*   **C++20:** _Not yet standardized_
*   **Abseil:** Initially supported July 31, 2020; see allowed/banned/TBD
    features below
    *   absl::StatusOr: Initially supported September 3, 2020
    *   absl::Cleanup: Initially supported February 4, 2021

[TOC]

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

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

### Inline Namespaces

```c++
inline namespace foo { ... }
```

**Description:** Allows better versioning of namespaces.

**Documentation:**
[Inline namespaces](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces)

**Notes:**
*** promo
Banned in the
[Google Style Guide](https://google.github.io/styleguide/cppguide.html#Namespaces).
Unclear how it will work with components.
***

### long long Type

```c++
long long var = value;
```

**Description:** An integer of at least 64 bits.

**Documentation:**
[Fundamental types](https://en.cppreference.com/w/cpp/language/types)

**Notes:**
*** promo
Use a stdint.h type if you need a 64-bit number.
[Discussion thread](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/RxugZ-pIDxk)
***

### User-Defined Literals

```c++
type var = literal_value_type;
```

**Description:** Allows user-defined literal expressions.

**Documentation:**
[User-defined literals](https://en.cppreference.com/w/cpp/language/user_literal)

**Notes:**
*** promo
Banned in the
[Google Style Guide](https://google.github.io/styleguide/cppguide.html#Operator_Overloading).
***

### thread_local Storage Class

```c++
thread_local int foo = 1;
```

**Description:** Puts variables into thread local storage.

**Documentation:**
[Storage duration](https://en.cppreference.com/w/cpp/language/storage_duration)

**Notes:**
*** promo
Some surprising effects on Mac
([discussion](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/2msN8k3Xzgs),
[fork](https://groups.google.com/a/chromium.org/forum/#!topic/cxx/h7O5BdtWCZw)).
Use `base::SequenceLocalStorageSlot` for sequence support, and
`base::ThreadLocal`/`base::ThreadLocalStorage` otherwise.
***

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

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

### Bind Operations

```c++
std::bind(function, args, ...)
```

**Description:** Declares a function object bound to certain arguments

**Documentation:**
[std::bind](https://en.cppreference.com/w/cpp/utility/functional/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](https://groups.google.com/a/chromium.org/forum/#!topic/cxx/SoEj7oIDNuA)
***

### C Floating-Point Environment

```c++
#include <cfenv>
#include <fenv.h>
```

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

**Documentation:**
[Standard library header "cfenv"](https://en.cppreference.com/w/cpp/header/cfenv)

**Notes:**
*** promo
Banned by the
[Google Style Guide](https://google.github.io/styleguide/cppguide.html#C++11)
due to concerns about compiler support.
***

### Date and time utilities

```c++
#include <chrono>
```

**Description:** A standard date and time library

**Documentation:**
[Date and time utilities](https://en.cppreference.com/w/cpp/chrono)

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

### Exceptions

```c++
#include <exception>
```

**Description:** Enhancements to exception throwing and handling

**Documentation:**
[Standard library header "exception"](https://en.cppreference.com/w/cpp/header/exception)

**Notes:**
*** promo
Exceptions are banned by the
[Google Style Guide](https://google.github.io/styleguide/cppguide.html#Exceptions)
and disabled in Chromium compiles. However, the `noexcept` specifier is
explicitly allowed.
[Discussion thread](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/8i4tMqNpHhg)
***

### Function Objects

```c++
std::function
```

**Description:** Wraps a standard polymorphic function

**Documentation:**
[std::function](https://en.cppreference.com/w/cpp/utility/functional/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](https://groups.google.com/a/chromium.org/forum/#!topic/cxx/SoEj7oIDNuA)
***

### Random Number Engines

*** 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](https://en.cppreference.com/w/cpp/numeric/random)

**Notes:**
*** promo
Do not use any random number engines from `<random>`. Instead, use
`base::RandomBitGenerator`.
[Discussion thread](https://groups.google.com/a/chromium.org/forum/#!topic/cxx/16Xmw05C-Y0)
***

### Ratio Template Class

```c++
std::ratio<numerator, denominator>
```

**Description:** Provides compile-time rational numbers

**Documentation:**
[std::ratio](https://en.cppreference.com/w/cpp/numeric/ratio/ratio)

**Notes:**
*** promo
Banned by the
[Google Style Guide](https://google.github.io/styleguide/cppguide.html#C++11)
due to concerns that this is tied to a more template-heavy interface style.
***

### Regular Expressions

```c++
#include <regex>
```

**Description:** A standard regular expressions library

**Documentation:**
[Regular expressions library](https://en.cppreference.com/w/cpp/regex)

**Notes:**
*** promo
Overlaps with many regular expression libraries in Chromium. When in doubt, use
`re2`.
***

### Shared Pointers

```c++
std::shared_ptr
```

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

**Documentation:**
[std::shared_ptr](https://en.cppreference.com/w/cpp/memory/shared_ptr)

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

*   [Google Style Guide](https://google.github.io/styleguide/cppguide.html#Ownership_and_Smart_Pointers).
*   [Discussion Thread](https://groups.google.com/a/chromium.org/forum/#!topic/cxx/aT2wsBLKvzI)
***

### String-Number Conversion Functions

```c++
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:**
*   [std::stoi, std::stol, std::stoll](https://en.cppreference.com/w/cpp/string/basic_string/stol),
*   [std::stoul, std::stoull](https://en.cppreference.com/w/cpp/string/basic_string/stoul),
*   [std::stof, std::stod, std::stold](https://en.cppreference.com/w/cpp/string/basic_string/stof),
*   [std::to_string](https://en.cppreference.com/w/cpp/string/basic_string/to_string)

**Notes:**
*** promo
The string-to-number conversions rely on exceptions to communicate failure,
while the number-to-string conversions have performance concerns and depend on
the locale. Use the routines in `base/strings/string_number_conversions.h`
instead.
***

### Thread Library

*** 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](https://en.cppreference.com/w/cpp/thread)

**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

```c++
std::weak_ptr
```

**Description:** Allows a weak reference to a `std::shared_ptr`

**Documentation:**
[std::weak_ptr](https://en.cppreference.com/w/cpp/memory/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

```c++
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](https://en.cppreference.com/w/cpp/chrono/operator%22%22s)

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

## Abseil Allowed Library Features {#absl-allowlist}

The following Abseil library features are allowed in the Chromium codebase.

### Optional

```c++
absl::optional
```

**Description:** Early adaptation of C++17 `std::optional`.

**Documentation:**
[std::optional](https://en.cppreference.com/w/cpp/utility/optional)

**Notes:**
*** promo
Replaces `base::Optional`.
[Discussion thread](https://groups.google.com/a/chromium.org/g/cxx/c/zUGqagX1NFU)
***

### Status

```c++
absl::Status
```

**Description:** Type for returning detailed errors.

**Documentation:**
[status.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/status/status.h)

**Notes:**
*** promo
Approved for use inside a wrapper type. Use
[abseil_string_conversions.h](https://source.chromium.org/chromium/chromium/src/+/main:base/strings/abseil_string_conversions.h)
to convert to and from
[absl::string_view](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/strings/string_view.h)
so the wrapper can expose
[base::StringPiece](https://source.chromium.org/chromium/chromium/src/+/main:base/strings/string_piece.h).
Use
[absl::Cord](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/strings/cord.h)
directly as minimally necessary to interface; do not expose in the wrapper type
API.

[Discussion thread](https://groups.google.com/a/chromium.org/g/cxx/c/ImdFCSZ-NMA)
***

### Variant

```c++
absl::variant
```

**Description:** Early adaptation of C++17 `std::variant`.

**Documentation:**
[std::variant](https://en.cppreference.com/w/cpp/utility/variant)

**Notes:**
*** promo
[Discussion thread](https://groups.google.com/a/chromium.org/g/cxx/c/DqvG-TpvMyU)
***

## Abseil Banned Library Features {#absl-blocklist}

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

### Any

```c++
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](https://en.cppreference.com/w/cpp/utility/any)

**Notes:**
*** promo
Banned since workaround for lack of RTTI isn't compatible with the component
build. ([Bug](https://crbug.com/1096380))
***

### Command line flags

```c++
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](https://abseil.io/docs/cpp/guides/flags)

**Notes:**
*** promo
Banned since workaround for lack of RTTI isn't compatible with the component
build. ([Bug](https://crbug.com/1096380)) Use `base::CommandLine` instead.
***

### Span

```c++
absl::Span
```

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

**Documentation:** [Using absl::Span](https://abseil.io/tips/93)

**Notes:**
*** promo
Banned due to being less std::-compliant than `base::span`. Keep using
`base::span`.
***

### string_view

```c++
absl::string_view
```

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

**Documentation:** [absl::string_view](https://abseil.io/tips/1)

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

## 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.

### 128bit integer

```c++
uint64_t a;
absl::uint128 v = a;
```

**Description:** Signed and unsigned 128-bit integer types meant to mimic
intrinsic types as closely as possible.

**Documentation:**
[Numerics](https://abseil.io/docs/cpp/guides/numeric)

**Notes:**
*** promo
None
***

### bind_front

```c++
absl::bind_front
```

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

**Documentation:**
*   [bind_front.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/functional/bind_front.h)
*   [Avoid std::bind](https://abseil.io/tips/108)

**Notes:**
*** promo
Overlaps with `base::Bind`.
***

### Cleanup

```c++
FILE* sink_file = fopen(sink_path, "w");
auto sink_closer = absl::MakeCleanup([sink_file] { fclose(sink_file); });
```

**Description:** Implements the scope guard idiom, invoking the contained
callback's `operator()() &&` on scope exit.

**Documentation:**
[cleanup.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/cleanup/cleanup.h)

**Notes:**
*** promo
Similar to `defer` in Golang.
***

### Containers

```c++
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:**
*   [Containers](https://abseil.io/docs/cpp/guides/container)
*   [Hash](https://abseil.io/docs/cpp/guides/hash)

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

### Container utilities

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

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

**Documentation:**
[container.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/algorithm/container.h)

**Notes:**
*** promo
Overlaps with `base/ranges/algorithm.h`.
***

### FunctionRef

```c++
absl::FunctionRef
```

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

**Documentation:**
[function_ref.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/functional/function_ref.h)

**Notes:**
*** promo
None
***

### Random

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

**Description:** Functions and utilities for generating pseudorandom data.

**Documentation:** [Random library](https://abseil.io/docs/cpp/guides/random)

**Notes:**
*** promo
Overlaps with `base/rand_util.h`.
***

### StatusOr

```c++
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](https://source.chromium.org/chromium/chromium/src/+/main:third_party/abseil-cpp/absl/status/statusor.h)

**Notes:**
*** promo
None
***

### String Formatting

```c++
absl::StrFormat
```

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

**Documentation:**
[String Formatting](https://abseil.io/docs/cpp/guides/format)

**Notes:**
*** promo
None
***

### Strings Library

```c++
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](https://abseil.io/docs/cpp/guides/strings)

**Notes:**
*** promo
Overlaps with `base/strings`.
***

### Synchronization

```c++
absl::Mutex
```

**Description:** Primitives for managing tasks across different threads.

**Documentation:**
[Synchronization](https://abseil.io/docs/cpp/guides/synchronization)

**Notes:**
*** promo
Overlaps with `Lock` in `base/synchronization/`.
***

### Time library

```c++
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](https://abseil.io/docs/cpp/guides/time)

**Notes:**
*** promo
Overlaps with `Time` APIs in `base/`.
***