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<!-- go/cmark -->
<!--* freshness: {owner: 'danilchap' reviewed: '2021-05-12'} *-->
# Using Abseil in WebRTC
You may use a subset of the utilities provided by the [Abseil][abseil]
library when writing WebRTC C++ code. Below, we list the explicitly
*allowed* and the explicitly *disallowed* subsets of Abseil; if you
find yourself in need of something that isn’t in either subset,
please add it to the *allowed* subset in this doc in the same CL that
adds the first use.
[abseil]: https://abseil.io/about/
## How to depend on Abseil
For build targets of type `rtc_library`, `rtc_source_set` and
`rtc_static_library`, dependencies on Abseil need to be listed in `absl_deps`
instead of `deps`.
This is needed in order to support the Abseil component build in Chromium. In
that build mode, WebRTC will depend on a monolithic Abseil build target that
will generate a shared library.
## **Allowed**
* `absl::AnyInvocable`
* `absl::bind_front`
* `absl::Cleanup`
* `absl::InlinedVector`
* `absl::Nonnull` and `absl::Nullable`
* `absl::WrapUnique`
* `absl::optional` and related stuff from `absl/types/optional.h`.
* `absl::string_view`
* The functions in `absl/strings/ascii.h`, `absl/strings/match.h`,
and `absl/strings/str_replace.h`.
* The functions in `absl/strings/escaping.h`.
* `absl::is_trivially_copy_constructible`,
`absl::is_trivially_copy_assignable`, and
`absl::is_trivially_destructible` from `absl/meta/type_traits.h`.
* `absl::variant` and related stuff from `absl/types/variant.h`.
* The functions in `absl/algorithm/algorithm.h` and
`absl/algorithm/container.h`.
* `absl/base/const_init.h` for mutex initialization.
* The macros in `absl/base/attributes.h`, `absl/base/config.h` and
`absl/base/macros.h`.
* `absl/numeric/bits.h`
## **Disallowed**
### `absl::make_unique`
*Use `std::make_unique` instead.*
### `absl::Mutex`
*Use `webrtc::Mutex` instead.*
Chromium has a ban on new static initializers, and `absl::Mutex` uses
one. To make `absl::Mutex` available, we would need to nicely ask the
Abseil team to remove that initializer (like they already did for a
spinlock initializer). Additionally, `absl::Mutex` handles time in a
way that may not be compatible with the rest of WebRTC.
### `absl::Span`
*Use `rtc::ArrayView` instead.*
`absl::Span` differs from `rtc::ArrayView` on several points, and both
of them differ from the `std::span` that was voted into
C++20—and `std::span` is likely to undergo further changes
before C++20 is finalized. We should just keep using `rtc::ArrayView`
and avoid `absl::Span` until C++20 is finalized and the Abseil team
has decided if they will change `absl::Span` to match.
[Bug](https://bugs.webrtc.org/9214).
### `absl::StrCat`, `absl::StrAppend`, `absl::StrJoin`, `absl::StrSplit`
*Use `rtc::SimpleStringBuilder` to build strings.*
These are optimized for speed, not binary size. Even `StrCat` calls
with a modest number of arguments can easily add several hundred bytes
to the binary.
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