aosp12/external/webrtc/abseil-in-webrtc.md

Using Abseil in WebRTC

You may use a subset of the utilities provided by the 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.

Allowed

• absl::InlinedVector
• 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.
• 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.

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.

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.

How to depend on Abseil

For build targets 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 such build mode, WebRTC will depend on a unique Abseil build target what will generate a shared library.