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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&rsquo;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 deps.

The GN templates will take care of generating the proper dependency when used within Chromium or standalone. 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
• [Hash tables, and B-tree ordered][abseil-containers] containers
• absl::InlinedVector
• absl_nonnull and absl_nullable
• absl::WrapUnique
• 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.
• The functions in absl/algorithm/algorithm.h and

absl/algorithm/container.h.

• The macros in absl/base/attributes.h, absl/base/config.h and

absl/base/macros.h.

• absl/numeric/bits.h
• Single argument absl::StrCat

• ABSL_FLAG is allowed in tests and tools, but disallowed in in non-test code.

[abseil-containers]: https://abseil.io/docs/cpp/guides/container

**Disallowed**

`absl::make_unique`

Use `std::make_unique` instead.

`absl::Mutex`

Use `webrtc::Mutex` instead.

`absl::optional` and `absl::variant`

Use `std::optional` and `std::variant` directly.

`absl::Span`

Use `webrtc::ArrayView` instead.

absl::Span differs from webrtc::ArrayView on several points, and both of them differ from the std::span introduced in C++20. We should just keep using webrtc::ArrayView and avoid absl::Span. Note that we are planning to replace webrtc::ArrayView with std::span rather than with absl::Span, see https://bugs.webrtc.org/439801349

`absl::StrCat`, `absl::StrAppend`, `absl::StrJoin`, `absl::StrSplit`

Use `webrtc::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.

Exception: Single-argument absl::StrCat is allowed in order to make it easy to use AbslStringify. See TOTW #215 for details on AbslStringify.