aosp12/external/webrtc/api/README.md

How to write code in the api/ directory

Mostly, just follow the regular style guide, but:

• Note that api/ code is not exempt from the “.h and .cc files come in pairs” rule, so if you declare something in api/path/to/foo.h, it should be defined in api/path/to/foo.cc.
• Headers in api/ should, if possible, not #include headers outside api/. It’s not always possible to avoid this, but be aware that it adds to a small mountain of technical debt that we’re trying to shrink.
• .cc files in api/, on the other hand, are free to #include headers outside api/.

That is, the preferred way for api/ code to access non-api/ code is to call it from a .cc file, so that users of our API headers won’t transitively #include non-public headers.

For headers in api/ that need to refer to non-public types, forward declarations are often a lesser evil than including non-public header files. The usual rules still apply, though.

.cc files in api/ should preferably be kept reasonably small. If a substantial implementation is needed, consider putting it with our non-public code, and just call it from the api/ .cc file.