api/README.md

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# How to write code in the `api/` directory

Mostly, just follow the regular [style guide](/g3doc/style-guide.md), 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/`.
- Avoid structs in api, prefer classes.

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-`api/` types, forward
declarations are often a lesser evil than including non-public header files. The
usual [rules](/g3doc/style-guide.md#forward-declarations) 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.

Avoid defining api with structs as it makes harder for the api to evolve. Your
struct may gain invariant, or change how it represents data. Evolving struct
from the api is particularly challenging as it is designed to be used in other
code bases and thus needs to be updated independently from its usage. Class with
accessors and setters makes such migration safer. See
[Google C++ style guide](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes)
for more.

If you need to evolve an existing struct in api, prefer first to convert it into
a class.