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# How to write code in the `api/` directory
Mostly, just follow the regular [style guide](../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/`.
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](../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
