docs/upb/style-guide.md

# C style guide

<!--*
# Document freshness: For more information, see go/fresh-source.
freshness: { owner: 'haberman' reviewed: '2024-02-05' }
*-->

Since upb is written in pure C, we supplement the
[Google C++ style guide](https://google.github.io/styleguide/cppguide.html) with
some C-specific guidance.

Everything written here is intended to follow the spirit of the C++ style guide.

upb is currently inconsistent about following these conventions. It is intended
that all code will be updated to match these guidelines. The priority is
converting public interfaces as these are more difficult to change later.

## Naming

### Functions and Types

C does not have namespaces. Anywhere you would normally use a namespace
separator (`::`) in C++, we use an underscore (`_`) in C:

```c++
// C equivalent for upb::Arena::New()
upb_Arena* upb_Arena_New();
```

Since we rely on `_` to be our namespace separator, we never use it to merely
separate words in function or type names:

```c++
// BAD: this would be interpreted as upb::FieldDef::has::default().
bool upb_FieldDef_has_default(const upb_FieldDef* f);

// GOOD: this is equivalent to upb::FieldDef::HasDefault().
bool upb_FieldDef_HasDefault(const upb_FieldDef* f);
```

For multi-word namespaces, we use `PascalCase`:

```c++
// `PyUpb` is the namespace.
PyObject* PyUpb_CMessage_GetAttr(PyObject* _self, PyObject* attr);
```

### Private Functions and Members

Since we do not have `private` in C++, we use the `UPB_PRIVATE()` macro to mark
internal functions and variables that should only be accessed from upb:

```c++
// Internal-only function.
int64_t UPB_PRIVATE(upb_Int64_FromLL)();

// Internal-only members. Underscore prefixes are only necessary when the
// structure is defined in a header file.
typedef struct {
  const int32_t* UPB_PRIVATE(values);
  uint64_t UPB_PRIVATE(mask);
  int UPB_PRIVATE(value_count);
} upb_MiniTableEnum;

// Using these members in an internal function.
int upb_SomeFunction(const upb_MiniTableEnum* e) {
  return e->UPB_PRIVATE(value_count);
}
```

This prevents users from accessing these symbols, because `UPB_PRIVATE()`
is defined in `port_def.inc` and undefined in `port_undef.inc`, and these
textual headers are not accessible by users.

It is only necessary to use `UPB_PRIVATE()` for things defined in headers.
For symbols in `.c` files, it is sufficient to mark private functions with
`static`.