HarfBuzz features two kinds of data types: non-opaque, pass-by-value types and opaque, heap-allocated types. This kind of separation is common in C libraries that have to provide API/ABI compatibility (almost) indefinitely.

Value types: The non-opaque, pass-by-value types include integer types, enums, and small structs. Exposing a struct in the public API makes it impossible to expand the struct in the future. As such, exposing structs is reserved for cases where it’s extremely inefficient to do otherwise.

In HarfBuzz, several structs, like hb_glyph_info_t and hb_glyph_position_t, fall into that efficiency-sensitive category and are non-opaque.

For all non-opaque structs where future extensibility may be necessary, reserved members are included to hold space for possible future members. As such, it’s important to provide equal(), and hash() methods for such structs, allowing users of the API do effectively deal with the type without having to adapt their code to future changes.

Important value types provided by HarfBuzz include the structs for working with Unicode code points, glyphs, and tags for font tables and features, as well as the enums for many Unicode and OpenType properties.