Unsized Types

rkyv supports unsized types out of the box and ships with implementations for the most common unsized types (strs and slices). Trait objects can also be supported with rkyv_dyn, see "Trait Objects" for more details.


The core concept that enables unsized types is metadata. In rust, pointers to types can be different sizes, in contrast with languages like C and C++ where all pointers are the same size. This is important for the concept of sizing, which you may have encountered through rust's Sized trait.

Pointers are composed of two pieces: a data address and some metadata. The data address is what most people think of when they think about pointers; it's the location of the pointed data. The metadata for a pointer is some extra data that is needed to work safely with the data at the pointed location. It can be almost anything, or nothing at all for Sized types. Pointers with no extra metadata are sometimes called "thin" pointers, and pointers with metadata are sometimes called "wide" or "fat" pointers.

rkyv uses the ptr_meta crate to perform these conversions safely. In the future, these may be incorporated as part of the standard library.

Fundamentally, the metadata of a pointer exists to provide the program enough information to safely access, drop, and deallocate structures that are pointed to. For slices, the metadata carries the length of the slice, for trait objects it carries the virtual function table (vtable) pointer, and for custom unsized structs it carries the metadata of the single trailing unsized member.

Archived Metadata

For unsized types, the metadata for a type is archived separately from the relative pointer to the data. This mirrors how rust works internally to support archiving shared pointers and other exotic use cases. This does complicate things somewhat, but for most people the metadata archiving process will end up as just filling out a few functions and returning ().

This is definitely one of the more complicated parts of the library, and can be difficult to wrap your head around. Reading the documentation for ArchiveUnsized may help you understand how the system works by working through an example.