[kentonv]

https://capnproto.org/encoding.html

Records in Cap'n Proto are laid out like C structs. All fields are at fixed offsets from the start of the structure. For variable-width values, the struct contains a pointer to data elsewhere in the message.

Each new object is added to the end of the message, so that the message stays contiguous. This does imply that if you resize a variable-width object, then it may have to be moved to the end of the message, and the old space it occupied becomes a hole full of zeros that can't really be reused. This is definitely a down-side of this approach: Cap'n Proto does not work great for data structures that are modified over time. It's best for write-once messages. FlatBuffers has similar limitations, IIRC.