Sui Objects

For Sui, key is used to signify an object. Objects the only way to store data in Sui--allowing the data to persist between transactions.

For more details, see the Sui documentation on

• The Object Model
• Move Rules for Objects
• Transferring Objects

Object Rules

An object is a struct with the key ability. The first field of the struct must be id: sui::object::UID. This 32-byte field (a strongly typed wrapper around an address) is then used to uniquely identify the object.

Note that since sui::object::UID has only the store ability (it does not have copy or drop), no object has copy or drop.

Transfer Rules

Objects can be have their ownership changed and transferred in the sui::transfer module. Many functions in the module have "public" and "private" variant, where the "private" variant can only be called inside of the module that defines the object's type. The "public" variants can be called only if the object has store.

For example if we had two objects A and B defined in the module my_module:

module a::my_module {
    public struct A has key {
        id: sui::object::UID,
    }
    public struct B has key, store {
        id: sui::object::UID,
    }
}

A can only be transferred using the sui::transfer::transfer inside of a::my_module, while B can be transferred anywhere using sui::transfer::public_transfer. These rules are enforced by a custom type system (bytecode verifier) rule in Sui.