Structs and Resources

Move structs are plain typed data. Structs become resources when they carry the right abilities and are stored in global storage.

Simple structs

module 0x2::example {
    struct Foo { x: u64, y: bool }
    struct Bar { foo: Foo }
}

By default, structs are module-private. If another module needs access, expose functions instead of assuming public field access.

The four abilities

1. key: lets the struct live at the top level of global storage.
2. store: lets the struct live inside another stored struct.
3. drop: lets values be discarded when they go out of scope.
4. copy: lets values be copied implicitly.

struct Message has copy, drop {
    text: String,
}

struct Billboard has key {
    messages: vector<Message>,
}

Why resources matter

If a struct has key, it can be stored under an account or object address and becomes persistent on-chain state. That is the Move replacement for much of what Solidity developers mentally file under “contract storage”.

The migration habit to build early is: decide carefully which structs should not have copy, and which ones should be durable resources versus temporary values.