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Module 0x2::bag

A bag is a heterogeneous map-like collection. The collection is similar to iota::table in that its keys and values are not stored within the Bag value, but instead are stored using Iota's object system. The Bag struct acts only as a handle into the object system to retrieve those keys and values. Note that this means that Bag values with exactly the same key-value mapping will not be equal, with ==, at runtime. For example

let bag1 = bag::new();
let bag2 = bag::new();
bag::add(&mut bag1, 0, false);
bag::add(&mut bag1, 1, true);
bag::add(&mut bag2, 0, false);
bag::add(&mut bag2, 1, true);
// bag1 does not equal bag2, despite having the same entries
assert!(&bag1 != &bag2);

At it's core, iota::bag is a wrapper around UID that allows for access to iota::dynamic_field while preventing accidentally stranding field values. A UID can be deleted, even if it has dynamic fields associated with it, but a bag, on the other hand, must be empty to be destroyed.

use 0x2::dynamic_field;
use 0x2::object;
use 0x2::tx_context;

Resource Bag​

struct Bag has store, key
Fields
id: object::UID

the ID of this bag

size: u64

the number of key-value pairs in the bag

Constants​

const EBagNotEmpty: u64 = 0;

Function new​

Creates a new, empty bag

public fun new(ctx: &mut tx_context::TxContext): bag::Bag
Implementation
public fun new(ctx: &mut TxContext): Bag {
Bag {
id: object::new(ctx),
size: 0,
}
}

Function add​

Adds a key-value pair to the bag bag: &mut Bag Aborts with iota::dynamic_field::EFieldAlreadyExists if the bag already has an entry with that key k: K.

public fun add<K: copy, drop, store, V: store>(bag: &mut bag::Bag, k: K, v: V)
Implementation
public fun add<K: copy + drop + store, V: store>(bag: &mut Bag, k: K, v: V) {
field::add(&mut bag.id, k, v);
bag.size = bag.size + 1;
}

Function borrow​

Immutable borrows the value associated with the key in the bag bag: &Bag. Aborts with iota::dynamic_field::EFieldDoesNotExist if the bag does not have an entry with that key k: K. Aborts with iota::dynamic_field::EFieldTypeMismatch if the bag has an entry for the key, but the value does not have the specified type.

public fun borrow<K: copy, drop, store, V: store>(bag: &bag::Bag, k: K): &V
Implementation
public fun borrow<K: copy + drop + store, V: store>(bag: &Bag, k: K): &V {
field::borrow(&bag.id, k)
}

Function borrow_mut​

Mutably borrows the value associated with the key in the bag bag: &mut Bag. Aborts with iota::dynamic_field::EFieldDoesNotExist if the bag does not have an entry with that key k: K. Aborts with iota::dynamic_field::EFieldTypeMismatch if the bag has an entry for the key, but the value does not have the specified type.

public fun borrow_mut<K: copy, drop, store, V: store>(bag: &mut bag::Bag, k: K): &mut V
Implementation
public fun borrow_mut<K: copy + drop + store, V: store>(bag: &mut Bag, k: K): &mut V {
field::borrow_mut(&mut bag.id, k)
}

Function remove​

Mutably borrows the key-value pair in the bag bag: &mut Bag and returns the value. Aborts with iota::dynamic_field::EFieldDoesNotExist if the bag does not have an entry with that key k: K. Aborts with iota::dynamic_field::EFieldTypeMismatch if the bag has an entry for the key, but the value does not have the specified type.

public fun remove<K: copy, drop, store, V: store>(bag: &mut bag::Bag, k: K): V
Implementation
public fun remove<K: copy + drop + store, V: store>(bag: &mut Bag, k: K): V {
let v = field::remove(&mut bag.id, k);
bag.size = bag.size - 1;
v
}

Function contains​

Returns true iff there is an value associated with the key k: K in the bag bag: &Bag

public fun contains<K: copy, drop, store>(bag: &bag::Bag, k: K): bool
Implementation
public fun contains<K: copy + drop + store>(bag: &Bag, k: K): bool {
field::exists_<K>(&bag.id, k)
}

Function contains_with_type​

Returns true iff there is an value associated with the key k: K in the bag bag: &Bag with an assigned value of type V

public fun contains_with_type<K: copy, drop, store, V: store>(bag: &bag::Bag, k: K): bool
Implementation
public fun contains_with_type<K: copy + drop + store, V: store>(bag: &Bag, k: K): bool {
field::exists_with_type<K, V>(&bag.id, k)
}

Function length​

Returns the size of the bag, the number of key-value pairs

public fun length(bag: &bag::Bag): u64
Implementation
public fun length(bag: &Bag): u64 {
bag.size
}

Function is_empty​

Returns true iff the bag is empty (if length returns 0)

public fun is_empty(bag: &bag::Bag): bool
Implementation
public fun is_empty(bag: &Bag): bool {
bag.size == 0
}

Function destroy_empty​

Destroys an empty bag Aborts with EBagNotEmpty if the bag still contains values

public fun destroy_empty(bag: bag::Bag)
Implementation
public fun destroy_empty(bag: Bag) {
let Bag { id, size } = bag;
assert!(size == 0, EBagNotEmpty);
id.delete()
}