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The ink_storage crate acts as the standard storage library for ink! smart contracts. It provides all the necessary tools and data structures to organize and operate the contract's storage intuitively and efficiently.

You can find the crates documentation for all ink! data structures here.

At the moment we provide these data structures:

  • BinaryHeap: A priority queue implemented with a binary heap.
  • BitStash A stash for bits operating on the contract storage.
  • Bitvec A storage bit vector.
  • HashMap A hash map operating on the contract storage.
  • SmallVec A contiguous growable array type.
  • Stash A stash data structure operating on contract storage.
  • Vec A contiguous growable array type, written Vec<T> but pronounced 'vector'.

Data structures provided by the ink_storage crate are inherently lazy; they are either high-level lazy or low-level lazy data structures. The difference between high-level and low-level lies in the distinction in how these data structures are aware of the elements that they operate on.

For high-level data structures they are fully aware about the elements they contain, do all the clean-up by themselves so the user can concentrate on the business logic.

For low-level data structures the responsibility about the elements lies in the hands of the contract author. Also they operate on cells (Option<T>) instead of entities of type T. But what does that mean exactly?

The ink_storage::Lazy type caches their entities and acts lazily on the storage. This means that a read or write operation is only performed when it really needs to in order to satisfy other inputs.

Data types such as Rust primitives i32 or Rust's very own Vec or data structures can also be used to operate on the contract's storage, however, they will load their contents eagerly which is often not what you want.

An example follows with the below contract storage and a message that operates on either of the two fields.

#[ink(storage)]pub struct TwoValues {    offset: i32,    a: i32,    b: i32,}
impl TwoValues {    #[ink(message)]    pub fn set(&mut self, which: bool, new_value: i32) {        match which {            true  => { self.a = self.offset + new_value; },            false => { self.b = self.offset + new_value; },        }    }}

Whenever we call TwoValues::set always both a and b are loaded despite the fact the we only operate on one of them at a time. This is very costly since storage accesses are in fact database look-ups. In order to prevent this eager loading of storage contents we can make use of ink_storage::Lazy or other lazy data structures defined in that crate:

#[ink(storage)]pub struct TwoValues {    offset: i32,    a: ink_storage::Lazy<i32>,    b: ink_storage::Lazy<i32>,}
impl TwoValues {    #[ink(message)]    pub fn set(&mut self, which: bool, new_value: i32) {        match which {            true  => { self.a = offset + new_value; },            false => { self.b = offset + new_value; },        }    }}

Now a and b are only loaded when the contract really needs their values. Note that offset remained i32 since it is always needed and could spare the minor overhead of the ink_storage::Lazy wrapper.