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Version: 6.x

Working with StorageVec

In this section we demonstrate how to work with ink!'s StorageVec.

Example: Using a StorageVec​

Here is an example of an append-only on-chain log:

#[ink(storage)]
pub struct MyContract {
    on_chain_log: ink::storage::StorageVec<String>,
}

The following example contract utilizes a StorageVec to log each operation on chain (similar to emitting events but the contract can access them).

#![cfg_attr(not(feature = "std"), no_std, no_main)]

#[ink::contract]
mod mycontract {
    use ink::prelude::{format, string::String};
    use ink::storage::StorageVec;

    #[ink(storage)]
    pub struct MyContract {
        on_chain_log: StorageVec<String>,
    }

    impl MyContract {
        #[ink(constructor)]
        pub fn new() -> Self {
            Self {
                on_chain_log: Default::default(),
            }
        }

        /// Donate money to the contract.
        #[ink(message, payable)]
        pub fn donate(&mut self) {
            let caller = self.env().caller();
            let endowment = self.env().transferred_value();

            let log_message = format!("{caller:?} donated {endowment}");

            self.on_chain_log.push(&log_message);
        }

        /// How many donations had the contract so far?
        #[ink(message)]
        pub fn log_length(&self) -> u32 {
            self.on_chain_log.len()
        }

        /// What was the last donation to the contract?
        #[ink(message)]
        pub fn last_donation(&self) -> Option<String> {
            self.on_chain_log.peek()
        }
    }
}

Difference between StorageVec and Rusts Vec type​

Any Rust Vec<T> will exhibit Packed storage layout; where StorageVec stores each value under it's own storage key.

Hence, any read or write from or to a Vec on storage will load or store all of its elements.

This can be undesirable: The cost of reading or writing a single element grows linearly corresponding to the number of elements in the vector (its length). Additionally, the maximum capacity of the whole vector is limited by the size of ink!'s static buffer used during ABI encoding and decoding (default 16 KiB).

StorageVec on the other hand allows to access each element individually. Thus, it can theoretically grow to infinite size. However, we currently limit the length at 2 ^ 32 elements. In practice, even if the vector elements are single bytes, it'll allow to store more than 4 GB data in blockchain storage.

Caveats​

Iterators are not provided. StorageVec is expected to be used to store a lot elements, where iterating through the elements would be rather inefficient. Manually iterating over the elements using a loop is possible but considered an anti-pattern for most cases.

For the same reason, operations which would require re-ordering stored elements are not supported. Examples include inserting and deleting elements at arbitrary positions or sorting elements.

The decision whether to use Vec<T> or StorageVec can be seen as an optimization problem with several factors:

  • How large you expect the vector to grow
  • The size of individual elements being stored
  • How frequently reads, writes and iterations happen

For example, if a vector is expected to stay small but is frequently iterated over. Choosing a Vec<T> instead of StorageVec will be preferred as individual storage reads are much more expensive as opposed to retrieving and decoding the whole collection with a single storage read.

Storage Layout​

At given StorageKey K, the length of the StorageVec is hold. Each element E is then stored under a combination of the StorageVec key K and the elements index.

Given StorageVec under key K, the storage key E of the Nth element is calculated as follows:

E = scale::Encode((K, N))

Considerations when using the StorageVec type​

StorageVec is a Lazy type similar to Mapping.

Hence, the same considerations apply to StorageVec too:

Rust Docs​

See here for the Rust documentation of this data structure: StorageVec.