pub trait StorageValue<T: FullCodec> {
    type Query;

Show 13 methods fn hashed_key() -> [u8; 32]; fn exists() -> bool; fn get() -> Self::Query; fn try_get() -> Result<T, ()>; fn translate<O: Decode, F: FnOnce(Option<O>) -> Option<T>>(
        f: F
    ) -> Result<Option<T>, ()>; fn put<Arg: EncodeLike<T>>(val: Arg); fn set(val: Self::Query); fn mutate<R, F: FnOnce(&mut Self::Query) -> R>(f: F) -> R; fn try_mutate<R, E, F: FnOnce(&mut Self::Query) -> Result<R, E>>(
        f: F
    ) -> Result<R, E>; fn kill(); fn take() -> Self::Query; fn append<Item, EncodeLikeItem>(item: EncodeLikeItem)
        Item: Encode,
        EncodeLikeItem: EncodeLike<Item>,
        T: StorageAppend<Item>
; fn decode_len() -> Option<usize>
        T: StorageDecodeLength
, { ... }
Expand description

A trait for working with macro-generated storage values under the substrate storage API.

Details on implementation can be found at [generator::StorageValue].

Required Associated Types

The type that get/take return.

Required Methods

Get the storage key.

Does the value (explicitly) exist in storage?

Load the value from the provided storage instance.

Try to get the underlying value from the provided storage instance.

Returns Ok if it exists, Err if not.

Translate a value from some previous type (O) to the current type.

f: F is the translation function.

Returns Err if the storage item could not be interpreted as the old type, and Ok, along with the new value if it could.

NOTE: This operates from and to Option<_> types; no effort is made to respect the default value of the original type.


This function must be used with care, before being updated the storage still contains the old type, thus other calls (such as get) will fail at decoding it.


This would typically be called inside the module implementation of on_runtime_upgrade, while ensuring no usage of this storage are made before the call to on_runtime_upgrade. (More precisely prior initialized modules doesn’t make use of this storage).

Store a value under this key into the provided storage instance.

Store a value under this key into the provided storage instance; this uses the query type rather than the underlying value.

Mutate the value

Mutate the value if closure returns Ok

Clear the storage value.

Take a value from storage, removing it afterwards.

Append the given item to the value in the storage.

T is required to implement StorageAppend.


If the storage item is not encoded properly, the storage item will be overwritten and set to [item]. Any default value set for the storage item will be ignored on overwrite.

Provided Methods

Read the length of the storage value without decoding the entire value.

T is required to implement StorageDecodeLength.

If the value does not exists or it fails to decode the length, None is returned. Otherwise Some(len) is returned.


None does not mean that get() does not return a value. The default value is completly ignored by this function.