Struct frame_support::pallet_prelude::StorageValue

pub struct StorageValue<Prefix, Value, QueryKind = OptionQuery, OnEmpty = GetDefault>(_);

A type that allow to store a value.

Each value is stored at:

Twox128(Prefix::pallet_prefix()) ++ Twox128(Prefix::STORAGE_PREFIX)

Implementations

impl<Prefix, Value, QueryKind, OnEmpty> StorageValue<Prefix, Value, QueryKind, OnEmpty> where
    Prefix: StorageInstance,
    Value: FullCodec,
    QueryKind: QueryKindTrait<Value, OnEmpty>,
    OnEmpty: Get<QueryKind::Query> + 'static, 

pub fn hashed_key() -> [u8; 32]

Get the storage key.

pub fn exists() -> bool

Does the value (explicitly) exist in storage?

pub fn get() -> QueryKind::Query

Load the value from the provided storage instance.

pub fn try_get() -> Result<Value, ()>

Try to get the underlying value from the provided storage instance; Ok if it exists, Err if not.

pub fn translate<O: Decode, F: FnOnce(Option<O>) -> Option<Value>>(
    f: F
) -> Result<Option<Value>, ()>

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.

Warning

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.

Usage

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).

pub fn put<Arg: EncodeLike<Value>>(val: Arg)

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

pub fn set(val: QueryKind::Query)

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

this uses the query type rather than the underlying value.

pub fn mutate<R, F: FnOnce(&mut QueryKind::Query) -> R>(f: F) -> R

Mutate the value

pub fn try_mutate<R, E, F: FnOnce(&mut QueryKind::Query) -> Result<R, E>>(
    f: F
) -> Result<R, E>

Mutate the value if closure returns Ok

pub fn kill()

Clear the storage value.

pub fn take() -> QueryKind::Query

Take a value from storage, removing it afterwards.

pub fn append<Item, EncodeLikeItem>(item: EncodeLikeItem) where
    Item: Encode,
    EncodeLikeItem: EncodeLike<Item>,
    Value: StorageAppend<Item>, 

Append the given item to the value in the storage.

Value is required to implement StorageAppend.

Warning

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.

pub fn decode_len() -> Option<usize> where
    Value: StorageDecodeLength, 

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

Value 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.

Warning

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

Trait Implementations

impl<Prefix, Value, QueryKind, OnEmpty> StorageValueMetadata for StorageValue<Prefix, Value, QueryKind, OnEmpty> where
    Prefix: StorageInstance,
    Value: FullCodec,
    QueryKind: QueryKindTrait<Value, OnEmpty>,
    OnEmpty: Get<QueryKind::Query> + 'static, 

const MODIFIER: StorageEntryModifier

const NAME: &'static str

const DEFAULT: DefaultByteGetter