Struct SingleValueMapper 

pub struct SingleValueMapper<SA, T, A = CurrentStorage>
where
    SA: StorageMapperApi,
    A: StorageAddress<SA>,
    T: TopEncode + TopDecode + 'static,
{ /* private fields */ }

A storage mapper for managing a single serializable value with atomic read/write operations.

Storage Layout

The SingleValueMapper stores a single value directly at its storage key:

• base_key → encoded value of type T

This is the simplest storage mapper - just one key storing one value.

Main Operations

• Write: set(value) - Stores a value (accepts owned or borrowed). O(1).
• Read: get() - Retrieves the stored value. O(1).
• Conditional write: set_if_empty(value) - Stores only if empty. O(1).
• Update: update(|v| {...}) - Read-modify-write in one operation. O(1).
• Replace: replace(new_value) - Swaps value and returns old one. O(1).
• Take: take() - Retrieves value and clears storage. O(1).
• Clear: clear() - Removes value from storage. O(1).
• Check: is_empty() - Returns true if no value stored. O(1).

Value Semantics

• Empty storage: Returns the default/zero value when calling get()
• Setting a value: Overwrites any existing value
• Clearing: Removes the value entirely from storage
• Zero values: A value set to its zero/default may be indistinguishable from empty storage

Trade-offs

• Pros: Simplest storage pattern; minimal overhead; direct key-value mapping; very efficient.
• Cons: Only one value per mapper; no built-in indexing or collections.

Use Cases

• Configuration values (flags, thresholds, addresses)
• Global counters or accumulators
• Contract state variables (owner, paused status, etc.)
• Cached computed values
• Simple on-chain variables that don’t need collection semantics

Example

// Set a value
mapper.set(&owner);

// Check if empty
assert!(!mapper.is_empty());

// Get the value
let current_owner = mapper.get();
assert_eq!(current_owner, owner);

// Conditional set (only if empty)
mapper.set_if_empty(&owner);  // Does nothing, already set

// Update in place
mapper.update(|addr| {
    // Modify the value
    *addr = owner.clone();
});

// Replace and get old value
let old_owner = mapper.replace(&new_owner);
assert_eq!(old_owner, owner);

// Take value (get and clear)
let taken = mapper.take();
assert!(mapper.is_empty());

// Clear storage
mapper.set(&owner);
mapper.clear();
assert!(mapper.is_empty());

Numeric Counter Example

// Initialize counter
counter.set(0u64);

// Increment using update
counter.update(|value| *value += 1);
assert_eq!(counter.get(), 1);

// Increment and return new value
let new_value = counter.update(|value| {
    *value += 1;
    *value
});
assert_eq!(new_value, 2);

Implementations

impl<SA, T, A> SingleValueMapper<SA, T, A>

where SA: StorageMapperApi, A: StorageAddress<SA>, T: TopEncode + TopDecode,

pub fn get(&self) -> T

Retrieves current value from storage.

pub fn is_empty(&self) -> bool

Returns whether the storage managed by this mapper is empty.

pub fn raw_byte_length(&self) -> usize

impl<SA, T> SingleValueMapper<SA, T, CurrentStorage>

where SA: StorageMapperApi, T: TopEncode + TopDecode,

pub fn set<BT>(&self, new_value: BT)

where BT: Borrow<T>,

Saves argument to storage.

Accepts owned item of type T, or any borrowed form of it, such as &T.

pub fn set_if_empty<BT>(&self, value: BT)

where BT: Borrow<T>,

Saves argument to storage only if the storage is empty. Does nothing otherwise.

pub fn clear(&self)

Clears the storage for this mapper.

pub fn update<R, F: FnOnce(&mut T) -> R>(&self, f: F) -> R

Syntactic sugar, to more compactly express a get, update and set in one line. Takes whatever lies in storage, apples the given closure and saves the final value back to storage. Propagates the return value of the given function.

pub fn take(&self) -> T

Takes the value out of the storage, clearing it in the process.

pub fn replace<BT>(&self, new_value: BT) -> T

where BT: Borrow<T>,

Trait Implementations

impl<SA, T> StorageMapper<SA> for SingleValueMapper<SA, T, CurrentStorage>

where SA: StorageMapperApi, T: TopEncode + TopDecode,

fn new(base_key: StorageKey<SA>) -> Self

Will be called automatically by the #[storage_mapper] annotation generated code.

impl<SA, T> StorageMapperFromAddress<SA> for SingleValueMapper<SA, T, ManagedAddress<SA>>

where SA: StorageMapperApi, T: TopEncode + TopDecode,

fn new_from_address( address: ManagedAddress<SA>, base_key: StorageKey<SA>, ) -> Self

Will be called automatically by the #[storage_mapper_from_address] annotation generated code.

impl<SA, T> TopEncodeMulti for SingleValueMapper<SA, T, CurrentStorage>

where SA: StorageMapperApi, T: TopEncode + TopDecode,

fn multi_encode_or_handle_err<O, H>( &self, output: &mut O, h: H, ) -> Result<(), H::HandledErr>

where O: TopEncodeMultiOutput, H: EncodeErrorHandler,

Version of top_encode that can handle errors as soon as they occur. For instance in can exit immediately and make sure that if it returns, it is a success. By not deferring error handling, this can lead to somewhat smaller bytecode.

fn multi_encode<O>(&self, output: &mut O) -> Result<(), EncodeError>

where O: TopEncodeMultiOutput,

Attempt to serialize the value to output.

impl<SA, T> TypeAbi for SingleValueMapper<SA, T, CurrentStorage>

where SA: StorageMapperApi, T: TopEncode + TopDecode + TypeAbi,

type Unmanaged = <T as TypeAbi>::Unmanaged

fn type_name() -> TypeName

fn type_name_rust() -> TypeName

fn provide_type_descriptions<TDC: TypeDescriptionContainer>( accumulator: &mut TDC, )

A type can provide more than its own name. For instance, a struct can also provide the descriptions of the type of its fields. TypeAbi doesn’t care for the exact accumulator type, which is abstracted by the TypeDescriptionContainer trait.

fn type_names() -> TypeNames

impl<SA, T> TypeAbiFrom<SingleValueMapper<SA, T>> for PlaceholderOutput

where SA: StorageMapperApi, T: TopEncode + TopDecode,

impl<SA, T, R> TypeAbiFrom<SingleValueMapper<SA, T>> for SingleValue<R>

where SA: StorageMapperApi, T: TopEncode + TopDecode, R: TopDecode + TypeAbiFrom<T>,

impl<SA, T> TypeAbiFrom<SingleValueMapper<SA, T>> for SingleValueMapper<SA, T, CurrentStorage>

where SA: StorageMapperApi, T: TopEncode + TopDecode + TypeAbi,