Contract Storage

The most common way for interacting with a contract’s storage is through storage variables. As stated previously, storage variables allow you to store data that will be stored in the contract's storage that is itself stored on the blockchain. These data are persistent and can be accessed and modified anytime once the contract is deployed.

Storage variables in Starknet contracts are stored in a special struct called Storage:

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

A Storage Struct

The storage struct is a struct like any other, except that it must be annotated with #[storage]. This annotation tells the compiler to generate the required code to interact with the blockchain state, and allows you to read and write data from and to storage. Moreover, this allows you to define storage mappings using the LegacyMap type.

Each variable stored in the storage struct is stored in a different location in the contract's storage. The storage address of a variable is determined by the variable's name, and the eventual keys of the variable if it is a mapping.

Storage Addresses

The address of a storage variable is computed as follows:

• If the variable is a single value (not a mapping), the address is the sn_keccak hash of the ASCII encoding of the variable's name. sn_keccak is Starknet's version of the Keccak256 hash function, whose output is truncated to 250 bits.
• If the variable is a mapping, the address of the value at key k_1,...,k_n is h(...h(h(sn_keccak(variable_name),k_1),k_2),...,k_n) where ℎ is the Pedersen hash and the final value is taken mod (2^251) − 256.
• If it is a mapping to complex values (e.g., tuples or structs), then this complex value lies in a continuous segment starting from the address calculated in the previous point. Note that 256 field elements is the current limitation on the maximal size of a complex storage value.

You can access the address of a storage variable by calling the address function on the variable, which returns a StorageBaseAddress value.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Interacting with Storage Variables

Variables stored in the storage struct can be accessed and modified using the read and write functions, and you can get their address in storage using the addr function. These functions are automatically generated by the compiler for each storage variable.

To read the value of the owner storage variable, which is a single value, we call the read function on the owner variable, passing in no parameters.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Calling the read function on the owner variable

To read the value of the storage variable names, which is a mapping from ContractAddress to felt252, we call the read function on the names variable, passing in the key address as a parameter. If the mapping had more than one key, we would pass in the other keys as parameters as well.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Calling the read function on the names variable

To write a value to a storage variable, we call the write function passing in the eventual keys the value as arguments. As with the read function, the number of arguments depends on the number of keys - here, we only pass in the value to write to the owner variable as it is a simple variable.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Calling the write function on the owner variable

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Calling the write function on the names variable

Storing custom types

The Store trait, defined in the starknet::storage_access module, is used to specify how a type should be stored in storage. In order for a type to be stored in storage, it must implement the Store trait. Most types from the core library, such as unsigned integers (u8, u128, u256...), felt252, bool, ContractAddress, etc. implement the Store trait and can thus be stored without further action.

But what if you wanted to store a type that you defined yourself, such as an enum or a struct? In that case, you have to explicitly tell the compiler how to store this type.

In our example, we want to store a Person struct in storage, which is possible by implementing the Store trait for the Person type. This can be achieved by simply adding a #[derive(starknet::Store)] attribute on top of our struct definition.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Similarly, Enums can be written to storage if they implement the Store trait, which can be trivially derived as long as all associated types implement the Store trait.

use starknet::ContractAddress;

#[starknet::interface]
trait INameRegistry<TContractState> {
    fn store_name(
        ref self: TContractState, name: felt252, registration_type: NameRegistry::RegistrationType
    );
    fn get_name(self: @TContractState, address: ContractAddress) -> felt252;
    fn get_owner(self: @TContractState) -> NameRegistry::Person;
}


#[starknet::contract]
mod NameRegistry {
    use starknet::{ContractAddress, get_caller_address};

    #[storage]
    struct Storage {
        names: LegacyMap::<ContractAddress, felt252>,
        registration_type: LegacyMap::<ContractAddress, RegistrationType>,
        total_names: u128,
        owner: Person
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        StoredName: StoredName,
    }

    #[derive(Drop, starknet::Event)]
    struct StoredName {
        #[key]
        user: ContractAddress,
        name: felt252
    }

    #[derive(Copy, Drop, Serde, starknet::Store)]
    struct Person {
        name: felt252,
        address: ContractAddress
    }

    #[derive(Drop, Serde, starknet::Store)]
    enum RegistrationType {
        finite: u64,
        infinite
    }

    #[constructor]
    fn constructor(ref self: ContractState, owner: Person) {
        self.names.write(owner.address, owner.name);
        self.total_names.write(1);
        self.owner.write(owner);
    }

    #[external(v0)]
    impl NameRegistry of super::INameRegistry<ContractState> {
        fn store_name(ref self: ContractState, name: felt252, registration_type: RegistrationType) {
            let caller = get_caller_address();
            self._store_name(caller, name, registration_type);
        }

        fn get_name(self: @ContractState, address: ContractAddress) -> felt252 {
            let name = self.names.read(address);
            name
        }
        fn get_owner(self: @ContractState) -> Person {
            let owner = self.owner.read();
            owner
        }
    }

    #[generate_trait]
    impl InternalFunctions of InternalFunctionsTrait {
        fn _store_name(
            ref self: ContractState,
            user: ContractAddress,
            name: felt252,
            registration_type: RegistrationType
        ) {
            let mut total_names = self.total_names.read();
            self.names.write(user, name);
            self.registration_type.write(user, registration_type);
            self.total_names.write(total_names + 1);
            self.emit(StoredName { user: user, name: name });

        }
    }

    fn get_contract_name() -> felt252 {
        'Name Registry'
    }

    fn get_owner_storage_address(self: @ContractState) -> starknet::StorageBaseAddress {
        self.owner.address()
    }
}

Structs storage layout

On Starknet, structs are stored in storage as a sequence of primitive types. The elements of the struct are stored in the same order as they are defined in the struct definition. The first element of the struct is stored at the base address of the struct, which is computed as specified in Storage Addresses and can be obtained by calling var.address(), and subsequent elements are stored at addresses contiguous to the first element. For example, the storage layout for the owner variable of type Person will result in the following layout:

Enums storage layout

When you store an enum variant, what you're essentially storing is the variant's index and an eventual associated values. This index starts at 0 for the first variant of your enum and increments by 1 for each subsequent variant. If your variant has an associated value, it's stored starting from the address immediately following the base address. For example, suppose we have the RegistrationType enum with the finite variant, which carries an associated limit date. The storage layout would look like this:

Storage mappings

Storage mappings are similar to hash tables in that they allow mapping keys to values. However, unlike a typical hash table, the key data itself is not stored - only its hash is used to look up the associated value in the contract's storage. Mappings do not have a concept of length or whether a key/value pair is set. The only way to remove a mapping is to set its value to the default zero value.

Mappings are only used to compute the location of data in the storage of a contract given certain keys. They are thus only allowed as storage variables. They cannot be used as parameters or return parameters of contract functions, and cannot be used as types inside structs.

Mapping keys to storage values

To declare a mapping, use the LegacyMap type enclosed in angle brackets <>, specifying the key and value types.

You can also create more complex mappings with multiple keys. You can find one in Listing 99-2bis like the popular allowances storage variable in the ERC20 Standard which maps an owner and an allowed spender to its allowance amount using multiple keys passed inside a tuple:

    #[storage]
    struct Storage {
        allowances: LegacyMap::<(ContractAddress, ContractAddress), u256>
    }

Listing 99-2bis: Storing mappings

The address in storage of a variable stored in a mapping is computed according to the description in the Storage Addresses section. If the key of a mapping is a struct, each element of the struct constitutes a key. Moreover, the struct should implement the Hash trait, which can be derived with the #[derive(Hash)] attribute. For example, if you have struct with two fields, the address will be h(h(sn_keccak(variable_name),k_1),k_2) - where k_1 and k_2 are the values of the two fields of the struct.

Similarly, in the case of a nested mapping such as LegacyMap((ContractAddress, ContractAddress), u8), the address will be computed in the same way: h(h(sn_keccak(variable_name),k_1),k_2).

For more details about the contract storage layout in the Starknet Documentation