Storage and Mappings
NeoVM uses key-value storage. Neo DevPack for Solidity therefore maps Solidity's slot-based storage model onto deterministic byte keys instead of contiguous 256-bit slots.
State Variables
Simple state variables use the hash of the variable name:
storage_key = SHA256(variable_name)Renaming a state variable in an upgrade changes the key and makes previously stored data unreachable unless migration code moves it.
Mappings
Mappings use iterative hashing over serialized keys:
slot_hash = SHA256("balances")
storage_key = SHA256(serialize(account) || slot_hash)Nested mappings repeat the process from outer key to inner key:
slot_hash = SHA256("approvals")
level_1 = SHA256(serialize(owner) || slot_hash)
level_2 = SHA256(serialize(spender) || level_1)level_2 is the final key passed to System.Storage.Get or System.Storage.Put.
Arrays
Storage arrays keep one key for length and one derived key per element:
length_key = SHA256(variable_name)
element_key(i) = SHA256(serialize(i) || SHA256(variable_name))push() reads the current length, writes the new element at the derived element key, and updates the length key.
Structs
Structs stored as state variables are serialized as a single binary payload with StdLib.serialize(). Reading one field requires loading and deserializing the whole struct. Use separate state variables for frequently updated hot fields.
Key Serialization
| Type | Storage-Key Encoding |
|---|---|
| Integers | Big-endian bytes padded to declared width; signed values are sign-extended. |
| Booleans | 0x00 for false, 0x01 for true. |
| Address | Raw 20-byte UInt160. |
| Fixed bytes | Raw fixed-length bytes. |
| Dynamic bytes / string | Raw bytes, no length prefix. |
| Struct values | StdLib.serialize() payload when persisted as values. |