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Beyond primitives like uint and address, Solidity lets you define your own types. Structs group related fields, and enums represent a fixed set of named states.

A struct bundles several variables under one name, perfect for modeling real-world entities like a user or an order.

You can construct a struct by positional arguments or by naming each field. Named arguments are clearer and less error prone.

A struct in storage persists on chain; in memory it is a temporary copy. Assigning a storage struct to a memory variable copies it, so changes do not write back.

To modify persisted data, work with a storage reference and assign to its fields directly.

Structs shine when stored in a mapping keyed by address or id, giving each user their own record.

An enum defines a small set of named options. It improves readability over raw numbers for representing state.

Enum members are stored as uint starting at 0. You compare and assign them by name, which makes contract logic self-documenting.

Enums are ideal for modeling a state machine. Guard transitions with require so an order can only move to a valid next state.

Put an enum field inside a struct to track each record state, for example storing a Status on every order struct so each order tracks its own lifecycle.

Each struct field is a storage slot, and writing storage is expensive. Order fields to pack smaller types together, and use enums (one byte range) instead of strings for fixed states to save gas.

You learned Solidity custom types:

• Structs group related fields and are often stored in mappings
• Mind storage vs memory when copying or updating structs
• Enums model fixed states as integers and power state machines
• Combine them and pack fields to save gas

Custom types make contracts more expressive and maintainable.