> ## Documentation Index
> Fetch the complete documentation index at: https://developers.stellar.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Types

> Struct, union, and enum types defined by contracts.

Custom types are struct, union, and enum types defined by contracts. They are usable everywhere primitives types can be used: as contract inputs, outputs, or for storage.

> [!NOTE]
>
> The [custom types example] demonstrates how to define your own types.
>
> [custom types example]: ../../../../build/smart-contracts/example-contracts/custom-types

> [!NOTE]
>
> Error enum types are another type contracts can define that have some unique behaviors. See [Errors](https://developers.stellar.org/docs/learn/fundamentals/contract-development/errors-and-debugging/errors.md) for more information.

## Structs (with Named Fields)

Structs with named fields are stored on ledger as a map of key-value pairs, where the key is a 32 character string representing the field name, and the value is the value encoded.

Field names must be no more than 32 characters. Fields with names no longer than 9 characters are slightly more efficient at runtime, though the difference should be marginal most of the time.

```rust theme={"theme":{"light":"github-light","dark":"github-dark"}}
#[contracttype]
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct State {
    pub count: u32,
    pub last_incr: u32,
}
```

When converted to XDR, the value becomes an `ScVal`, containing an `ScMap`, containing an array of key-value pairs.

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{
  "map": [
    { "key": { "symbol": "count" }, "val": { "u32": 0 } },
    { "key": { "symbol": "last_incr" }, "val": { "u32": 0 } }
  ]
}
```

## Structs (with Unnamed Fields)

Structs with unnamed fields are stored on ledger as a vector of values, and are interchangeable with tuples and vectors. The elements are placed in the vector in order that they appear in the field list.

```rust theme={"theme":{"light":"github-light","dark":"github-dark"}}
#[contracttype]
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct State(pub u32, pub u32);
```

When converted to XDR, the value becomes an `ScVal`, containing an `ScVec`, containing an array of values.

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{ "vec": [{ "u32": 0 }, { "u32": 0 }] }
```

## Enum (Unit and Tuple Variants)

Enums containing unit and tuple variants are are stored on ledger as a two element vector, where the first element is the name of the enum variant as a string up to 32 characters in length, and the value is the value if the variant has one.

Only unit variants and tuple variants, like `A` and `B` below, are supported.

```rust theme={"theme":{"light":"github-light","dark":"github-dark"}}
#[contracttype]
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum Enum {
    A,
    B(u32),
}
```

When a unit variant, such as `Enum::A`, is converted to XDR, the value becomes an `ScVal`, containing an `ScVec`, containing an array with a single value, the symbol containing the variant name.

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{ "vec": [{ "symbol": "A" }] }
```

When a tuple variant, such as `Enum::B`, is converted to XDR, the value becomes an `ScVal`, containing an `ScVec`, containing an array with two values, the symbol containing the variant name and the tuple value.

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{ "vec": [{ "symbol": "B" }, { "u32": 0 }] }
```

When tuple variants containing multiple values are implemented, the values will be included into the vector.

## Enum (Integer Variants)

Enums containing integer values are stored on ledger as the `u32` value.

```rust theme={"theme":{"light":"github-light","dark":"github-dark"}}
#[contracttype]
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
#[repr(u32)]
pub enum Enum {
    A = 1,
    B = 2,
}
```

When converted to XDR, the value becomes an `ScVal`, containing a `U32`.

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{ "u32": 1 }
```