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

<AgentInstructions>

## Submitting Feedback

If you encounter incorrect, outdated, or confusing documentation on this page, submit feedback:

POST https://docs.ton.org/feedback

```json
{
  "path": "/tolk/syntax/structures-fields",
  "feedback": "Description of the issue"
}
```

Only submit feedback when you have something specific and actionable to report.

</AgentInstructions>

# Structures and fields

Tolk supports [structures](/languages/tolk/types/structures) — similar to TypeScript classes.

## Structure syntax

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct Demo {
    allowExotic: bool = false
    customData: tuple
}
```

* Every field has a required type and an optional default value.
* Fields are separated by newlines, which are preferred, or by commas or semicolons.
* Fields can be `private` and `readonly`.

## Object creation

Use `{ ... }` when the type is clear, or `StructName { ... }` explicitly.

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
fun demo() {
    // the type is clear from the assignment
    var o1: Demo = { customData: someTuple };
    o1.someMethod();

    // alternatively, `Demo { ... }`
    Demo{customData: someTuple}.someMethod();
}
```

## Shorthand object syntax

The shorthand `{ a, b }` expands to `{ a: a, b: b }`. This syntax is similar to object shorthand in TypeScript.

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
fun createDemo(allowExotic: bool, customData: tuple) {
    return Demo { allowExotic, customData }
}
```

## Methods for structures

[Methods](/languages/tolk/syntax/functions-methods) are declared separately as extension functions:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
fun Demo.hasData(self): bool {
    return self.customData.size() != 0
}
```

## Empty structures

A structure without fields is used as a grouping construct for static methods. For example, standard functions such as `blockchain.now()` are declared this way:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct blockchain

fun blockchain.now(): int
    asm "NOW"

fun blockchain.logicalTime(): int
    asm "LTIME"
```

These methods are static because they do not accept `self`.

## Default values for fields

Fields with default values may be missed out of a literal:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct DefDemo {
    f1: int = 0
    f2: int? = null
    f3: (int, coins) = (0, ton("0.05"))
}

fun demo() {
    var d: DefDemo = {};         // ok
    var d: DefDemo = { f2: 5 };  // ok
}
```

## Private and readonly fields

* `private` – accessible only within methods.
* `readonly` – immutable after object creation.

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct PositionInTuple {
    private readonly t: tuple
    currentIndex: int
}

fun PositionInTuple.create(t: tuple): PositionInTuple {
    return { t, currentIndex: 0 }
}

fun PositionInTuple.next(mutate self) {
    // self.t cannot be modified: it is readonly
    self.currentIndex += 1;
}

fun demo() {
    var p = PositionInTuple.create(someTuple);
    // p.t is unavailable here: it is private
}
```

An object with a private field can only be constructed by a static method or an assembler function.

## Generic structs

[Generics](/languages/tolk/types/generics) exist only at the type level and incur no runtime cost.

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct Nullable<T> {
    value: T?
}
```

## Methods for generic structs

When parsing the receiver in `fun <receiver>.f()`, the compiler treats unknown symbols as type parameters:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
fun Nullable<T>.isNull(self) {
    return self.value == null
}
```

It's a generalization. For example, `fun T.copy()` is valid for any receiver.

Method overloading or partial specialization is also allowed:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
fun Nullable<map<K, V>>.isNull(self) {
    return self.value.isEmpty()
}
```

## Serialization prefixes and opcodes

The syntax `struct (PREFIX) Name { ... }` specifies a [serialization prefix for the structure](/languages/tolk/types/overall-serialization#structures). For messages, 32-bit prefixes are typically called opcodes.

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
struct (0x7362d09c) TransferNotification {
    queryId: uint64
    // ...
}
```

An outgoing message starts with the hex number:

```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
// message will be '0x7362d09c0000000000000000' + ...
createMessage({
    // ...
    body: TransferNotification {
        queryId: 0,
        // ...
    }
});
```

Prefixes are not restricted to 32 bits:

* `0x000F` — 16-bit hexadecimal prefix, value 15.
* `0b010` — 3-bit binary prefix, value 2.

Non‑32‑bit prefixes are useful for controlling union types when expressing TL‑B's multiple constructors.