> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Swift Read-Only Subscript
A read-only subscript in Swift is a custom indexing interface that allows retrieval of values from an instance of a class, structure, or enumeration, but explicitly prohibits modification. It is defined by implementing only a getter within the `subscript` declaration and omitting the setter.
## Syntax
Swift provides two ways to declare a read-only subscript: using an explicit `get` block, or using a shorthand syntax that omits the `get` keyword entirely.
**Explicit Getter Syntax:**
```swift theme={"dark"}
subscript(parameter: ParameterType) -> ReturnType {
get {
// Return an appropriate value of ReturnType
}
}
```
**Implicit Getter (Shorthand) Syntax:**
Because read-only subscripts are common, Swift allows the omission of the `get` block. The body of the subscript acts directly as the getter.
```swift theme={"dark"}
subscript(parameter: ParameterType) -> ReturnType {
// Return an appropriate value of ReturnType
}
```
## Technical Characteristics
* **Immutability Enforcement:** Attempting to assign a value to a read-only subscript results in a compile-time error (`Cannot assign through subscript: subscript is get-only`).
* **Parameter Constraints:** Subscripts can accept any number of input parameters of any type, including variadic parameters. However, they cannot use `inout` parameters or provide default parameter values.
* **Return Type:** Unlike functions, subscripts must explicitly declare a return type. The getter must return an instance matching this declared type.
* **Overloading:** You can define multiple read-only subscripts on the same type. The Swift compiler resolves which subscript to invoke based on the parameter signature passed at the call site.
* **Type Subscripts:** By prefixing the `subscript` keyword with `static` (or `class` for class types), a read-only subscript can be applied to the type itself rather than an instance of the type.
## Implementation Example
The following demonstrates a structure utilizing the shorthand read-only subscript syntax to expose internal storage without allowing external mutation.
```swift theme={"dark"}
struct AbstractContainer {
private var internalStorage: [Int]
init(elements: [Int]) {
self.internalStorage = elements
}
// Read-only subscript
subscript(index: Int) -> Int {
return internalStorage[index]
}
}
let container = AbstractContainer(elements: [10, 20, 30])
let value = container[1] // Valid: Retrieves 20
// container[1] = 50 // Invalid: Compile-time error
```
Tired of Poor Swift Skills? Fix That With Deep Grasping!
Learn More
