---
title: "init(capacity:initializingWith:)"
framework: Swift
role: symbol
role_heading: Initializer
platforms: [iOS 27.0+, iPadOS 27.0+, Mac Catalyst 27.0+, macOS 27.0+, tvOS 27.0+, visionOS 27.0+, watchOS 27.0+]
path: "swift/uniquearray/init(capacity:initializingwith:)"
---

# init(capacity:initializingWith:)

Creates a new array with the specified capacity, directly initializing its storage using an output span.

## Declaration

```swift
init<E>(capacity: Int, initializingWith body: @_lifetime(0: copy 0) (inout OutputSpan<Element>) throws(E) -> Void) throws(E) where E : Error
```

## Parameters

- `capacity`: The storage capacity of the new array.
- `body`: A callback that gets called at most once to directly populate newly reserved storage within the array. The function is allowed to add fewer than capacity items. The array is initialized with however many items the callback adds to the output span before it returns (or before it throws an error).