---
title: "init(unsafeUninitializedCapacity:initializingWith:)"
framework: swift
role: symbol
role_heading: Initializer
path: "swift/contiguousarray/init(unsafeuninitializedcapacity:initializingwith:)"
---

# init(unsafeUninitializedCapacity:initializingWith:)

Creates an array with the specified capacity, and then calls the given closure with a buffer covering the array’s uninitialized memory.

## Declaration

```swift
init<E>(unsafeUninitializedCapacity: Int, initializingWith initializer: (inout UnsafeMutableBufferPointer<Element>, inout Int) throws(E) -> Void) throws(E) where E : Error
```

## Parameters

- `unsafeUninitializedCapacity`: The number of elements to allocate space for in the new array.
- `initializer`: A closure that initializes elements and sets the count of the new array. Parameters: buffer: A buffer covering uninitialized memory with room for the specified number of elements. initializedCount: The count of initialized elements in the array, which begins as zero. Set initializedCount to the number of elements you initialize.

## Discussion

Discussion Inside the closure, set the initializedCount parameter to the number of elements that are initialized by the closure. The memory in the range buffer[0..<initializedCount] must be initialized at the end of the closure’s execution, and the memory in the range buffer[initializedCount...] must be uninitialized. This postcondition must hold even if the initializer closure throws an error. note: While the resulting array may have a capacity larger than the requested amount, the buffer passed to the closure will cover exactly the requested number of elements.