UnsafeMutablePointer

struct UnsafeMutablePointer

A pointer for accessing and manipulating data of a specific type.

Associated Types
public typealias Distance = Int

You use instances of the UnsafeMutablePointer type to access data of a specific type in memory. The type of data that a pointer can access is the pointer's Pointee type. UnsafeMutablePointer provides no automated memory management or alignment guarantees. You are responsible for handling the life cycle of any memory you work with through unsafe pointers to avoid leaks or undefined behavior.

Memory that you manually manage can be either untyped or bound to a specific type. You use the UnsafeMutablePointer type to access and manage memory that has been bound to a specific type.

Understanding a Pointer's Memory State

The memory referenced by an UnsafeMutablePointer instance can be in one of several states. Many pointer operations must only be applied to pointers with memory in a specific state---you must keep track of the state of the memory you are working with and understand the changes to that state that different operations perform. Memory can be untyped and uninitialized, bound to a type and uninitialized, or bound to a type and initialized to a value. Finally, memory that was allocated previously may have been deallocated, leaving existing pointers referencing unallocated memory.

Uninitialized Memory

Memory that has just been allocated through a typed pointer or has been deinitialized is in an uninitialized state. Uninitialized memory must be initialized before it can be accessed for reading.

You can use methods like initialize(to:count:), initialize(from:count:), and moveInitialize(from:count:) to initialize the memory referenced by a pointer with a value or series of values.

Initialized Memory

Initialized memory has a value that can be read using a pointer's pointee property or through subscript notation. In the following example, ptr is a pointer to memory initialized with a value of 23:

let ptr: UnsafeMutablePointer<Int> = ...
// ptr.pointee == 23
// ptr[0] == 23

Accessing a Pointer's Memory as a Different Type

When you access memory through an UnsafeMutablePointer instance, the Pointee type must be consistent with the bound type of the memory. If you do need to access memory that is bound to one type as a different type, Swift's pointer types provide type-safe ways to temporarily or permanently change the bound type of the memory, or to load typed instances directly from raw memory.

An UnsafeMutablePointer<UInt8> instance allocated with eight bytes of memory, uint8Pointer, will be used for the examples below.

var bytes: [UInt8] = [39, 77, 111, 111, 102, 33, 39, 0]
let uint8Pointer = UnsafeMutablePointer<UInt8>.allocate(capacity: 8)
uint8Pointer.initialize(from: &bytes, count: 8)

When you only need to temporarily access a pointer's memory as a different type, use the withMemoryRebound(to:capacity:) method. For example, you can use this method to call an API that expects a pointer to a different type that is layout compatible with your pointer's Pointee. The following code temporarily rebinds the memory that uint8Pointer references from UInt8 to Int8 to call the imported C strlen function.

// Imported from C
func strlen(_ __s: UnsafePointer<Int8>!) -> UInt

let length = uint8Pointer.withMemoryRebound(to: Int8.self, capacity: 8) {
    return strlen($0)
}
// length == 7

When you need to permanently rebind memory to a different type, first obtain a raw pointer to the memory and then call the bindMemory(to:capacity:) method on the raw pointer. The following example binds the memory referenced by uint8Pointer to one instance of the UInt64 type:

let uint64Pointer = UnsafeMutableRawPointer(uint8Pointer)
                          .bindMemory(to: UInt64.self, capacity: 1)

After rebinding the memory referenced by uint8Pointer to UInt64, accessing that pointer's referenced memory as a UInt8 instance is undefined.

var fullInteger = uint64Pointer.pointee          // OK
var firstByte = uint8Pointer.pointee             // undefined

Alternatively, you can access the same memory as a different type without rebinding through untyped memory access, so long as the bound type and the destination type are trivial types. Convert your pointer to an UnsafeMutableRawPointer instance and then use the raw pointer's load(fromByteOffset:as:) and storeBytes(of:toByteOffset:as:) methods to read and write values.

let rawPointer = UnsafeMutableRawPointer(uint64Pointer)
fullInteger = rawPointer.load(as: UInt64.self)   // OK
firstByte = rawPointer.load(as: UInt8.self)      // OK

Performing Typed Pointer Arithmetic

Pointer arithmetic with a typed pointer is counted in strides of the pointer's Pointee type. When you add to or subtract from an UnsafeMutablePointer instance, the result is a new pointer of the same type, offset by that number of instances of the Pointee type.

// 'intPointer' points to memory initialized with [10, 20, 30, 40]
let intPointer: UnsafeMutablePointer<Int> = ...

// Load the first value in memory
let x = intPointer.pointee
// x == 10

// Load the third value in memory
let offsetPointer = intPointer + 2
let y = offsetPointer.pointee
// y == 30

You can also use subscript notation to access the value in memory at a specific offset.

let z = intPointer[2]
// z == 30

Implicit Casting and Bridging

When calling a function or method with an UnsafeMutablePointer parameter, you can pass an instance of that specific pointer type or use Swift's implicit bridging to pass a compatible pointer.

For example, the printInt(atAddress:) function in the following code sample expects an UnsafeMutablePointer<Int> instance as its first parameter:

func printInt(atAddress p: UnsafeMutablePointer<Int>) {
    print(p.pointee)
}

As is typical in Swift, you can call the printInt(atAddress:) function with an UnsafeMutablePointer instance. This example passes intPointer, a mutable pointer to an Int value, to print(address:).

printInt(atAddress: intPointer)
// Prints "42"

Alternatively, you can use Swift's implicit bridging to pass a pointer to an instance or to the elements of an array. The following example passes a pointer to the value variable by using inout syntax:

var value: Int = 23
printInt(atAddress: &value)
// Prints "23"

A mutable pointer to the elements of an array is implicitly created when you pass the array using inout syntax. This example uses implicit bridging to pass a pointer to the elements of numbers when calling printInt(atAddress:).

var numbers = [5, 10, 15, 20]
printInt(atAddress: &numbers)
// Prints "5"

No matter which way you call printInt(atAddress:), Swift's type safety guarantees that you can only pass a pointer to the type required by the function---in this case, a pointer to an Int.

Important:

var number = 5
let numberPointer = UnsafeMutablePointer<Int>(&number)
// Accessing 'numberPointer' is undefined behavior.

Initializers

init init(_:) Required

Creates an immutable typed pointer referencing the same memory as the given mutable pointer.

  • Parameter other: The pointer to convert.

Declaration

public init(_ other: UnsafeMutablePointer<Pointee>)
init init(mutating:) Required

Creates a mutable typed pointer referencing the same memory as the given immutable pointer.

  • Parameter other: The immutable pointer to convert.

Declaration

public init(mutating other: UnsafePointer<Pointee>)
init init?(_:) Required

Creates an immutable typed pointer referencing the same memory as the given mutable pointer.

  • Parameter other: The pointer to convert. If other is nil, the result is nil.

Declaration

public init?(_ other: UnsafeMutablePointer<Pointee>?)
init init?(mutating:) Required

Creates a mutable typed pointer referencing the same memory as the given immutable pointer.

  • Parameter other: The immutable pointer to convert. If other is nil, the result is nil.

Declaration

public init?(mutating other: UnsafePointer<Pointee>?)

Instance Variables

var customPlaygroundQuickLook Required

A custom playground Quick Look for this instance.

If this type has value semantics, the PlaygroundQuickLook instance should be unaffected by subsequent mutations.

Declaration

var customPlaygroundQuickLook: PlaygroundQuickLook

Instance Methods

func assign(from source: UnsafePointer<Pointee>, count: Int) Required

Replaces this pointer's initialized memory with the specified number of instances from the given pointer's memory.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be initialized or Pointee must be a trivial type. After calling assign(from:count:), the region is initialized.

Note: Returns without performing work if self and source are equal.

Declaration

@inlinable public func assign(from source: UnsafePointer<Pointee>, count: Int)
func assign(repeating repeatedValue: Pointee, count: Int) Required

Replaces this pointer's memory with the specified number of consecutive copies of the given value.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be initialized or Pointee must be a trivial type. After calling assign(repeating:count:), the region is initialized.

Declaration

@inlinable public func assign(repeating repeatedValue: Pointee, count: Int)
func deallocate() Required

Deallocates the memory block previously allocated at this pointer.

This pointer must be a pointer to the start of a previously allocated memory block. The memory must not be initialized or Pointee must be a trivial type.

Declaration

@inlinable public func deallocate()
func deinitialize(count: Int) -> UnsafeMutableRawPointer Required

Deinitializes the specified number of values starting at this pointer.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be initialized. After calling deinitialize(count:), the memory is uninitialized, but still bound to the Pointee type.

  • Parameter count: The number of instances to deinitialize. count must not be negative.

Declaration

@inlinable public func deinitialize(count: Int) -> UnsafeMutableRawPointer
func initialize(from source: UnsafePointer<Pointee>, count: Int) Required

Initializes the memory referenced by this pointer with the values starting at the given pointer.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be uninitialized or Pointee must be a trivial type. After calling initialize(from:count:), the region is initialized.

Declaration

@inlinable public func initialize(from source: UnsafePointer<Pointee>, count: Int)
func initialize(repeating repeatedValue: Pointee, count: Int) Required

Initializes this pointer's memory with the specified number of consecutive copies of the given value.

The destination memory must be uninitialized or the pointer's Pointee must be a trivial type. After a call to initialize(repeating:count:), the memory referenced by this pointer is initialized.

Declaration

@inlinable public func initialize(repeating repeatedValue: Pointee, count: Int)
func initialize(to value: Pointee) Required

Initializes this pointer's memory with a single instance of the given value.

The destination memory must be uninitialized or the pointer's Pointee must be a trivial type. After a call to initialize(to:), the memory referenced by this pointer is initialized. Calling this method is roughly equivalent to calling initialize(repeating:count:) with a count of 1.

Declaration

@inlinable public func initialize(to value: Pointee)
func move() -> Pointee Required

Retrieves and returns the referenced instance, returning the pointer's memory to an uninitialized state.

Calling the move() method on a pointer p that references memory of type T is equivalent to the following code, aside from any cost and incidental side effects of copying and destroying the value:

let value: T = {
    defer { p.deinitialize(count: 1) }
    return p.pointee
}()

The memory referenced by this pointer must be initialized. After calling move(), the memory is uninitialized.

Declaration

@inlinable public func move() -> Pointee
func moveAssign(from source: UnsafeMutablePointer<Pointee>, count: Int) Required

Replaces the memory referenced by this pointer with the values starting at the given pointer, leaving the source memory uninitialized.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be initialized or Pointee must be a trivial type. After calling moveAssign(from:count:), the region is initialized and the memory region source..<(source + count) is uninitialized.

Declaration

@inlinable public func moveAssign(from source: UnsafeMutablePointer<Pointee>, count: Int)
func moveInitialize(from source: UnsafeMutablePointer<Pointee>, count: Int) Required

Moves instances from initialized source memory into the uninitialized memory referenced by this pointer, leaving the source memory uninitialized and the memory referenced by this pointer initialized.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be uninitialized or Pointee must be a trivial type. After calling moveInitialize(from:count:), the region is initialized and the memory region source..<(source + count) is uninitialized.

Declaration

@inlinable public func moveInitialize(from source: UnsafeMutablePointer<Pointee>, count: Int)
func withMemoryRebound(to type: T.Type, capacity count: Int, _ body: (UnsafeMutablePointer<T>) throws -> Result) rethrows -> Result Required

Executes the given closure while temporarily binding the specified number of instances to the given type.

Use this method when you have a pointer to memory bound to one type and you need to access that memory as instances of another type. Accessing memory as a type T requires that the memory be bound to that type. A memory location may only be bound to one type at a time, so accessing the same memory as an unrelated type without first rebinding the memory is undefined.

The region of memory starting at this pointer and covering count instances of the pointer's Pointee type must be initialized.

The following example temporarily rebinds the memory of a UInt64 pointer to Int64, then accesses a property on the signed integer.

let uint64Pointer: UnsafeMutablePointer<UInt64> = fetchValue()
let isNegative = uint64Pointer.withMemoryRebound(to: Int64.self) { ptr in
    return ptr.pointee < 0
}

Because this pointer's memory is no longer bound to its Pointee type while the body closure executes, do not access memory using the original pointer from within body. Instead, use the body closure's pointer argument to access the values in memory as instances of type T.

After executing body, this method rebinds memory back to the original Pointee type.

Note: Only use this method to rebind the pointer's memory to a type with the same size and stride as the currently bound Pointee type. To bind a region of memory to a type that is a different size, convert the pointer to a raw pointer and use the bindMemory(to:capacity:) method.

Declaration

@inlinable public func withMemoryRebound<T, Result>(to type: T.Type, capacity count: Int, _ body: (UnsafeMutablePointer<T>) throws -> Result) rethrows -> Result

Type Methods

func allocate(capacity count: Int) -> UnsafeMutablePointer<Pointee> Required

Allocates uninitialized memory for the specified number of instances of type Pointee.

The resulting pointer references a region of memory that is bound to Pointee and is count * MemoryLayout<Pointee>.stride bytes in size.

The following example allocates enough new memory to store four Int instances and then initializes that memory with the elements of a range.

let intPointer = UnsafeMutablePointer<Int>.allocate(capacity: 4)
for i in 0..<4 {
    (intPointer + i).initialize(to: i)
}
print(intPointer.pointee)
// Prints "0"

When you allocate memory, always remember to deallocate once you're finished.

intPointer.deallocate()
  • Parameter count: The amount of memory to allocate, counted in instances of Pointee.

Declaration

@inlinable public static func allocate(capacity count: Int) -> UnsafeMutablePointer<Pointee>