UnsafeRawPointer

The UnsafeRawPointer type provides no automated memory management, no type safety, and no 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 UnsafeRawPointer type to access and manage raw bytes in memory, whether or not that memory has been bound to a specific type.

Understanding a Pointer’s Memory State

The memory referenced by an UnsafeRawPointer 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.

Raw, Uninitialized Memory

Raw memory that has just been allocated is in an uninitialized, untyped state. Uninitialized memory must be initialized with values of a type before it can be used with any typed operations.

To bind uninitialized memory to a type without initializing it, use the bindMemory(to:count:) method. This method returns a typed pointer for further typed access to the memory.

Typed Memory

Memory that has been bound to a type, whether it is initialized or uninitialized, is typically accessed using typed pointers—instances of UnsafePointer and UnsafeMutablePointer. Initialization, assignment, and deinitialization can be performed using UnsafeMutablePointer methods.

Memory that has been bound to a type can be rebound to a different type only after it has been deinitialized or if the bound type is a trivial type. Deinitializing typed memory does not unbind that memory’s type. The deinitialized memory can be reinitialized with values of the same type, bound to a new type, or deallocated.

When reading from memory as raw bytes when that memory is bound to a type, you must ensure that you satisfy any alignment requirements.

Raw Pointer Arithmetic

Pointer arithmetic with raw pointers is performed at the byte level. When you add to or subtract from a raw pointer, the result is a new raw pointer offset by that number of bytes. The following example allocates four bytes of memory and stores 0xFF in all four bytes:

let bytesPointer = UnsafeMutableRawPointer.allocate(byteCount: 4, alignment: 4)
bytesPointer.storeBytes(of: 0xFFFF_FFFF, as: UInt32.self)

// Load a value from the memory referenced by 'bytesPointer'
let x = bytesPointer.load(as: UInt8.self)       // 255

// Load a value from the last two allocated bytes
let offsetPointer = bytesPointer + 2
let y = offsetPointer.load(as: UInt16.self)     // 65535

The code above stores the value 0xFFFF_FFFF into the four newly allocated bytes, and then loads the first byte as a UInt8 instance and the third and fourth bytes as a UInt16 instance.

Always remember to deallocate any memory that you allocate yourself.

bytesPointer.deallocate()

Implicit Casting and Bridging

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

For example, the print(address:as:) function in the following code sample takes an UnsafeRawPointer instance as its first parameter:

func print<T>(address p: UnsafeRawPointer, as type: T.Type) {
    let value = p.load(as: type)
    print(value)
}

As is typical in Swift, you can call the print(address:as:) function with an UnsafeRawPointer instance. This example passes rawPointer as the initial parameter.

// 'rawPointer' points to memory initialized with `Int` values.
let rawPointer: UnsafeRawPointer = ...
print(address: rawPointer, as: Int.self)
// Prints "42"

Because typed pointers can be implicitly cast to raw pointers when passed as a parameter, you can also call print(address:as:) with any mutable or immutable typed pointer instance.

let intPointer: UnsafePointer<Int> = ...
print(address: intPointer, as: Int.self)
// Prints "42"

let mutableIntPointer = UnsafeMutablePointer(mutating: intPointer)
print(address: mutableIntPointer, as: Int.self)
// Prints "42"

Alternatively, you can use Swift’s implicit bridging to pass a pointer to an instance or to the elements of an array. Use inout syntax to implicitly create a pointer to an instance of any type. The following example uses implicit bridging to pass a pointer to value when calling print(address:as:):

var value: Int = 23
print(address: &value, as: Int.self)
// Prints "23"

An immutable pointer to the elements of an array is implicitly created when you pass the array as an argument. This example uses implicit bridging to pass a pointer to the elements of numbers when calling print(address:as:).

let numbers = [5, 10, 15, 20]
print(address: numbers, as: Int.self)
// Prints "5"

You can also use inout syntax to pass a mutable pointer to the elements of an array. Because print(address:as:) requires an immutable pointer, although this is syntactically valid, it isn’t necessary.

var mutableNumbers = numbers
print(address: &mutableNumbers, as: Int.self)