MemoryLayout

enum MemoryLayout<T>

The memory layout of a type, describing its size, stride, and alignment.

You can use MemoryLayout as a source of information about a type when allocating or binding memory using unsafe pointers. The following example declares a Point type with x and y coordinates and a Boolean isFilled property.

struct Point {
    let x: Double
    let y: Double
    let isFilled: Bool
}

The size, stride, and alignment of the Point type are accessible as static properties of MemoryLayout<Point>.

// MemoryLayout<Point>.size == 17
// MemoryLayout<Point>.stride == 24
// MemoryLayout<Point>.alignment == 8

Always use a multiple of a type's stride instead of its size when allocating memory or accounting for the distance between instances in memory. This example allocates untyped, uninitialized memory with space for four instances of Point.

let count = 4
let pointPointer = UnsafeMutableRawPointer.allocate(
        bytes: count * MemoryLayout<Point>.stride,
        alignedTo: MemoryLayout<Point>.alignment)
Import import Swift

Static Variables

static var alignment: Int

The default memory alignment of T, in bytes.

Use the alignment property for a type when allocating memory using an unsafe pointer. This value is always positive.

Declaration

static var alignment: Int { get }
static var size: Int

The contiguous memory footprint of T, in bytes.

A type's size does not include any dynamically allocated or out of line storage. In particular, MemoryLayout<T>.size, when T is a class type, is the same regardless of how many stored properties T has.

When allocating memory for multiple instances of T using an unsafe pointer, use a multiple of the type's stride instead of its size.

Declaration

static var size: Int { get }
static var stride: Int

The number of bytes from the start of one instance of T to the start of the next when stored in contiguous memory or in an Array<T>.

This is the same as the number of bytes moved when an UnsafePointer<T> instance is incremented. T may have a lower minimal alignment that trades runtime performance for space efficiency. This value is always positive.

Declaration

static var stride: Int { get }

Static Methods

static func alignment(ofValue:)

Returns the default memory alignment of T.

Use a type's alignment when allocating memory using an unsafe pointer.

When you have a type instead of an instance, use the MemoryLayout<T>.stride static property instead.

let x: Int = 100

// Finding the alignment of a value's type
let s = MemoryLayout.alignment(ofValue: x)
// s == 8

// Finding the alignment of a type directly
let t = MemoryLayout<Int>.alignment
// t == 8

value: A value representative of the type to describe. Returns: The default memory alignment, in bytes, of the given value's type. This value is always positive.

Declaration

static func alignment(ofValue value: T) -> Int
static func offset(of:)

Returns the offset of an inline stored property of T within the in-memory representation of T.

If the given key refers to inline, directly addressable storage within the in-memory representation of T, then the return value is a distance in bytes that can be added to a pointer of type T to get a pointer to the storage referenced by key.

If the return value of this method is non-nil, then accessing the value by key path or by an offset pointer are equivalent. For example, for a variable root of type T, value of type U, and a key path key of type WritableKeyPath<T, U>:

// Mutation through the key path
root[keyPath: key] = value

// Mutation through the offset pointer
withUnsafeMutableBytes(of: &root) { bytes in
    let rawPointerToValue = bytes.baseAddress! + MemoryLayout<T>.offset(of: key)!
    let pointerToValue = rawPointerToValue.assumingMemoryBound(to: U.self)
    pointerToValue.pointee = value
}

A property has inline, directly addressable storage when it is a stored property for which no additional work is required to extract or set the value. Properties are not directly accessible if they trigger any didSet or willSet accessors, perform any representation changes such as bridging or closure reabstraction, or mask the value out of overlapping storage as for packed bitfields. In addition, because class instance properties are always stored out-of-line, their positions are not accessible using offset(of:).

For example, in the ProductCategory type defined here, only \.updateCounter, \.identifier, and \.identifier.name refer to properties with inline, directly addressable storage:

struct ProductCategory {
    struct Identifier {
        var name: String              // addressable
    }

    var identifier: Identifier        // addressable
    var updateCounter: Int            // addressable
    var products: [Product] {         // not addressable: didSet handler
        didSet { updateCounter += 1 }
    }
    var productCount: Int {           // not addressable: computed property
        return products.count
    }
}

When using offset(of:) with a type imported from a library, don't assume that future versions of the library will have the same behavior. If a property is converted from a stored property to a computed property, the result of offset(of:) changes to nil. That kind of conversion is non-breaking in other contexts, but would trigger a runtime error if the result of offset(of:) is force-unwrapped.

key: A key path referring to storage that can be accessed through a value of type T. Returns: The offset in bytes from a pointer to a value of type T to a pointer to the storage referenced by key, or nil if no such offset is available for the storage referenced by key, such as because key is computed, has observers, requires reabstraction, or overlaps storage with other properties.

Declaration

static func offset(of key: PartialKeyPath<T>) -> Int?
static func size(ofValue:)

Returns the contiguous memory footprint of the given instance.

The result does not include any dynamically allocated or out of line storage. In particular, pointers and class instances all have the same contiguous memory footprint, regardless of the size of the referenced data.

When you have a type instead of an instance, use the MemoryLayout<T>.size static property instead.

let x: Int = 100

// Finding the size of a value's type
let s = MemoryLayout.size(ofValue: x)
// s == 8

// Finding the size of a type directly
let t = MemoryLayout<Int>.size
// t == 8

value: A value representative of the type to describe. Returns: The size, in bytes, of the given value's type.

Declaration

static func size(ofValue value: T) -> Int
static func stride(ofValue:)

Returns the number of bytes from the start of one instance of T to the start of the next when stored in contiguous memory or in an Array<T>.

This is the same as the number of bytes moved when an UnsafePointer<T> instance is incremented. T may have a lower minimal alignment that trades runtime performance for space efficiency. The result is always positive.

When you have a type instead of an instance, use the MemoryLayout<T>.stride static property instead.

let x: Int = 100

// Finding the stride of a value's type
let s = MemoryLayout.stride(ofValue: x)
// s == 8

// Finding the stride of a type directly
let t = MemoryLayout<Int>.stride
// t == 8

value: A value representative of the type to describe. Returns: The stride, in bytes, of the given value's type.

Declaration

static func stride(ofValue value: T) -> Int