offset(of:)

Declaration

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

Parameters

• key:

  A key path referring to storage that can be accessed through a value of type T.

Return Value

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. If the value is nil, it can be because key is computed, has observers, requires reabstraction, or overlaps storage with other properties.

Discussion

You can use this method to find the distance in bytes that can be added to a pointer of type T to get a pointer to the property referenced by key. The offset is available only if the given key refers to inline, directly addressable storage within the in-memory representation of T.

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, a key path key of type WritableKeyPath<T, U>, and a value of type U:

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

// Mutation through the offset pointer
withUnsafeMutableBytes(of: &root) { bytes in
    let offset = MemoryLayout<T>.offset(of: key)!
    let rawPointerToValue = bytes.baseAddress! + offset
    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 nonbreaking in other contexts, but would trigger a runtime error if the result of offset(of:) is force-unwrapped.