如何从不可变的非类值中获取Swift UnsafePointer？

Question

I want to be able to create an UnsafePointer from immutable values.

The simplest reproduction of what I've tried to do is as follows:

let number : Int = 42;
var pointer = UnsafePointer<Int>(&number);
                                 ^
                                 | Could not make `inout Int` from immutable
                                   value.

Since Int does not conform to AnyObject , I cannot use unsafeAddressOf() .

Answer 1

According to a message in the [swift-evolution] mailing list by Joe Groff , you can simply wrap the variable in an array when passing the pointer to another function. This method creates a temporary array with a copy of the target value and even works in Swift 2 .

let number : Int = 42
my_c_function([number])

In Swift 3 , you can construct an UnsafePointer directly. However, you must ensure the array's lifecycle matches that of the UnsafePointer . Otherwise, the array and the copied value may be overwritten.

let number : Int = 42
// Store the "temporary" array in a variable to ensure it is not overwritten.
let array = [number]
var pointer = UnsafePointer(array)

// Perform operations using the pointer.

Answer 2

You can use withUnsafePointer.

var number : Int = 42
withUnsafePointer(to: &number) { (pointer: UnsafePointer<Int>) in
    // use pointer inside this block 
    pointer.pointee
}

Alternately, you can use the "$0" shorthand argument to access the pointer within the block.

var number : Int = 42
withUnsafePointer(to: &number) { 
    // use pointer inside this block 
    $0.pointee 
}

Here are some good notes about using UnsafePointer: https://developer.apple.com/reference/swift/unsafepointer