Swift 取得變數的 Pointer/Address
==

# 工具

* `withUnsafePointer(to:)` [Doc](https://developer.apple.com/documentation/swift/2431879-withunsafepointer)
> `func withUnsafePointer<T, Result>(to value: inout T, _ body: (UnsafePointer<T>) throws -> Result) rethrows -> Result`
>
* `withUnsafePointer(to:_:)` [Doc](https://developer.apple.com/documentation/swift/2429788-withunsafemutablepointer)
> `func withUnsafeMutablePointer<T, Result>(to value: inout T, _ body: (UnsafeMutablePointer<T>) throws -> Result) rethrows -> Result`
* `Array<T>::withUnsafeBufferPointer(_:)` [Doc](https://developer.apple.com/documentation/swift/array/2994771-withunsafebufferpointer)
> `func withUnsafeBufferPointer<R>(_ body: (UnsafeBufferPointer<Element>) throws -> R) rethrows -> R`
* Swift Compiler's implicit bridge for Array

# Single Variable

* `withUnsafeMutablePointer` has `inout` in first parameter, it need leading `&` when passing

var x = 1

withUnsafePointer(to: x) { ptr in 
    print(type(of: ptr))
    print(ptr.pointee)
}

withUnsafeMutablePointer(to: &x) { ptr in 
    print(type(of: ptr))
    print(ptr.pointee)
}

func seeAddr<T>(_ ptr: UnsafePointer<T>) {
    print(type(of: ptr))
    print(ptr.pointee)
}
seeAddr(&x)

## Output

UnsafePointer<Int>
1
UnsafeMutablePointer<Int>
1
UnsafePointer<Int>
1

# Array

> * need use inout syntax(&) for `withUnsafeMutablePointer`
> * `withUnsafePointer` is used to get `Array<T>` instead of `T` in `Array`
> * To extract `T` from `Array<T>`, need to use `Array<T>`'s method `withUnsafeBufferPointer`

var a = [1,2,3]

// withUnsafePointer(to: T, (UnsafePointer->Result)) -> Result
withUnsafePointer(to: a) { ptr in 
    print(type(of: ptr))
}

// withUnsafeMutablePointer(to: inout T, (UnsafeMutablePointer->Result)) -> Result
withUnsafeMutablePointer(to: &a) { ptr in
    print(type(of: ptr))
}

// Array<T>::withUnsafeBufferPointer((UnsafeBufferPointer->Result)) -> Result
a.withUnsafeBufferPointer { ptr in
    print(type(of: ptr))
}

## Output

UnsafePointer<Array<Int>>
UnsafeMutablePointer<Array<Int>>
UnsafeBufferPointer<Int>

# Get address as `UnsafePointer<T>` from Array

> `withUnsafePointer` seems is unable to get correct address

var a = [2,3,5]

withUnsafePointer(to: a[0]) { ptr in
    print(type(of: ptr))    
    print(ptr[0]) // print 1
    print(ptr[1]) // still print 1, it seems ptr does not point to the original one
}

withUnsafeMutablePointer(to: &a[0]) { ptr in
    print(type(of: ptr))
    print(ptr[0])
    print(ptr[1])
    print(ptr[2])
}

a.withUnsafeBufferPointer{ ptr in
    print(type(of: ptr))
    print(ptr[0])
    print(ptr[1])
    print(ptr[2])

    let addrOpt = ptr.baseAddress
    print(type(of: addrOpt))
    
    let addr = addrOpt!
    print(type(of: addr))
    print(addr[0])
    print(addr[1])
    print(addr[2])
}

## Output

UnsafePointer<Int>
2
2
UnsafeMutablePointer<Int>
2
3
5
UnsafeBufferPointer<Int>
2
3
5
Optional<UnsafePointer<Int>>
UnsafePointer<Int>
2
3
5

# Implicit Bridge Array to `UnsafePointer<T>` when passing as function parameter

::: warning
According to https://developer.apple.com/documentation/swift/unsafepointer

> The pointer created through implicit bridging of an instance or of an array’s elements is only valid during the execution of the called function. Escaping the pointer to use after the execution of the function is undefined behavior. In particular, do not use implicit bridging when calling an UnsafePointer initializer.

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

:::

var a = [2,3,5]

func seeAddr<T>(_ ptr: UnsafePointer<T>) {
    print(ptr[0])
    print(ptr[1])
    print(ptr[2])
}

print("implicit bridge Array to UnsafePointer<T>")
seeAddr(a) // implicit bridge Array<T> to UnsafePointer<T>

print("implicit bridge Array with inout syntax(&) to UnsafePointer<T>")
seeAddr(&a) // implicit bridge Array<T> with inout syntax to UnsafePointer<T>

## Output

implicit bridge Array to UnsafePointer<T>
2
3
5
implicit bridge Array with inout syntax(&) to UnsafePointer<T>
2
3
5

# Reference

* [UnsafePointer](https://developer.apple.com/documentation/swift/unsafepointer)