Using pointer in Swift 3

Swift3_Pointer.swift

//: Playground - noun: a place where people can play

import Foundation

var str = "Hello"
let data1 = str.data(using: String.Encoding.utf8)!

data1.withUnsafeBytes { (bytes: UnsafePointer<CChar>) -> Void in
    print(bytes.pointee)
    let arr = UnsafeBufferPointer(start: bytes, count: 5)
    arr.forEach({
        print($0)
    })
}

/*
let a = UnsafeMutablePointer<Int>.allocate(capacity: 1)
a.pointee = 4
print("a = \(a.pointee)")
a.deallocate(capacity: 1)
*/

/*
// & operator with UnsafePointer
func receive(pointer: UnsafePointer<Int>) {
    print("param value is: \(pointer.pointee)")
}

var a = 42
receive(pointer: &a)
*/

/*
// & operator with UnsafeMutablePointer
func receive(mutablePointer: UnsafeMutablePointer<Int>) {
    mutablePointer.pointee *= 2
}

var a = 10
receive(mutablePointer: &a)
print("a = \(a)")
*/
//var a = 1024
//withUnsafeMutablePointer(to: &a) {
//    $0.pointee *= 2
//}

// 指针转换
var addrIn = sockaddr_in()
// Fill sockaddr_in fields
withUnsafePointer(to: &addrIn) { ptr in
    ptr.withMemoryRebound(to: sockaddr.self, capacity: 1, { ptrSockAddr in
        bind(1, UnsafePointer(ptrSockAddr), socklen_t(MemoryLayout<sockaddr>.size))
    })
}

let intPtr = UnsafeMutablePointer<Int>.allocate(capacity: 1)
let voidPtr = UnsafeRawPointer(intPtr)
let intPtrAgain = voidPtr.assumingMemoryBound(to: Int.self)

// 数组指针
let size = 10
var a = UnsafeMutablePointer<Int>.allocate(capacity: size)
for idx in 0..<10 {
    a.advanced(by: idx).pointee = idx*2
}

a.advanced(by: 1).pointee
a.advanced(by: 9).pointee

// UnsafeBufferPointer 这个结构体是一个Swift数组和指针的桥梁
var b = UnsafeBufferPointer(start: a, count: size)
b.forEach {
    print("\($0)")
}

var c = [1, 2, 3, 4, 5, 6]
c.withUnsafeBufferPointer({ ptr in
    ptr.forEach({ print("\($0)") }) // 1, 2, 3...
})

// 内存管理带来的危害
// 然而我们我们是使用 unsafe 引用来混合两个世界（不需要内存管理和手动内存管理）
var collectionPtr: UnsafeMutableBufferPointer<Int>?
func duplicateElements(inArray: UnsafeMutableBufferPointer<Int>) {
    for i in 0..<inArray.count {
        inArray[i] *= 2
    }
}

repeat {
    var collection = [1, 2, 3]
    collection.withUnsafeMutableBufferPointer({
        collectionPtr = $0
    })
    
    collectionPtr?.forEach({
        print($0) // 正常
    })
} while false

collectionPtr?.forEach({
    print($0)   // error
})

duplicateElements(inArray: collectionPtr!)

a.deallocate(capacity: size)

// 使用 bitPattern 来修改指针的值
//pass

// 透明指针
// no data here

// see more here, http://technology.meronapps.com/2016/09/27/Swift-3-0-unsafe-world-2/