Swift指针为结构体表示分为两种，一种是带有格式的，一种是不带格式的。
如：
UnsafePointer这种是带格式的，带格式的指针实现了Pointee，所以可以使用.pointee来取值；
UnsafeRawPointer这种是不带格式的 ，可以理解为一段的纯粹的二进制，要对UnsafeRawPointer这种指针取值，需要将其转化为带格式的指针。
带有mutale的为值可变指针，即可以对指针对应的值赋值，不带的mutale的为只读指针
指针初始化
注意：使用alloc初始化的指针，释放的时候都要调用deallocate来释放
方法一:
var str = "test"
let pointer = withUnsafePointer(to: &str, {
return $0
})
得到的是一个UnsafePointer
方法二：
let pointer = UnsafeMutablePointer.allocate(capacity: 8)
得到的是一段长度为8个字节的指针UnsafeMutablePointer
如果要得到一个空指针怎么办呢？
在oc中我们通常会用：(float *)NULL来表示
swift 提供了类似的函数使用：UnsafePointer(bitPattern: 0)
方法三：
先初始化无格式指针，再将无格式指针转化为有格式指针
let rawPointer = calloc(8, MemoryLayout<UInt8>.size)!
let pointer = rawPointer.assumingMemoryBound(to: UInt32)
方法四：
使用&,只能用于函数传递，例如：
var testStr = "test"
test(&testStr)
print(testStr)
func test(_ pointer: UnsafeMutablePointer) {
pointer.pointee = "changed"
}
指针类型变化
如果我有一个UnsafeMutablePointer<UInt8>想按照UnsafeMutablePointer<UInt32>读取怎么办？
let point = UnsafeMutablePointer<UInt8>.allocate(capacity: 8)
let newPointer = point.withMemoryRebound(to: UInt32, capacity: 2, {
return $0
})
赋值和取值
var testStr = "123456789"
let testP = withUnsafeMutablePointer(to: &testStr, {
return $0
})
print(testP.pointee)
testP.pointee = "abcdefg"
print(testP)
data与bytes
let str = "aaaaaaabbbbbbbccccccc"
var data = str.data(using: .utf8)!
data.withUnsafeMutableBytes { (bytes: UnsafeMutablePointer) -> Void in
var bytesStart = bytes
for _ in 0..
print(bytesStart.pointee)
bytesStart += 1
}
}
UnsafeBufferPointer（类似于bytes），用法与UnsafePointer差不多，实现了Sequence
let str = "aaaaaaabbbbbbbccccccc"
var data = str.data(using: .utf8)!
let pointer = UnsafeMutableBufferPointer.allocate(capacity: data.count)
data.copyBytes(to: pointer)
for i in 0..10
print(pointer[i])
}
