Swift：无法将“UnsafeMutablePointer”类型的值转换为预期的参数类型“UnsafeMutablePointer”

Posted 2023-03-14

【中文标题】Swift：无法将“UnsafeMutablePointer”类型的值转换为预期的参数类型“UnsafeMutablePointer”

【英文标题】：Swift: Cannot convert value of type 'UnsafeMutablePointer' to expected argument type 'UnsafeMutablePointer'

【发布时间】：2016-11-02 10:29:06

【问题描述】：

更新到 Swift 3 后我的代码有点问题。我在转换之前有这个代码：

extension NSData 
  func castToCPointer<T>() -> T 
    let mem = UnsafeMutablePointer<T>.alloc(sizeof(T.Type))
    self.getBytes(mem, length: sizeof(T.Type))
    return mem.move()
  

我把它转换成这个代码，在第三行我得到一个错误

...无法将“UnsafeMutablePointer”类型的值转换为预期的参数类型“UnsafeMutablePointer”

extension Data 
  func castToCPointer<T>() -> T
    let mem = UnsafeMutablePointer<T>.allocate(capacity: MemoryLayout<T.Type>.size)
    self.copyBytes(to: mem, count: MemoryLayout<T.Type>.size)
    //self.copyBytes(to: mem, count: MemoryLayout<T.Type>.size)
    return mem.move()
  

有谁知道如何摆脱这个？

【参考方案1】：

copyBytes 需要 UnsafeMutableBufferPointer 作为参数：

extension Data 
    func castToCPointer<T>() -> T 
        let mem = UnsafeMutablePointer<T>.allocate(capacity: 1)
        _ = self.copyBytes(to: UnsafeMutableBufferPointer(start: mem, count: 1))
        return mem.move()
    

(allocate() 将“项目”的数量作为参数，而不是数量 字节。）

但请注意，您的方法会泄漏内存，分配的内存 被取消初始化（使用move()），但也必须是 解除分配：

extension Data 
    func castToCPointer<T>() -> T 
        let mem = UnsafeMutablePointer<T>.allocate(capacity: 1)
        _ = self.copyBytes(to: UnsafeMutableBufferPointer(start: mem, count: 1))
        let val =  mem.move()
        mem.deallocate(capacity: 1)
        return val
    

一个更简单的解决方案是（从 round trip Swift number types to/from Data):

extension Data 
    func castToCPointer<T>() -> T 
        return self.withUnsafeBytes  $0.pointee 
    

【参考方案2】：

您也可以在扩展中使用以下语法

extension Data 
    func castToCPointer<T>() -> T 
        var bytes = self.bytes
        var val = withUnsafePointer(to: &bytes)  (temp) in
            return unsafeBitCast(temp, to: T.self)
        
        return val
    

---

var data:NSData/NSMutableData
var bytes = data.bytes
var val = withUnsafePointer(to: &bytes)  (temp) in
   return unsafeBitCast(temp, to: T.self)