swift - 如何在 Swift 中重新解释_cast？

Question

我们的项目依赖于一个 C 库，该库声明了一个通用的 struct

typedef struct
{
  SomeType a_field;
  char payload[248];
} GeneralStruct

还有一个更具体的:

typedef struct
{
  SomeType a_field;
  OtherType other_field;
  AnotherType another_file;
  YetAnotherType yet_another_field;
} SpecificStruct

我们有一些它在 C++ 中的用法示例，在某些情况下，需要将通用的转换为特定的，例如:

GeneralStruct generalStruct = // ...
SpecificStruct specificStruct = reinterpret_cast<SpecificStruct&>(generalStruct)

Swift 中是否有类似 reinterpret_cast 的东西？我想我可以手动从 payload 中读取字节，但我正在寻找一种惯用的方式

Answer 1

withMemoryRebound(to:capacity:_:)可以用

... when you have a pointer to memory bound to one type and you need to access that memory as instances of another type.

示例:获取通用结构的地址，然后重新绑定(bind)并取消引用指针:

let general = GeneralStruct()

let specific = withUnsafePointer(to: general) {
    $0.withMemoryRebound(to: SpecificStruct.self, capacity: 1) {
        $0.pointee
    }
}

如果两种类型具有相同的大小和兼容的内存布局，那么您还可以使用 unsafeBitCast(_:to:) :

Use this function only to convert the instance passed as x to a layout-compatible type when conversion through other means is not possible.

Warning: Calling this function breaks the guarantees of the Swift type system; use with extreme care.

例子:

let specific = unsafeBitCast(general, to: SpecificStruct.self)