A mutable nonowning collection interface to the bytes in a region of memory.
SDK
- Xcode 8.0+
Framework
- Swift Standard Library
Declaration
@frozen struct UnsafeMutableRawBufferPointer
Overview
You can use an Unsafe instance in low-level operations to eliminate uniqueness checks and release mode bounds checks. Bounds checks are always performed in debug mode.
An Unsafe instance is a view of the raw bytes in a region of memory. Each byte in memory is viewed as a UInt8 value independent of the type of values held in that memory. Reading from and writing to memory through a raw buffer are untyped operations. Accessing this collection’s bytes does not bind the underlying memory to UInt8.
In addition to its collection interface, an Unsafe instance also supports the following methods provided by Unsafe, including bounds checks in debug mode:
load(fromByte Offset: as:) storeBytes(of: to Byte Offset: as:) copyMemory(from:)
To access the underlying memory through typed operations, the memory must be bound to a trivial type.
UnsafeMutableRawBufferPointer Semantics
An Unsafe instance is a view into memory and does not own the memory that it references. Copying a variable or constant of type Unsafe does not copy the underlying memory. However, initializing another collection with an Unsafe instance copies bytes out of the referenced memory and into the new collection.
The following example uses some, an Unsafe instance, to demonstrate the difference between assigning a buffer pointer and using a buffer pointer as the source for another collection’s elements. Here, the assignment to dest creates a new, nonowning buffer pointer covering the first n bytes of the memory that some references—nothing is copied:
var destBytes = someBytes[0..<n]
Next, the bytes referenced by dest are copied into byte, a new [UInt] array, and then the remainder of some is appended to byte:
var byteArray: [UInt8] = Array(destBytes)
byteArray += someBytes[n..<someBytes.count]
Assigning into a ranged subscript of an Unsafe instance copies bytes into the memory. The next n bytes of the memory that some references are copied in this code:
destBytes[0..<n] = someBytes[n..<(n + n)]