A pointer for accessing data of a specific type.
- Xcode 6.0.1+
- Swift Standard Library
You use instances of the
Unsafe type to access data of a specific type in memory. The type of data that a pointer can access is the pointer’s
Unsafe provides no automated memory management or alignment guarantees. You are responsible for handling the life cycle of any memory you work with through unsafe pointers to avoid leaks or undefined behavior.
Memory that you manually manage can be either untyped or bound to a specific type. You use the
Unsafe type to access and manage memory that has been bound to a specific type.
Understanding a Pointer’s Memory State
The memory referenced by an
Unsafe instance can be in one of several states. Many pointer operations must only be applied to pointers with memory in a specific state—you must keep track of the state of the memory you are working with and understand the changes to that state that different operations perform. Memory can be untyped and uninitialized, bound to a type and uninitialized, or bound to a type and initialized to a value. Finally, memory that was allocated previously may have been deallocated, leaving existing pointers referencing unallocated memory.
Memory that has just been allocated through a typed pointer or has been deinitialized is in an uninitialized state. Uninitialized memory must be initialized before it can be accessed for reading.
Initialized memory has a value that can be read using a pointer’s
pointee property or through subscript notation. In the following example,
ptr is a pointer to memory initialized with a value of
Accessing a Pointer’s Memory as a Different Type
When you access memory through an
Unsafe instance, the
Pointee type must be consistent with the bound type of the memory. If you do need to access memory that is bound to one type as a different type, Swift’s pointer types provide type-safe ways to temporarily or permanently change the bound type of the memory, or to load typed instances directly from raw memory.
Unsafe instance allocated with eight bytes of memory,
uint8Pointer, will be used for the examples below.
When you only need to temporarily access a pointer’s memory as a different type, use the
with method. For example, you can use this method to call an API that expects a pointer to a different type that is layout compatible with your pointer’s
Pointee. The following code temporarily rebinds the memory that
uint8Pointer references from
Int8 to call the imported C
When you need to permanently rebind memory to a different type, first obtain a raw pointer to the memory and then call the
bind method on the raw pointer. The following example binds the memory referenced by
uint8Pointer to one instance of the
After rebinding the memory referenced by
UInt64, accessing that pointer’s referenced memory as a
UInt8 instance is undefined.
Alternatively, you can access the same memory as a different type without rebinding through untyped memory access, so long as the bound type and the destination type are trivial types. Convert your pointer to an
Unsafe instance and then use the raw pointer’s
load(from method to read values.
Performing Typed Pointer Arithmetic
Pointer arithmetic with a typed pointer is counted in strides of the pointer’s
Pointee type. When you add to or subtract from an
Unsafe instance, the result is a new pointer of the same type, offset by that number of instances of the
You can also use subscript notation to access the value in memory at a specific offset.
Implicit Casting and Bridging
When calling a function or method with an
Unsafe parameter, you can pass an instance of that specific pointer type, pass an instance of a compatible pointer type, or use Swift’s implicit bridging to pass a compatible pointer.
For example, the
Unsafe instance as its first parameter:
As is typical in Swift, you can call the
Unsafe instance. This example passes
int, a pointer to an
Int value, to
Because a mutable typed pointer can be implicitly cast to an immutable pointer with the same
Pointee type when passed as a parameter, you can also call
Alternatively, you can use Swift’s implicit bridging to pass a pointer to an instance or to the elements of an array. The following example passes a pointer to the
value variable by using inout syntax:
An immutable pointer to the elements of an array is implicitly created when you pass the array as an argument. This example uses implicit bridging to pass a pointer to the elements of
numbers when calling
You can also use inout syntax to pass a mutable pointer to the elements of an array. Because
No matter which way you call