Pointers in Swift
What’s is a pointer? A pointer is a variable that shops the reminiscence deal with of a referenced object. As I discussed this in my earlier article, in regards to the reminiscence format of varied objects in Swift, a reminiscence deal with is only a hexadecimal illustration of a knowledge situated someplace within the reminiscence. You utilize situations of varied unsafe pointer varieties to entry information of a particular kind in reminiscence.
Why can we need to use these type of pointers on the first place? By default you do not have to write down unsafe Swift code, and in many of the instances you may dwell with out unsafe reminiscence pointers. These pointers come useful if you must interoperate with different “unsafe” languages, resembling C. There are different low stage or legacy APIs that require using guide reminiscence administration, however you should not be afraid, when you get aware of unsafe Swift pointer varieties you will know much more about how reminiscence works and you may see how skinny is the layer between C libraries and Swift. 😱
What sort of pointers are there? With a purpose to perceive pointer varieties higher in Swift, let’s get again to C only for a second. Contemplate the next C code instance:
#embody <stdio.h>
int important () {
int x = 20;
int* xPointer = &x;
printf("x deal with: `%p`n", &x);
printf("x worth: `%u`n", x);
printf("pointer deal with: `%p`n", &xPointer);
printf("pointer reference: `%p`n", xPointer); // equals the deal with of x
printf("pointer reference worth: `%u`n", *xPointer);
*xPointer = 420;
printf("x worth: `%u`n", x);
printf("pointer reference worth: `%u`n", *xPointer);
x = 69;
printf("x worth: `%u`n", x);
printf("pointer reference worth: `%u`n", *xPointer);
return 0;
}
It can save you this code snippet utilizing the important.c identify, then compile & run it utilizing the clang important.c -o important && ./important command. It would present a fairly comparable output.
$ clang important.c -o important && ./important
x deal with: `0x16b0c7a38`
x worth: `20`
pointer deal with: `0x16b0c7a30`
pointer reference: `0x16b0c7a38`
pointer reference worth: `20`
pointer worth `20`
tib@~: clang important.c -o important && ./important
x deal with: `0x16d52fa38`
x worth: `20`
pointer deal with: `0x16d52fa30`
pointer reference: `0x16d52fa38`
pointer reference worth: `20`
x worth: `420`
pointer reference worth: `420`
x worth: `69`
pointer reference worth: `69`
So what is going on on right here? Effectively, we merely created an integer variable and a pointer variable with an integer kind. We used the deal with of our x variable (&x) to affiliate our pointer with the reminiscence deal with of x. Now each variables factors to the identical reminiscence deal with.
We will affirm this by logging the reminiscence deal with of each variables. We will additionally alter the worth of x by updating the referenced worth of the pointer (we are able to use the * character for this) or go together with the standard make x = one thing line. We have merely logged the modified values to verify that the pointer worth replace additionally modified the worth of x. Lets say that xPointer is only a reference to x.
Now how can we obtain the identical factor in Swift? First we’ve to learn to outline a pointer kind. Here is a fast listing of the entire unsafe pointer objects accessible within the Swift normal library:
You might need seen a sample right here: Unsafe|[Mutable][Raw][Buffer]Pointer[<T>].
Unsafe pointers are simply direct reminiscence addresses. The whole lot that’s mutable will be modified, in different phrases you may write to that deal with. Uncooked signifies that there isn’t a related (generic, T) kind to the given pointer, it is only a blob of uncooked bytes. Buffers are batches (collections) of pointers.
Don’t be concerned if these varieties are fairly complicated for you proper now, it will all make sense in a couple of minutes. Let’s get again to our authentic C pattern code and port it to Swift actual fast.
var x: Int = 20
var xPointer: UnsafeMutablePointer<Int> = .init(&x)
print("x deal with:", UnsafeRawPointer(&x));
print("x worth:", x);
print("pointer deal with:", UnsafeRawPointer(&xPointer));
print("pointer reference:", xPointer);
print("pointer reference worth:", xPointer.pointee);
xPointer.pointee = 420;
print("x worth:", x);
print("pointer reference worth:", xPointer.pointee);
x = 69;
print("x worth:", x);
print("pointer reference worth:", xPointer.pointee);We have created an UnsafeMutablePointer<Int> reference to our x worth, that is principally an int* kind if we return to the C instance. We will use the identical ampersand (&) character to create pointers from variables. We have created a typed mutable pointer, since we might like to vary the worth of the referenced integer object (by means of the pointee property) afterward.
To print the reminiscence deal with of a variable we are able to merely use an UnsafeRawPointer kind, as a result of we do not actually care in regards to the underlying “pointee” worth, however we simply want the deal with of the reference. In case you print a pointer kind the debug description will comprise the underlying reminiscence deal with of the referenced object. On this case the deal with of x and xPointer. 🤔
Working with typed pointers in Swift
How can we retailer some values at “unsafe” reminiscence addresses in Swift? The most straightforward method is that we begin with a generic mutable pointer. We will allocate pointers utilizing the required capability, since we’re working with unsafe reminiscence, we additionally must deallocate reminiscence after we have completed utilizing it. We additionally must manually initialize pointer reference values, unsafe pointers can already comprise some type of leftover information, so the protected method is to initialize them with a brand new default worth.
let numbers = [4, 8, 15, 16, 23, 42]
let pointer = UnsafeMutablePointer<Int>.allocate(capability: numbers.rely)
pointer.initialize(repeating: 0, rely: numbers.rely)
defer {
pointer.deinitialize(rely: numbers.rely)
pointer.deallocate()
}
for (index, worth) in numbers.enumerated() {
pointer.superior(by: index).pointee = worth
}
print(pointer.superior(by: 5).pointee);
let bufferPointer = UnsafeBufferPointer(begin: pointer, rely: numbers.rely)
for (index, worth) in bufferPointer.enumerated() {
print(index, "-", worth)
}
let bufferPointer = UnsafeMutableBufferPointer(begin: pointer, rely: numbers.rely)
for (index, _) in bufferPointer.enumerated() {
bufferPointer[index] = index + 1
}After we’ve the allotted reminiscence storage, we are able to set the suitable pointee values, since we have allotted the pointer with a capability of six integer values, we are able to retailer as much as 6 numbers utilizing this pointer. You need to use the superior(by:) technique (pointer arithmetic (pointer + 5).pointee = 42) works as properly) to maneuver to the following deal with and set the pointee worth of it.
The very last item I might wish to let you realize is that you need to use a typed buffer pointer to iterate by means of these quantity references. You may consider buffer pointers as an array of pointer references. It’s doable to enumerate by means of pointer values and indexes immediately. You may replace buffer pointer values through the use of the subscript syntax on a mutable buffer pointer. 💡
We already talked in regards to the UnsafePointer, UnsafeMutablePointer, UnsafeRawPointer, UnsafeBufferPointer and UnsafeMutableBufferPointer kind let’s dive in to uncooked pointers.
Reminiscence administration utilizing uncooked pointers
Typed pointers present some type of security if it involves pointers, however how can we work with uncooked pointers? We have already seen how straightforward is to print out an deal with of a given worth kind utilizing an UnsafeRawPointer reference, now it is time to join the dots and allocate some unsafe uncooked reminiscence. If you have to know extra about reminiscence format in Swift, please learn my earlier article.
To start with, we’ll must understand how a lot reminiscence to allocate. We will use the MemoryLayout struct to get data a few worth kind. We will use the stride and the variety of objects to rely how a lot byte is required to retailer our information kind utilizing a uncooked reminiscence storage.
let numbers = [4, 8, 15, 16, 23, 42]
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * numbers.rely
let pointer = UnsafeMutableRawPointer.allocate(byteCount: byteCount, alignment: alignment)
defer {
pointer.deallocate()
}
for (index, worth) in numbers.enumerated() {
pointer.superior(by: stride * index).storeBytes(of: worth, as: Int.self)
}
let bufferPointer = UnsafeRawBufferPointer(begin: pointer, rely: byteCount)
for index in 0..<numbers.rely {
let worth = bufferPointer.load(fromByteOffset: stride * index, as: Int.self)
print(index, "-", worth)
}After we have allotted the required area, we are able to use the pointer and the superior(by:) technique to retailer byte values of a given kind (storeBytes(of:as:)) as uncooked bytes. We will load a given kind utilizing the load(as:) technique. It’s value to say that if the reminiscence doesn’t comprise a worth that may be represented because the given kind, incompatible worth varieties can crash your app. ☠️
Anyway, when you saved a number of values utilizing a pointer you need to use the uncooked buffer assortment to iterate by means of them and cargo again the kinds as values from a given byte offset. In case you enumerate by means of a uncooked byte buffer you may also print the byte illustration for the pointer.
If you wish to know extra about methods to Safely handle pointers in Swift, I extremely suggest watching the linked WWDC video. It is a contemporary one, the pattern code is appropriate with Swift 5. 💪
Reminiscence binding will be harmful
You need to use the bindMemory and the asssumingMemoryBound strategies to transform a uncooked pointer to a typed pointer. The primary will truly bind the reminiscence to a given kind, however the second perform simply returns a referenced pointer assuming it is already sure to the desired kind. You may learn extra about the important thing variations right here or verify the unique UnsafeRawPointer API proposal.
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let rely = 1
let byteCount = stride * rely
let rawPointer = UnsafeMutableRawPointer.allocate(byteCount: byteCount, alignment: alignment)
defer {
rawPointer.deallocate()
}
let pointer = rawPointer.bindMemory(to: Int.self, capability: rely)
pointer.initialize(repeating: 0, rely: rely)
defer {
pointer.deinitialize(rely: rely)
}
pointer.pointee = 42
print(rawPointer.load(as: Int.self))
rawPointer.storeBytes(of: 69, toByteOffset: 0, as: Int.self)
print(pointer.pointee)Binding reminiscence will be harmful, there are a few guidelines that it’s best to comply with:
- By no means return the pointer from a
withUnsafeBytesname - Solely bind to 1 kind at a time
- Watch out with off-by-one errors
This text lists the problems that may occur when you re-bind a reminiscence deal with.
let badPointer = rawPointer.bindMemory(to: Bool.self, capability: rely)
print(badPointer.pointee)
pointer.withMemoryRebound(to: Bool.self, capability: rely) { boolPointer in
print(boolPointer.pointee)
}
withUnsafeBytes(of: &pointer.pointee) { pointer -> Void in
for byte in pointer {
print(byte)
}
}
let bufferPointer = UnsafeRawBufferPointer(begin: pointer, rely: byteCount + 1)
for byte in bufferPointer {
print(byte)
}I additionally suggest checking this text about reminiscence administration and byte computation in Swift. Additionally it is doable to repeat or transfer a reminiscence to a given vacation spot utilizing the assign(from:rely:) or moveAssign(from:rely:) strategies. You may learn extra about these features right here.
Opaque and managed Swift pointers
If unsafe pointers weren’t simply sufficient, it’s best to know that Swift has a number of different pointer varieties.
As Vadim Bulavin describes this in his article, with the assistance of the Unmanaged kind you may bypass Computerized Reference Counting (ARC) that’s in any other case enforced to each Swift class. The opposite case is to transform objects between opaque pointers forwards and backwards.
class MyPoint {
let x: Int
let y: Int
init(x: Int, y: Int) {
self.x = x
self.y = y
}
deinit {
print("deinit", x, y)
}
}
let unmanaged = Unmanaged.passRetained(MyPoint(x: 4, y: 20))
unmanaged.launch()
_ = Unmanaged.passUnretained(MyPoint(x: 6, y: 9))
let opaque = Unmanaged.passRetained(MyPoint(x: 1, y: 0)).toOpaque()
Unmanaged<MyPoint>.fromOpaque(opaque).launch()Opaque pointers are used when you must work with incomplete C information constructions which can’t be represented in Swift. For instance in case you have a struct that accommodates a pointer kind, that variable goes to be imported to Swift as an OpaquePointer kind when interacting with C code.
ManagedBufferPointer and the ManagedBuffer kind means that you can implement your personal copy-on-write information construction. This manner you may obtain the very same habits because the built-in array, set or dictionary varieties have. Russ Bishop has an ideal publish associated to this subject.
AutoreleasingUnsafeMutablePointer is a pointer that factors to an Goal-C reference that does not personal its goal. you may learn extra about it right here by Keith Harrison
The CVaListPointer is an easy wrapper round a C va_list pointer.