{"id":1923,"date":"2015-08-23T18:43:08","date_gmt":"2015-08-24T00:43:08","guid":{"rendered":"http:\/\/jamesonquave.com\/blog\/?p=1923"},"modified":"2015-09-01T14:21:33","modified_gmt":"2015-09-01T20:21:33","slug":"objective-c-pointers-and-swift","status":"publish","type":"post","link":"https:\/\/jamesonquave.com\/blog\/objective-c-pointers-and-swift\/","title":{"rendered":"Objective-C Pointers and Swift 2: A Simple Guide"},"content":{"rendered":"

This post written on August 23, 2015 to be compatible with Xcode 7 Beta and Swift 2<\/i><\/b><\/p>\n

Objective-C Pointers and Swift<\/h2>\n

<\/p>\n

Reading C pointers from Swift<\/h3>\n

Let’s say I have a C Int object as a Swift pointer.<\/p>\n

For example this Objective-C method would return an int pointer, or (int *) in C terminology:<\/p>\n

\r\n
\r\n\r\n\r\n\r\n
\r\n
\r\n
@interface<\/code> PointerBridge<\/code> : <\/code>NSObject<\/code> {<\/code><\/div>\r\n
    <\/code>int count;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
- (int *) getCountPtr;<\/code><\/div>\r\n
@end<\/code><\/div>\r\n
 <\/div>\r\n
@implementation<\/code> PointerBridge<\/code><\/div>\r\n
- (instancetype) <\/code>init<\/code> {<\/code><\/div>\r\n
    <\/code>self<\/code> = [<\/code>super<\/code> init<\/code>];<\/code><\/div>\r\n
    <\/code>if<\/code>(<\/code>self<\/code>) {<\/code><\/div>\r\n
        <\/code>count = 23;<\/code><\/div>\r\n
    <\/code>}<\/code><\/div>\r\n
    <\/code>return<\/code> self<\/code>;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
- (int *) getCountPtr {<\/code><\/div>\r\n
    <\/code>return<\/code> &count;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
@end<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
If you’re unfamiliar with Obj-C or C syntax, the above basically just declares a class called PointerBridge, and it has a single method called getCountPtr, which returns the memory address<\/em> of an int whose value is 23. The Int is just a variable on the instance called count<\/em>, which gets set to 23 in the constructor, init.<\/p>\n
I put this in an Objective-C h-file and import it in to my bridging header in order to expose it to Swift. Then in Swift I create the PointerBridge class as a new instance called bridge, and get the result of the getCountPtr() method…<\/p>\n
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\r\n
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let<\/code> bridge = <\/code>PointerBridge<\/code>()<\/code><\/div>\r\n
let<\/code> theInt = bridge.getCountPtr()<\/code><\/div>\r\n
print<\/code>(theInt)<\/code><\/div>\r\n
print<\/code>(theInt.memory)<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
Examine the type of theInt<\/em> by holding the Option key and clicking the variable name in Xcode, and you’ll find it’s Swift type is a UnsafeMutablePointer<Int32><\/em>. This is a pointer<\/em> to an Int, which is not the same thing as being an Int… It just points<\/em> to it.<\/p>\n
If we run this small program and allow this Swift to execute, we’ll see theInt<\/em> printed to the console as a memory address, something like 0x00007f8bdb508ef8<\/em>. Then, we’ll see the value 23 printed from the memory<\/em> member. Accessing memory<\/em> on a pointer generally returns the underlying object, in this case just the original 32-bit int (brought in to Swift as an Int32)<\/p>\n
Now consider our Objective-C class also allows us to set the value of count.<\/p>\n
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\r\n
\r\n
@interface<\/code> PointerBridge<\/code> : <\/code>NSObject<\/code> {<\/code><\/div>\r\n
    <\/code>int count;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
- (int *) getCountPtr;<\/code><\/div>\r\n
- (void) setCount:(int)newCount;<\/code><\/div>\r\n
@end<\/code><\/div>\r\n
 <\/div>\r\n
@implementation<\/code> PointerBridge<\/code><\/div>\r\n
- (instancetype) <\/code>init<\/code> {<\/code><\/div>\r\n
    <\/code>self<\/code> = [<\/code>super<\/code> init<\/code>];<\/code><\/div>\r\n
    <\/code>if<\/code>(<\/code>self<\/code>) {<\/code><\/div>\r\n
        <\/code>count = 23;<\/code><\/div>\r\n
    <\/code>}<\/code><\/div>\r\n
    <\/code>return<\/code> self<\/code>;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
- (int *) getCountPtr {<\/code><\/div>\r\n
    <\/code>return<\/code> &count;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
- (void) setCount:(int)newCount {<\/code><\/div>\r\n
    <\/code>count = newCount;<\/code><\/div>\r\n
}<\/code><\/div>\r\n
@end<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
Now, we could modify the value for count by calling setCount(). Since theInt<\/em> is a pointer, updating the count value using setCount should also update theInt.memory<\/em>. The memory address won’t change after all, just the value.<\/p>\n
In other words, the following should log to the console the number 23, and then the number 1000.<\/p>\n
\r\n
\r\n\r\n\r\n\r\n
\r\n
\r\n
let<\/code> bridge = <\/code>PointerBridge<\/code>()<\/code><\/div>\r\n
let<\/code> theInt = bridge.getCountPtr()<\/code><\/div>\r\n
print<\/code>(theInt.memory) <\/code>\/\/ 23<\/code><\/div>\r\n
bridge.setCount(1000)<\/code><\/div>\r\n
print<\/code>(theInt.memory) <\/code>\/\/ 1000<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
A shortcut you might want to take, to avoid writing .memory all the time, is to assign .memory to a variable:<\/p>\n
\r\n
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\r\n
\r\n
let<\/code> bridge = <\/code>PointerBridge<\/code>()<\/code><\/div>\r\n
let<\/code> theInt = bridge.getCountPtr()<\/code><\/div>\r\n
let<\/code> countVal = theInt.memory<\/code><\/div>\r\n
print<\/code>(countVal) <\/code>\/\/ 23<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
As before, 23 is printed to the console. However, if I do the same thing we did before, and modify the underlying count value by calling setCount(), we see a problem crop up:<\/p>\n
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\r\n\r\n\r\n\r\n
\r\n
\r\n
let<\/code> bridge = <\/code>PointerBridge<\/code>()<\/code><\/div>\r\n
let<\/code> theInt = bridge.getCountPtr()<\/code><\/div>\r\n
let<\/code> countVal = theInt.memory<\/code><\/div>\r\n
print<\/code>(countVal) <\/code>\/\/ 23<\/code><\/div>\r\n
 <\/div>\r\n
bridge.setCount(1000)<\/code><\/div>\r\n
print<\/code>(countVal) <\/code>\/\/ 23<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
What’s happening here is that countVal is being assigned by value<\/em>. At the time of assignment the value<\/em> is 23, so countVal actually has it’s own memory address where it stores 23 permanently, thus losing it’s pointer nature. countVal<\/em> is just a regular Int32 now.<\/p>\n
Creating C pointers from Swift Objects<\/h3>\n
What if we wanted to do the inverse of what we did above? Instead of setting the value of count with an Int, what if we needed to pass in a pointer?<\/p>\n
So in the Objective-C let’s say there was a method like this:<\/p>\n
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\r\n\r\n\r\n\r\n
\r\n
\r\n
- (void) setCountPtr:(int *)newCountPtr {<\/code><\/div>\r\n
    <\/code>count = *newCountPtr;<\/code><\/div>\r\n
}<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
This method is pretty contrived, and just ends up re-assigning the value of the newCountPtr, but in your Swift development you will face real-world scenarios of needing to pass in pointers. This one is contrived simply to demonstrate how to create the pointer type from Swift, and pass it in.<\/p>\n
You might think you could simply pass in an Int value, with a reference operator like &. This is how you might do this same thing in C. Or for example in Objective-C you would write this:<\/p>\n
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int mcount = 500;<\/code><\/div>\r\n
[<\/code>self<\/code> setCountPtr:&mcount];<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
Which would update the count value to be 500 successfully. However in Swift, you may see just from auto-complete that it wants something a little more complicated (and verbose). It’s looking for a newCountPtr of type UnsafeMutablePointer<Int32>.<\/p>\n
I know this type is a mouthful, and it looks really complicated. However, it’s actually fairly simple, especially if you know pointers from Obj-C. In order to create an object with type UnsafeMutablePointer<Int32>, we just call the constructor and for it’s only argument we pass in the size of the pointer in units of the number of objects.<\/p>\n
\r\n
\r\n\r\n\r\n\r\n
\r\n
\r\n
let<\/code> bridge = <\/code>PointerBridge<\/code>()<\/code><\/div>\r\n
let<\/code> theInt = bridge.getCountPtr()<\/code><\/div>\r\n
print<\/code>(theInt.memory) <\/code>\/\/ 23<\/code><\/div>\r\n
 <\/div>\r\n
let<\/code> newIntPtr = <\/code>UnsafeMutablePointer<\/code><<\/code>Int32<\/code>>.alloc(1)<\/code><\/div>\r\n
newIntPtr.memory = 100<\/code><\/div>\r\n
bridge.setCountPtr(newIntPtr)<\/code><\/div>\r\n
 <\/div>\r\n
print<\/code>(theInt.memory) <\/code>\/\/ 100<\/code><\/div>\r\n<\/div><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/code><\/pre>\n
The only argument to the UnsafeMutablePointer<Int32> constructor is how many objects worth of space will be allocated, so we can just pass in 1 since we only need 1 Int32 object. Then, simply inverting what we did before with the memory<\/em> property, we can assign a value to our newly created pointer. Finally, we can simply pass in the newIntPtr to the setCountrPtr method. Printing out the value of the original theInt<\/em> pointer, we see the value has indeed been updated to 100.<\/p>\n
Conclusion<\/h3>\n
The UnsafeMutablePointer<T> type, and it’s sibling UnsafePointer<T> are basically just abstractions of C pointers. If it helps to wrap your head around them, think of them sort of like Swift Optionals, in that they do not directly equate to a certain value, but instead act as an abstraction layer on top of a contained variable. The type is generic, and allows for other types to be used than just Int32. For example if you needed to work with a Float object you might need an UnsafeMutablePointer<Float>.<\/float><\/p>\n
The important thing to remember is that you do not cast<\/em> from an Int to an UnsafeMutablePointer<Int>, because a pointer is simply not the same as an Int value. So instead, be prepared to create a new object, calling it’s constructor UnsafeMutablePointer<Int>(count: Int)<\/code>.<\/int><\/p>\n
\nWe’ll follow-up this post with a deeper dive in to the details of function pointers, and how to take advantage of these features to work with legacy C and Objective-C APIs. Make sure to sign up for our newsletter so you don’t miss it<\/a>!<\/p>\n
Subscribe For Free Now »<\/h1>\n
<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
This post written on August 23, 2015 to be compatible with Xcode 7 Beta and Swift 2 Objective-C Pointers and Swift Reading C pointers from Swift Let’s say I have a C Int object as a Swift pointer. For example this Objective-C method would return an int pointer, or (int *) in C terminology: @interface…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_links_to":"","_links_to_target":""},"categories":[1],"tags":[],"yoast_head":"\nObjective-C Pointers and Swift 2: A Simple Guide - Jameson Quave<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jamesonquave.com\/blog\/objective-c-pointers-and-swift\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Objective-C Pointers and Swift 2: A Simple Guide - Jameson Quave\" \/>\n<meta property=\"og:description\" content=\"This post written on August 23, 2015 to be compatible with Xcode 7 Beta and Swift 2 Objective-C Pointers and Swift Reading C pointers from Swift Let’s say I have a C Int object as a Swift pointer. 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