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Class vs Struct in Memory | iOS & Swift

kaydacode profile image Kim Arnett  ・3 min read

Hey everyone, long time no blog! What better time to get back at it then between the two week span of being laid off and facing a global pandemic, am I right? So, to stay grounded I offered the following, in which someone asked a great question, that I felt like deserved a blog post instead of a quick response. Mostly because I have to remind myself of the answer multiple times a year. :)

Question is: Mutability in Structs vs Classes

If you're cruising around an iOS codebase, chances are you've come across some objects declared as structs, and some declared as classes. What gives? If you've googled for an answer, you probably found something talking about reference types or value types and other boring programming words. I'm going to cover the basics of what you need to know. If you want to dive deeper, by all means please do! Here's a great starting point from Apple's Swift docs on Structures vs Classes.

High level: Structs copy data, classes mutate data.

Let's experiment with structs:

Here I've defined a simple example, one variable.
Full playground example down below

struct exampleStruct {
    var varA = 0
}
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If we create two instances of exampleStruct, we can see that changing one does not change the other.

var myFirstStruct = exampleStruct()
var mySecondStruct = exampleStruct()

myFirstStruct.varA = 100
print(myFirstStruct.varA)   //100
print(mySecondStruct.varA)  //0
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If we assign the first struct to the second struct, we might expect that it would point to the same memory instance as the first struct, and therefore change with it. Let's see..

mySecondStruct = myFirstStruct
myFirstStruct.varA = 200

print(myFirstStruct.varA)   //200
print(mySecondStruct.varA)  //100
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Whaaat. Actually what happened was the second struct took a copy of the first struct in a new memory slot. Meaning, it adopted the values that existed in first struct at the time of copy, but by saving them into a new memory slot, it copies/duplicates and therefore does not adopt any new values assigned to first struct.

Let's experiment with classes:

Another simple example, one variable.

class exampleClass {
    var varA = 0
}
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If we create two instances of exampleClass, we can see that changing one does not change the other, same as structs.

var myFirstClass = exampleClass()
var mySecondClass = exampleClass()

myFirstClass.varA = 100
print(myFirstClass.varA)    //100
print(mySecondClass.varA)   //0
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So far so good. If we assign the first class to the second class, will it point to the same memory instance as we expected before? Or will it copy like structs? Let's see..

mySecondClass = myFirstClass
myFirstClass.varA = 200

print(myFirstClass.varA)    //200
print(mySecondClass.varA)   //200
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The second class pointed to the same memory slot as the first class object, thus aligning them to any values that change going forward.

mySecondClass.varA = 300
print(myFirstClass.varA)    //300
print(mySecondClass.varA)   //300
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Thanks for checking this out! There are some other differences worth checking out, again a good place to start is the Swift docs linked above.

Full playground example here:

struct exampleStruct {
    var varA = 0
}

//STRUCT EXAMPLE - Start with two
var myFirstStruct = exampleStruct()
var mySecondStruct = exampleStruct()

myFirstStruct.varA = 100
print(myFirstStruct.varA)   //100
print(mySecondStruct.varA)  //0

mySecondStruct = myFirstStruct
myFirstStruct.varA = 200

print(myFirstStruct.varA)   //200
print(mySecondStruct.varA)  //100


print("-----------------")

class exampleClass {
    var varA = 0
}


//CLASS EXAMPLE
var myFirstClass = exampleClass()
var mySecondClass = exampleClass()

myFirstClass.varA = 100
print(myFirstClass.varA)    //100
print(mySecondClass.varA)   //0

mySecondClass = myFirstClass
myFirstClass.varA = 200
print(myFirstClass.varA)    //200
print(mySecondClass.varA)   //200

mySecondClass.varA = 300
print(myFirstClass.varA)    //300
print(mySecondClass.varA)   //300
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Discussion (2)

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Olivier Lance

👍
It might be worth mentioning that a struct with a class instance value will not duplicate the instance (i.e. you get the same pointer to the same instance) when duplicating the struct!

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Info Comment marked as low quality/non-constructive by the community. View code of conduct
hlc0000

Hello, I'm an IOS developer. Recently, when using swift to develop applications, I found that there are few caches written by pure swift. So I wrote a cache -- swiftlycache, a lightweight general-purpose IOS cache library using swift 5. If you are using swift for development, if you also need to use cache, maybe you can try swiftlycache, maybe you will like it, If you like, you can also introduce it to your friends. Thank you
github.com/hlc0000/SwiftlyCache