为什么选择结构而不是类？

Question

Playing around with Swift, coming from a Java background, why would you want to choose a Struct instead of a Class? Seems like they are the same thing, with a Struct offering less functionality. Why choose it then?

Answer 1

According to the very popular WWDC 2015 talk Protocol Oriented Programming in Swift ( video , transcript ), Swift provides a number of features that make structs better than classes in many circumstances.

Structs are preferable if they are relatively small and copiable because copying is way safer than having multiple references to the same instance as happens with classes. This is especially important when passing around a variable to many classes and/or in a multithreaded environment. If you can always send a copy of your variable to other places, you never have to worry about that other place changing the value of your variable underneath you.

With Structs, there is much less need to worry about memory leaks or multiple threads racing to access/modify a single instance of a variable. (For the more technically minded, the exception to that is when capturing a struct inside a closure because then it is actually capturing a reference to the instance unless you explicitly mark it to be copied).

Classes can also become bloated because a class can only inherit from a single superclass. That encourages us to create huge superclasses that encompass many different abilities that are only loosely related. Using protocols, especially with protocol extensions where you can provide implementations to protocols, allows you to eliminate the need for classes to achieve this sort of behavior.

The talk lays out these scenarios where classes are preferred:

• Copying or comparing instances doesn't make sense (eg, Window)
• Instance lifetime is tied to external effects (eg, TemporaryFile)
• Instances are just "sinks"--write-only conduits to external state (egCGContext)

It implies that structs should be the default and classes should be a fallback.

On the other hand, The Swift Programming Language documentation is somewhat contradictory:

Structure instances are always passed by value, and class instances are always passed by reference. This means that they are suited to different kinds of tasks. As you consider the data constructs and functionality that you need for a project, decide whether each data construct should be defined as a class or as a structure.

As a general guideline, consider creating a structure when one or more of these conditions apply:

• The structure's primary purpose is to encapsulate a few relatively simple data values.
• It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an instance of that structure.
• Any properties stored by the structure are themselves value types, which would also be expected to be copied rather than referenced.
• The structure does not need to inherit properties or behavior from another existing type.

Examples of good candidates for structures include:

• The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.
• A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.
• A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.

In all other cases, define a class, and create instances of that class to be managed and passed by reference. In practice, this means that most custom data constructs should be classes, not structures.

Here it is claiming that we should default to using classes and use structures only in specific circumstances. Ultimately, you need to understand the real world implication of value types vs. reference types and then you can make an informed decision about when to use structs or classes. Also, keep in mind that these concepts are always evolving and The Swift Programming Language documentation was written before the Protocol Oriented Programming talk was given.

Answer 2

Since struct instances are allocated on stack, and class instances are allocated on heap, structs can sometimes be drastically faster.

However, you should always measure it yourself and decide based on your unique use case.

Consider the following example, which demonstrates 2 strategies of wrapping Int data type using struct and class . I am using 10 repeated values are to better reflect real world, where you have multiple fields.

class Int10Class {
    let value1, value2, value3, value4, value5, value6, value7, value8, value9, value10: Int
    init(_ val: Int) {
        self.value1 = val
        self.value2 = val
        self.value3 = val
        self.value4 = val
        self.value5 = val
        self.value6 = val
        self.value7 = val
        self.value8 = val
        self.value9 = val
        self.value10 = val
    }
}

struct Int10Struct {
    let value1, value2, value3, value4, value5, value6, value7, value8, value9, value10: Int
    init(_ val: Int) {
        self.value1 = val
        self.value2 = val
        self.value3 = val
        self.value4 = val
        self.value5 = val
        self.value6 = val
        self.value7 = val
        self.value8 = val
        self.value9 = val
        self.value10 = val
    }
}

func + (x: Int10Class, y: Int10Class) -> Int10Class {
    return IntClass(x.value + y.value)
}

func + (x: Int10Struct, y: Int10Struct) -> Int10Struct {
    return IntStruct(x.value + y.value)
}

Performance is measured using

// Measure Int10Class
measure("class (10 fields)") {
    var x = Int10Class(0)
    for _ in 1...10000000 {
        x = x + Int10Class(1)
    }
}

// Measure Int10Struct
measure("struct (10 fields)") {
    var y = Int10Struct(0)
    for _ in 1...10000000 {
        y = y + Int10Struct(1)
    }
}

func measure(name: String, @noescape block: () -> ()) {
    let t0 = CACurrentMediaTime()

    block()

    let dt = CACurrentMediaTime() - t0
    print("\(name) -> \(dt)")
}

Code can be found at https://github.com/knguyen2708/StructVsClassPerformance

UPDATE (27 Mar 2018) :

As of Swift 4.0, Xcode 9.2, running Release build on iPhone 6S, iOS 11.2.6, Swift Compiler setting is -O -whole-module-optimization :

• class version took 2.06 seconds
• struct version took 4.17e-08 seconds (50,000,000 times faster)

(I no longer average multiple runs, as variances are very small, under 5%)

Note : the difference is a lot less dramatic without whole module optimization. I'd be glad if someone can point out what the flag actually does.

---

UPDATE (7 May 2016) :

As of Swift 2.2.1, Xcode 7.3, running Release build on iPhone 6S, iOS 9.3.1, averaged over 5 runs, Swift Compiler setting is -O -whole-module-optimization :

• class version took 2.159942142s
• struct version took 5.83E-08s (37,000,000 times faster)

Note : as someone mentioned that in real-world scenarios, there will be likely more than 1 field in a struct, I have added tests for structs/classes with 10 fields instead of 1. Surprisingly, results don't vary much.

---

ORIGINAL RESULTS (1 June 2014):

(Ran on struct/class with 1 field, not 10)

As of Swift 1.2, Xcode 6.3.2, running Release build on iPhone 5S, iOS 8.3, averaged over 5 runs

• class version took 9.788332333s
• struct version took 0.010532942s (900 times faster)

---

OLD RESULTS (from unknown time)

(Ran on struct/class with 1 field, not 10)

With release build on my MacBook Pro:

• The class version took 1.10082 sec
• The struct version took 0.02324 sec (50 times faster)

Answer 3

Similarities between structs and classes.

I created gist for this with simple examples. https://github.com/objc-swift/swift-classes-vs-structures

And differences

1. Inheritance.

structures can't inherit in swift. If you want

class Vehicle{
}

class Car : Vehicle{
}

Go for an class.

2. Pass By

Swift structures pass by value and class instances pass by reference.

Contextual Differences

Struct constant and variables

Example (Used at WWDC 2014)

struct Point{
 
   var x = 0.0;
   var y = 0.0;

} 

Defines a struct called Point.

var point = Point(x:0.0,y:2.0)

Now if I try to change the x. Its a valid expression.

point.x = 5

But if I defined a point as constant.

let point = Point(x:0.0,y:2.0)
point.x = 5 //This will give compile time error.

In this case entire point is immutable constant.

If I used a class Point instead this is a valid expression. Because in a class immutable constant is the reference to the class itself not its instance variables (Unless those variables defined as constants)

Answer 4

Assuming that we know Struct is a value type and Class is a reference type .

_{If you don't know what a value type and a reference type are then see What's the difference between passing by reference vs. passing by value?}

Based on mikeash's post :

... Let's look at some extreme, obvious examples first. Integers are obviously copyable. They should be value types. Network sockets can't be sensibly copied. They should be reference types. Points, as in x, y pairs, are copyable. They should be value types. A controller that represents a disk can't be sensibly copied. That should be a reference type.

Some types can be copied but it may not be something you want to happen all the time. This suggests that they should be reference types. For example, a button on the screen can conceptually be copied. The copy will not be quite identical to the original. A click on the copy will not activate the original. The copy will not occupy the same location on the screen. If you pass the button around or put it into a new variable you'll probably want to refer to the original button, and you'd only want to make a copy when it's explicitly requested. That means that your button type should be a reference type.

View and window controllers are a similar example. They might be copyable, conceivably, but it's almost never what you'd want to do. They should be reference types.

What about model types? You might have a User type representing a user on your system, or a Crime type representing an action taken by a User. These are pretty copyable, so they should probably be value types. However, you probably want updates to a User's Crime made in one place in your program to be visible to other parts of the program. This suggests that your Users should be managed by some sort of user controller which would be a reference type . eg

struct User {} class UserController { var users: [User] func add(user: User) { ... } func remove(userNamed: String) { ... } func ... }

Collections are an interesting case. These include things like arrays and dictionaries, as well as strings. Are they copyable? Obviously. Is copying something you want to happen easily and often? That's less clear.

Most languages say "no" to this and make their collections reference types. This is true in Objective-C and Java and Python and JavaScript and almost every other language I can think of. (One major exception is C++ with STL collection types, but C++ is the raving lunatic of the language world which does everything strangely.)

Swift said "yes," which means that types like Array and Dictionary and String are structs rather than classes. They get copied on assignment, and on passing them as parameters. This is an entirely sensible choice as long as the copy is cheap, which Swift tries very hard to accomplish. ...

_{I personally don't name my classes like that. I usually name mine UserManager instead of UserController but the idea is the same}

In addition don't use class when you have to override each and every instance of a function ie them not having any shared functionality.

So instead of having several subclasses of a class. Use several structs that conform to a protocol.

---

Another reasonable case for structs is when you want to do a delta/diff of your old and new model. With references types you can't do that out of the box. With value types the mutations are not shared.

Answer 5

Here are some other reasons to consider:

1. structs get an automatic initializer that you don't have to maintain in code at all.

    struct MorphProperty { var type : MorphPropertyValueType var key : String var value : AnyObject enum MorphPropertyValueType { case String, Int, Double } } var m = MorphProperty(type: .Int, key: "what", value: "blah")

To get this in a class, you would have to add the initializer, and maintain the intializer...

2. Basic collection types like Array are structs. The more you use them in your own code, the more you will get used to passing by value as opposed to reference. For instance:

    func removeLast(var array:[String]) { array.removeLast() println(array) // [one, two] } var someArray = ["one", "two", "three"] removeLast(someArray) println(someArray) // [one, two, three]

3. Apparently immutability vs. mutability is a huge topic, but a lot of smart folks think immutability -- structs in this case -- is preferable. Mutable vs immutable objects

Answer 6

Some advantages:

• automatically threadsafe due to not being shareable
• uses less memory due to no isa and refcount (and in fact is stack allocated generally)
• methods are always statically dispatched, so can be inlined (though @final can do this for classes)
• easier to reason about (no need to "defensively copy" as is typical with NSArray, NSString, etc...) for the same reason as thread safety

Answer 7

Structure is much more faster than Class. Also, if you need inheritance then you must use Class. Most important point is that Class is reference type whereas Structure is value type. for example,

class Flight {
    var id:Int?
    var description:String?
    var destination:String?
    var airlines:String?
    init(){
        id = 100
        description = "first ever flight of Virgin Airlines"
        destination = "london"
        airlines = "Virgin Airlines"
    } 
}

struct Flight2 {
    var id:Int
    var description:String
    var destination:String
    var airlines:String  
}

now lets create instance of both.

var flightA = Flight()

var flightB = Flight2.init(id: 100, description:"first ever flight of Virgin Airlines", destination:"london" , airlines:"Virgin Airlines" )

now lets pass these instance to two functions which modify the id, description, destination etc..

func modifyFlight(flight:Flight) -> Void {
    flight.id = 200
    flight.description = "second flight of Virgin Airlines"
    flight.destination = "new york"
    flight.airlines = "Virgin Airlines"
}

also,

func modifyFlight2(flight2: Flight2) -> Void {
    var passedFlight = flight2
    passedFlight.id = 200
    passedFlight.description = "second flight from virgin airlines" 
}

so,

modifyFlight(flight: flightA)
modifyFlight2(flight2: flightB)

now if we print the flightA's id and description, we get

id = 200
description = "second flight of Virgin Airlines"

Here, we can see the id and description of FlightA is changed because the parameter passed to the modify method actually points to the memory address of flightA object(reference type).

now if we print the id and description of FLightB instance we get,

id = 100
description = "first ever flight of Virgin Airlines"

Here we can see that the FlightB instance is not changed because in modifyFlight2 method, actual instance of Flight2 is passes rather than reference ( value type).

Answer 8

Structs are value type and Classes are reference type

• Value types are faster than Reference types
• Value type instances are safe in a multi-threaded environment as multiple threads can mutate the instance without having to worry about the race conditions or deadlocks
• Value type has no references unlike reference type; therefore there is no memory leaks.

Use a value type when:

• You want copies to have independent state, the data will be used in code across multiple threads

Use a reference type when:

• You want to create shared, mutable state.

Further information could be also found in the Apple documentation

https://docs.swift.org/swift-book/LanguageGuide/ClassesAndStructures.html

---

Additional Information

Swift value types are kept in the stack. In a process, each thread has its own stack space, so no other thread will be able to access your value type directly. Hence no race conditions, locks, deadlocks or any related thread synchronization complexity.

Value types do not need dynamic memory allocation or reference counting, both of which are expensive operations. At the same time methods on value types are dispatched statically. These create a huge advantage in favor of value types in terms of performance.

As a reminder here is a list of Swift

Value types:

• Struct
• Enum
• Tuple
• Primitives (Int, Double, Bool etc.)
• Collections (Array, String, Dictionary, Set)

Reference types:

• Class
• Anything coming from NSObject
• Function
• Closure

Answer 9

Answering the question from the perspective of value types vs reference types, from this Apple blog post it would appear very simple:

Use a value type [eg struct, enum] when:

• Comparing instance data with == makes sense
• You want copies to have independent state
• The data will be used in code across multiple threads

Use a reference type [eg class] when:

• Comparing instance identity with === makes sense
• You want to create shared, mutable state

As mentioned in that article, a class with no writeable properties will behave identically with a struct, with (I will add) one caveat: structs are best for thread-safe models -- an increasingly imminent requirement in modern app architecture.

Answer 10

With classes you get inheritance and are passed by reference, structs do not have inheritance and are passed by value.

There are great WWDC sessions on Swift, this specific question is answered in close detail in one of them. Make sure you watch those, as it will get you up to speed much more quickly then the Language guide or the iBook.

Answer 11

I wouldn't say that structs offer less functionality.

Sure, self is immutable except in a mutating function, but that's about it.

Inheritance works fine as long as you stick to the good old idea that every class should be either abstract or final.

Implement abstract classes as protocols and final classes as structs.

The nice thing about structs is that you can make your fields mutable without creating shared mutable state because copy on write takes care of that :)

That's why the properties / fields in the following example are all mutable, which I would not do in Java or C# or swift classes .

Example inheritance structure with a bit of dirty and straightforward usage at the bottom in the function named "example":

protocol EventVisitor
{
    func visit(event: TimeEvent)
    func visit(event: StatusEvent)
}

protocol Event
{
    var ts: Int64 { get set }

    func accept(visitor: EventVisitor)
}

struct TimeEvent : Event
{
    var ts: Int64
    var time: Int64

    func accept(visitor: EventVisitor)
    {
        visitor.visit(self)
    }
}

protocol StatusEventVisitor
{
    func visit(event: StatusLostStatusEvent)
    func visit(event: StatusChangedStatusEvent)
}

protocol StatusEvent : Event
{
    var deviceId: Int64 { get set }

    func accept(visitor: StatusEventVisitor)
}

struct StatusLostStatusEvent : StatusEvent
{
    var ts: Int64
    var deviceId: Int64
    var reason: String

    func accept(visitor: EventVisitor)
    {
        visitor.visit(self)
    }

    func accept(visitor: StatusEventVisitor)
    {
        visitor.visit(self)
    }
}

struct StatusChangedStatusEvent : StatusEvent
{
    var ts: Int64
    var deviceId: Int64
    var newStatus: UInt32
    var oldStatus: UInt32

    func accept(visitor: EventVisitor)
    {
        visitor.visit(self)
    }

    func accept(visitor: StatusEventVisitor)
    {
        visitor.visit(self)
    }
}

func readEvent(fd: Int) -> Event
{
    return TimeEvent(ts: 123, time: 56789)
}

func example()
{
    class Visitor : EventVisitor
    {
        var status: UInt32 = 3;

        func visit(event: TimeEvent)
        {
            print("A time event: \(event)")
        }

        func visit(event: StatusEvent)
        {
            print("A status event: \(event)")

            if let change = event as? StatusChangedStatusEvent
            {
                status = change.newStatus
            }
        }
    }

    let visitor = Visitor()

    readEvent(1).accept(visitor)

    print("status: \(visitor.status)")
}

Answer 12

In Swift, a new programming pattern has been introduced known as Protocol Oriented Programming.

Creational Pattern:

In swift, Struct is a value types which are automatically cloned. Therefore we get the required behavior to implement the prototype pattern for free.

Whereas classes are the reference type, which is not automatically cloned during the assignment. To implement the prototype pattern, classes must adopt the NSCopying protocol.

---

Shallow copy duplicates only the reference, that points to those objects whereas deep copy duplicates object's reference.

---

Implementing deep copy for each reference type has become a tedious task. If classes include further reference type, we have to implement prototype pattern for each of the references properties. And then we have to actually copy the entire object graph by implementing the NSCopying protocol.

class Contact{
  var firstName:String
  var lastName:String
  var workAddress:Address // Reference type
}

class Address{
   var street:String
   ...
} 

By using structs and enums , we made our code simpler since we don't have to implement the copy logic.

Answer 13

Struct vs Class

[Stack vs Heap]
[Value vs Reference type]

Struct is more preferable . But Struct does not solve all issues by default. Usually you can hear that value type is allocated on stack, but it is not always true. Only local variables are allocated on stack

//simple blocks
struct ValueType {}
class ReferenceType {}

struct StructWithRef {
    let ref1 = ReferenceType()
}

class ClassWithRef {
    let ref1 = ReferenceType()
}

func foo() {
    
    //simple  blocks
    let valueType1 = ValueType()
    let refType1 = ReferenceType()
    
    //RetainCount
    //StructWithRef
    let structWithRef1 = StructWithRef()
    let structWithRef1Copy = structWithRef1
    
    print("original:", CFGetRetainCount(structWithRef1 as CFTypeRef)) //1
    print("ref1:", CFGetRetainCount(structWithRef1.ref1)) //2 (originally 3)
    
    //ClassWithRef
    let classWithRef1 = ClassWithRef()
    let classWithRef1Copy = classWithRef1
    
    print("original:", CFGetRetainCount(classWithRef1)) //2 (originally 3)
    print("ref1:", CFGetRetainCount(classWithRef1.ref1)) //1 (originally 2)
     
}

*You should not use/rely on retainCount , because it does not say useful information

During compiling SIL(Swift Intermediate Language) can optimize you code

swiftc -emit-silgen -<optimization> <file_name>.swift
//e.g.
swiftc -emit-silgen -Onone file.swift

//emit-silgen -> emit-sil(is used in any case)
//-emit-silgen           Emit raw SIL file(s)
//-emit-sil              Emit canonical SIL file(s)
//optimization: O, Osize, Onone. It is the same as Swift Compiler - Code Generation -> Optimization Level

There you can find alloc_stack (allocation on stack) andalloc_box (allocation on heap)

Answer 14

Many Cocoa APIs require NSObject subclasses, which forces you into using class. But other than that, you can use the following cases from Apple's Swift blog to decide whether to use a struct / enum value type or a class reference type.

https://developer.apple.com/swift/blog/?id=10

Answer 15

One point not getting attention in these answers is that a variable holding a class vs a struct can be a let while still allowing changes on the object's properties, while you cannot do this with a struct.

This is useful if you don't want the variable to ever point to another object, but still need to modify the object, ie in the case of having many instance variables that you wish to update one after another. If it is a struct, you must allow the variable to be reset to another object altogether using var in order to do this, since a constant value type in Swift properly allows zero mutation, while reference types (classes) don't behave this way.

Answer 16

As struct are value types and you can create the memory very easily which stores into stack.Struct can be easily accessible and after the scope of the work it's easily deallocated from the stack memory through pop from the top of the stack. On the other hand class is a reference type which stores in heap and changes made in one class object will impact to other object as they are tightly coupled and reference type.All members of a structure are public whereas all the members of a class are private.

The disadvantages of struct is that it can't be inherited .

Answer 17

• Structure and class are user defied data types

• By default, structure is a public whereas class is private

• Class implements the principal of encapsulation

• Objects of a class are created on the heap memory

• Class is used for re usability whereas structure is used for grouping the data in the same structure

• Structure data members cannot be initialized directly but they can be assigned by the outside the structure

• Class data members can be initialized directly by the parameter less constructor and assigned by the parameterized constructor