Why Choose Struct Over Class

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

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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

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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  e g   Window    Instance lifetime is tied to external effects  e g   TemporaryFile    Instances are just  sinks --write-only conduits to external state  e g CGContext       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

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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  e g  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

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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

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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

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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

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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

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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  - gt  Void       flight id   200     flight description    second flight of Virgin Airlines      flight destination    new york      flight airlines    Virgin Airlines      also   func modifyFlight2 flight2  Flight2  - gt  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

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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  e g  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  e g  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

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Here are some other reasons to consider    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       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   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

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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

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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  - gt  Int10Class       return IntClass x value   y value     func    x  Int10Struct  y  Int10Struct  - gt  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     - gt            let t0   CACurrentMediaTime        block        let dt   CACurrentMediaTime   - t0     print    name  - gt    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

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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  i e  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

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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  - gt  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

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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

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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

[swift] Why Choose Struct Over Class?

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