Difference between and

Question

In swift there seem to be two equality operators  the double equals      and the triple equals        what is the difference between the two

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There are subtleties with Swifts     that go beyond mere pointer arithmetics  While in Objective-C you were able to compare any two pointers  i e  NSObject    with    this is no longer true in Swift since types play a much greater role during compilation   A Playground will give you  1     2                       false 1     1                       true let one   1                   1 1     one                     compile error  Type  Int  does not conform to protocol  AnyObject  1      one as AnyObject       true  surprisingly  to me at least     With strings we will have to get used to this   var st    123                                      123  var ns    st as NSString                           123  st    ns                                          true  content equality st     ns                                         compile error ns      st as NSString                            false  new struct ns      st as AnyObject                           false  new struct  st as NSString       st as NSString              false  new structs  bridging is not  free   as in  lunch   NSString string st      NSString string st        false  new structs var st1   NSString string st                       123  var st2   st1                                      123  st1     st2                                       true var st3    st as NSString                          123  st1     st3                                       false  st as AnyObject       st as AnyObject            false   but then you can also have fun as follows   var st4   st                 123  st4    st                   true st4     5                    1235  st4    st                   false  not quite a reference  copy on write semantics   I am sure you can think of a lot more funny cases  -   Update for Swift 3  as suggested by the comment from Jakub Truhl  r   1   2                                       Compiler error  binary operator       cannot be applied to two  Int  operands  1 as AnyObject       2 as AnyObject        false let two   2  2 as AnyObject       two as AnyObject      false  rather unpleasant   2 as AnyObject       2 as AnyObject        false  this makes it clear that there are new objects being generated    This looks a little more consistent with Type  Int  does not conform to protocol  AnyObject   however we then get  type of  1 as AnyObject                      SwiftTypePreservingNSNumber Type   but the explicit conversion makes clear that there might be something going on  On the String-side of things NSString will still be available as long as we import Cocoa  Then we will have  var st    123                                      123  var ns    st as NSString                           123  st    ns                                          Compile error with Fixit   NSString  is not implicitly convertible to  String   did you mean to use  as  to explicitly convert  st    ns as String                                true  content equality st     ns                                         compile error  binary operator       cannot be applied to operands of type  String  and  NSString  ns      st as NSString                            false  new struct ns      st as AnyObject                           false  new struct  st as NSString       st as NSString              false  new structs  bridging is not  free   as in  lunch   NSString string st      NSString string st        false  new objects var st1   NSString string st                       123  var st2   st1                                      123  st1     st2                                       true var st3    st as NSString                          123  st1     st3                                       false  st as AnyObject       st as AnyObject            false   It is still confusing to have two String classes  but dropping the implicit conversion will probably make it a little more palpable

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In swift 3 and above      or       Checks if the values are identical  both point to the same memory address   Comparing reference types  Like    in Obj-C  pointer equality        or      Checks if the values are the same  Comparing value types  Like the default isEqual  in Obj-C behavior   Here I compare three instances  class is a reference type  class Person     let person   Person   let person2   person let person3   Person    person     person2    true person     person3    false

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Swift 4  Another example using Unit Tests which only works with      Note  Test below fails with     works with      func test inputTextFields Delegate is ViewControllerUnderTest                instantiate viewControllerUnderTest from Main storyboard         let storyboard   UIStoryboard name   Main   bundle  nil          viewControllerUnderTest   storyboard instantiateViewController withIdentifier   StoryBoardIdentifier   as  ViewControllerUnderTest          let     viewControllerUnderTest view          XCTAssertTrue viewControllerUnderTest inputTextField delegate     viewControllerUnderTest           And the class being  class ViewControllerUnderTest  UIViewController  UITextFieldDelegate        IBOutlet weak var inputTextField  UITextField       override func viewDidLoad             super viewDidLoad           inputTextField delegate   self           The error in Unit Tests if you use    is  Binary operator      cannot be applied to operands of type  UITextFieldDelegate   and  ViewControllerUnderTest

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is used to check if two variables are equal i e  2    2  But in case of     it stands for equality i e if two instances referring  to the same object example in case of classes a reference is created which is held by many other instances

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and     are identity operators and are used to determine if two objects have the same reference      Swift also provides two identity operators      and       which you use to test whether two object references both refer to the same object instance    Excerpt From  Apple Inc     The Swift Programming Language     iBooks  https   itun es us jEUH0 l

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gt  gt  is arithmetic shift right   gt  gt  gt  is logical shift right   In an arithmetic shift  the sign bit is extended to preserve the signedness of the number     For example  -2 represented in 8 bits would be 11111110  because the most significant bit has negative weight    Shifting it right one bit using arithmetic shift would give you 11111111  or -1   Logical right shift  however  does not care that the value could possibly represent a signed number  it simply moves everything to the right and fills in from the left with 0s   Shifting our -2 right one bit using logical shift would give 01111111

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In short      operator checks if their instance values are equal   equal to       operator checks if the references point the same instance   identical to   Long Answer   Classes are reference types  it is possible for multiple constants and variables to refer to the same single instance of a class behind the scenes  Class references stay in Run Time Stack  RTS  and their instances stay in Heap area of Memory  When you control equality with    it means if their instances are equal to each other  It doesn t need to be same instance to be equal  For this you need to provide a equality criteria to your custom class  By default  custom classes and structures do not receive a default implementation of the equivalence operators  known as the    equal to    operator    and    not equal to    operator      To do this your custom class needs to conform Equatable protocol and it s static func     lhs   rhs   - gt  Bool function  Let s look at example   class Person   Equatable       let ssn  Int     let name  String      init ssn  Int  name  String            self ssn   ssn         self name   name            static func     lhs  Person  rhs  Person  - gt  Bool           return lhs ssn    rhs ssn           P S   Since ssn social security number  is a unique number  you don t need to compare if their name are equal or not   let person1   Person ssn  5  name   Bob   let person2   Person ssn  5  name   Bob    if person1    person2      print  the two instances are equal        Although person1 and person2 references point two different instances in Heap area  their instances are equal because their ssn numbers are equal  So the output will be the two instance are equal   if person1     person2        It does not enter here   else      print  the two instances are not identical            operator checks if the references point the same instance   identical to   Since person1 and person2 have two different instance in Heap area  they are not identical and the output the two instance are not identical   let person3   person1   P S  Classes are reference types and person1 s reference is copied to person3 with this assignment operation  thus both references point the same instance in Heap area   if person3     person1      print  the two instances are identical        They are identical and the output will be the two instances are identical

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gt  gt  gt  will always put a 0 in the left most bit  while  gt  gt  will put a 1 or a 0 depending on what the sign of it is

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Read more about Bitwise and Bit Shift Operators   gt  gt       Signed right shift  gt  gt  gt      Unsigned right shift   The bit pattern is given by the left-hand operand  and the number of positions to shift by the right-hand operand  The unsigned right shift operator  gt  gt  gt  shifts a zero into the leftmost position    while the leftmost position after  gt  gt  depends on sign extension   In simple words  gt  gt  gt  always shifts a zero into the leftmost position whereas  gt  gt  shifts based on sign of the number i e  1 for negative number and 0 for positive number     For example try with negative as well as positive numbers   int c   -153  System out printf   32s n  Integer toBinaryString c  gt  gt   2    System out printf   32s n  Integer toBinaryString c  lt  lt   2    System out printf   32s n  Integer toBinaryString c  gt  gt  gt   2    System out println Integer toBinaryString c  lt  lt   2     System out println     c   153  System out printf   32s n  Integer toBinaryString c  gt  gt   2    System out printf   32s n  Integer toBinaryString c  lt  lt   2    System out printf   32s n  Integer toBinaryString c  gt  gt  gt   2    System out printf   32s n  Integer toBinaryString c  lt  lt   2      output   11111111111111111111111111011001 11111111111111111111111101100100   111111111111111111111111011001 11111111111111111111111101100100                            100110                         10011000                           100110                         10011000

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Just a minor contribution related to the Any object   I was working with unit tests around NotificationCenter  which makes use of Any as a parameter that I wanted to compare for equality   However  since Any cannot be used in an equality operation  it was necessary to change it  Ultimately  I settled on the following approach  which allowed me to get equality in my specific situation  shown here with a simplistic example   func compareTwoAny a  Any  b  Any  - gt  Bool       return ObjectIdentifier a as AnyObject     ObjectIdentifier b as AnyObject      This function takes advantage of ObjectIdentifier  which provides a unique address for the object  allowing me to test   One item to note though about ObjectIdentifier per Apple at the above link      In Swift  only class instances and metatypes have unique identities    There is no notion of identity for structs  enums  functions  or   tuples

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The right shift logical operator   gt  gt  gt  N  shifts bits to the right by N positions  discarding the sign bit and padding the N left-most bits with 0 s  For example   -1  in 32-bit   11111111111111111111111111111111   after a  gt  gt  gt  1 operation becomes   2147483647  01111111111111111111111111111111   The right shift arithmetic operator   gt  gt  N  also shifts bits to the right by N positions  but preserves the sign bit and pads the N left-most bits with 1 s   For example   -2  in 32-bit   11111111111111111111111111111110   after a  gt  gt  1 operation becomes   -1  11111111111111111111111111111111

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For example  if you create two instances of a class e g  myClass   var inst1   myClass   var inst2   myClass     you can compare those instances    if inst1     inst2   cited      which you use to test whether two object references both refer to the   same object instance    Excerpt From  Apple Inc     The Swift Programming Language     iBooks  https   itun es sk jEUH0 l

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In both Objective-C and Swift  the    and    operators test for value equality for number values  e g   NSInteger  NSUInteger  int  in Objective-C and Int  UInt  etc  in Swift   For objects  NSObject NSNumber and subclasses in Objective-C and reference types in Swift      and    test that the objects reference types are the same identical thing -- i e   same hash value -- or are not the same identical thing  respectively   let a   NSObject   let b   NSObject   let c   a a    b    false a    c    true   Swift s identity equality operators      and      check referential equality -- and thus  should probably be called the referential equality operators IMO   a     b    false a     c    true   It s also worth pointing out that custom reference types in Swift  that do not subclass a class that conforms to Equatable  do not automatically implement the equal to operators  but the identity equality operators still apply  Also  by implementing        is automatically implemented   class MyClass  Equatable     let myProperty  String    init s  String        myProperty   s        func    lhs  MyClass  rhs  MyClass  - gt  Bool     return lhs myProperty    rhs myProperty    let myClass1   MyClass s   Hello   let myClass2   MyClass s   Hello   myClass1    myClass2    true myClass1    myClass2    false myClass1     myClass2    false myClass1     myClass2    true   These equality operators are not implemented for other types such as structures in either language  However  custom operators can be created in Swift  which would  for example  enable you to create an operator to check equality of a CGPoint   infix operator  lt    gt    precedence 130   func  lt    gt   lhs  CGPoint  rhs  CGPoint  - gt  Bool     return lhs x    rhs x  amp  amp  lhs y    rhs y    let point1   CGPoint x  1 0  y  1 0  let point2   CGPoint x  1 0  y  1 0  point1  lt    gt  point2    true

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In Swift we have     simbol which means is both objects are referring to the same reference same address  class SomeClass   var a  Int   init   a  Int        self a   a       var someClass1   SomeClass 4  var someClass2   SomeClass 4  someClass1     someClass2    false someClass2   someClass1 someClass1     someClass2    true

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The logical right shift  v  gt  gt  gt  n  returns a value in which the bits in v have been shifted to the right by n bit positions  and 0 s are shifted in from the left side  Consider shifting 8-bit values  written in binary   01111111  gt  gt  gt  2   00011111 10000000  gt  gt  gt  2   00100000   If we interpret the bits as an unsigned nonnegative integer  the logical right shift has the effect of dividing the number by the corresponding power of 2  However  if the number is in two s-complement representation  logical right shift does not correctly divide negative numbers  For example  the second right shift above shifts 128 to 32 when the bits are interpreted as unsigned numbers  But it shifts -128 to 32 when  as is typical in Java  the bits are interpreted in two s complement   Therefore  if you are shifting in order to divide by a power of two  you want the arithmetic right shift  v  gt  gt  n   It returns a value in which the bits in v have been shifted to the right by n bit positions  and copies of the leftmost bit of v are shifted in from the left side   01111111  gt  gt  2   00011111 10000000  gt  gt  2   11100000   When the bits are a number in two s-complement representation  arithmetic right shift has the effect of dividing by a power of two  This works because the leftmost bit is the sign bit  Dividing by a power of two must keep the sign the same

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gt  gt  gt  is unsigned-shift  it ll insert 0   gt  gt  is signed  and will extend the sign bit   JLS 15 19 Shift Operators     The shift operators include left shift  lt  lt   signed right shift  gt  gt   and unsigned right shift  gt  gt  gt        The value of n gt  gt s is n right-shifted s bit positions with sign-extension       The value of n gt  gt  gt s is n right-shifted s bit positions with zero-extension         System out println Integer toBinaryString -1           prints  11111111111111111111111111111111      System out println Integer toBinaryString -1  gt  gt  16           prints  11111111111111111111111111111111      System out println Integer toBinaryString -1  gt  gt  gt  16           prints  1111111111111111    To make things more clear adding positive counterpart  System out println Integer toBinaryString 121       prints  1111001  System out println Integer toBinaryString 121  gt  gt  1       prints  111100  System out println Integer toBinaryString 121  gt  gt  gt  1       prints  111100    Since it is positive both signed and unsigned shifts will add 0 to left most bit   Related questions   Right Shift to Perform Divide by 2 On -1 Is shifting bits faster than multiplying and dividing in Java   NET  what is c c   equivalent way of doing           as in java  unsigned right shift  Negative logical shift Java   s    versus     Operator  What is the difference between the Java operators    and      Difference between     and    operators What   s the reason high-level languages like C  Java mask the bit shift count operand     1  gt  gt  gt  32    1

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They are both right-shift  but  gt  gt  gt  is unsigned  From the documentation      The unsigned right shift operator       shifts a zero into the leftmost position  while the leftmost position after      depends on sign extension

[swift] Difference between == and ===

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