What exactly does Perl s bless do

Question

I understand one uses the  bless  keyword in Perl inside a class s  new  method   sub new       my  self   bless          return  self          But what exactly is  bless  doing to that hash reference

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Short version  it s marking that hash as attached to the current package namespace  so that that package provides its class implementation

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I ll provide an answer here since the ones here didn t quite click for me   Perl s bless function associates any reference to all functions inside a package   Why would we need this   Let s begin by expressing an example in JavaScript         gt         use strict        class Animal           constructor args                this name   args name              this sound   args sound                           WRONG   global scope corruption         var animal   Animal               name    Jeff               sound    bark                     console log animal name          animal sound      seems good        console log window name      my window s name is Jeff                  new is important      var animal   new Animal           name    Jeff               sound    bark              console log animal name          animal sound      still fine      console log window name      undefined         Now lets strip away the class construct and make do without it         gt         use strict        var Animal   function args            this name   args name          this sound   args sound          return this     implicit context hashmap                the  new  causes the Animal to be unbound from global context  and         rebinds it to an empty hash map before being constructed  The state is        now bound to animal  not the global scope      var animal   new Animal            name    Jeff            sound    bark              console log animal sound               The function takes a hash table of unordered properties since it makes no sense to have to write properties in a specific order in dynamic languages in 2016  and returns a hash table with those properties  or if you forgot to put the new keyword  it will return the whole global context eg window in browser or global in nodejs    Perl has no  this  nor  new  nor  class   but it can still have a function that behaves similarly  We won t have a constructor nor a prototype  but we will be able to create new animals at will and modify their individual properties     self contained scope   sub       my  Animal    sub           return                name    gt     0   name                 sound    gt     0   sound                           my  animal    Animal- gt             name    gt   Jeff            sound    gt   bark               print  animal- gt  sound     - gt       Now  we have a problem  What if we want the animal to perform the sounds by themselves instead of us printing what their voice is  That is  we want a function performSound that prints the animal s own sound   One way to do this is by teaching each individual Animal how to do it s sound  This means that each Cat has its own duplicate function to performSound     self contained scope   sub       my  Animal    sub            name      0   name             sound      0   sound             return                name    gt   name               sound    gt   sound               performSound    gt  sub                   print  sound     n                                         my  animal    Animal- gt             name    gt   Jeff            sound    gt   bark                animal- gt   performSound        - gt       This is bad because performSound is put as a completely new function object each time an animal is constructed  10000 animals means 10000 performSounds  We want to have a single function performSound that is used by all animals that looks up their own sound and prints it         gt         use strict           a function that creates an Animal constructor which can be used to create animals        var Animal         gt               function is important  as fat arrow does not have  this  and will not be bound to Animal             var InnerAnimal   function args                this name   args name              this sound   args sound                        defined once and all animals use the same single function call            InnerAnimal prototype performSound   function                 console log this name                       return InnerAnimal                    we re gonna create an animal with arguments in different order        because we want to be edgy         var animal   new Animal            sound    bark            name    Jeff              animal performSound       Jeff         Here is where the parallel to Perl kinda stops   JavaScript s new operator is not optional  without it   this  inside object methods corrupts global scope         gt            use strict      uncommenting this prevents corruption and raises an error instead       var Person   function             this name    Sam                var wrong   Person       oops  we have overwritten window name or global main        console log window name      my window s name is Sam      var correct   new Person     person s name is actually stored in the person now           We want to have one function for each Animal that looks up that animal s own sound rather than hardcoding it at construction   Blessing lets us use a package as the prototype of objects  This way  the object is aware of the  package  it is  referenced to   and in turn can have the functions in the package  reach into  the specific instances that were created from the constructor of that  package object    package Animal  sub new       my  packageRef      0       my  name      1 - gt   name        my  sound      1 - gt   sound         my  this              name    gt   name           sound    gt   sound                bless  this   packageRef       return  this       all animals use the same performSound to look up their sound  sub performSound       my  this   shift      my  sound    this- gt   sound        print  sound     n      package main  my  animal   Animal- gt new        name    gt   Cat        sound    gt   meow       animal- gt performSound      Summary TL DR   Perl has no  this    class   nor  new   blessing an object to a package gives that object a reference to the package  and when it calls functions in the package  their arguments will be offset by 1 slot  and the first argument    0  or shift  will be equivalent to javascript s  this   In turn  you can somewhat simulate JavaScript s prototype model    Unfortunately it makes it impossible to my understanding  to create  new classes  at runtime  as you need each  class  to have its own package  whereas in javascript  you don t need packages at all  as  new  keyword makes up an anonymous hashmap for you to use as a package at runtime to which you can add new functions and remove functions on the fly    There are some Perl libraries creating their own ways of bridging this limitation in expressiveness  such as Moose   Why the confusion    Because of packages  Our intuition tells us to bind the object to a hashmap containing its  prototype  This lets us create  packages  at runtime like JavaScript can  Perl does not have such flexibility at least not built in  you have to invent it or get it from other modules   and in turn your runtime expressiveness is hindered  Calling it  bless  doesn t do it much favors neither   What we want to do   Something like this  but have binding to the prototype map recursive  and be implicitly bound to the prototype rather than having to explicitly do it    Here is a naive attempt at it  the issue is that  call  does not know  what called it   so it may as well be a universal perl function  objectInvokeMethod object  method   which checks whether the object has the method  or its prototype has it  or its prototype has it  until it reaches the end and finds it or not  prototypical inheritence   Perl has nice eval magic to do it but I ll leave that for something I can try doing later   Anyway here is the idea    sub        my  Animal    sub           my  AnimalPrototype                  performSound    gt  sub                   return    0 - gt   sound                                      my  call   sub               my  this      0               my  proc      1                if  exists  this- gt   proc                     return  this- gt   proc - gt                   else                   return  this- gt  prototype - gt   proc - gt   this   proc                                     return sub               my  name      0 - gt  name               my  sound      0 - gt  sound                my  this                       this    gt   this                   name    gt   name                   sound    gt   sound                   prototype    gt   AnimalPrototype                   call    gt   call                                                 - gt          my  animal    Animal- gt             name    gt   Jeff            sound   gt   bark              print  animal- gt  call   animal   performSound       - gt       Anyway hopefully somebody will find this post useful

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bless associates a reference with a package    It doesn t matter what the reference is to  it can be to a hash  most common case   to an array  not so common   to a scalar  usually this indicates an inside-out object   to a regular expression  subroutine or TYPEGLOB  see the book Object Oriented Perl  A Comprehensive Guide to Concepts and Programming Techniques by Damian Conway for useful examples  or even a reference to a file or directory handle  least common case    The effect bless-ing has is that it allows you to apply special syntax to the blessed reference    For example  if a blessed reference is stored in  obj  associated by bless with package  Class    then  obj- gt foo  args  will call a subroutine foo and pass as first argument the reference  obj followed by the rest of the arguments   args   The subroutine should be defined in package  Class   If there is no subroutine foo in package  Class   a list of other packages  taken form the array  ISA in the package  Class   will be searched and the first subroutine foo found will be called

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Along with a number of good answers  what specifically distinguishes a bless-ed reference is that the SV  for it picks up an additional FLAGS  OBJECT  and a STASH  perl -MDevel  Peek -wE      package Pack    sub func   return   a  gt 1             package Class   sub new    return bless   A  gt 10              vp    Pack  func    print Dump  vp    say ---         obj   Class- gt new    print Dump  obj    Prints  with the same  and irrelevant for this  parts suppressed   SV   IV 0x12d5530  at 0x12d5540   REFCNT   1   FLAGS    ROK    RV   0x12a5a68   SV   PVHV 0x12ab980  at 0x12a5a68     REFCNT   1     FLAGS    SHAREKEYS                SV   IV 0x12a5ce0  at 0x12a5cf0       REFCNT   1       FLAGS    IOK pIOK        IV   1 --- SV   IV 0x12cb8b8  at 0x12cb8c8   REFCNT   1   FLAGS    PADMY ROK    RV   0x12c26b0   SV   PVHV 0x12aba00  at 0x12c26b0     REFCNT   1     FLAGS    OBJECT SHAREKEYS      STASH   0x12d5300    Class                SV   IV 0x12c26b8  at 0x12c26c8       REFCNT   1       FLAGS    IOK pIOK        IV   10   With that it s known that 1  it is an object 2  what package it belongs to  and this informs its use    For example  when dereferencing on that variable is encountered   obj- gt name   a sub with that name is sought in the package  or hierarchy   the object is passed as the first argument  etc

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bless associates a reference with a package    It doesn t matter what the reference is to  it can be to a hash  most common case   to an array  not so common   to a scalar  usually this indicates an inside-out object   to a regular expression  subroutine or TYPEGLOB  see the book Object Oriented Perl  A Comprehensive Guide to Concepts and Programming Techniques by Damian Conway for useful examples  or even a reference to a file or directory handle  least common case    The effect bless-ing has is that it allows you to apply special syntax to the blessed reference    For example  if a blessed reference is stored in  obj  associated by bless with package  Class    then  obj- gt foo  args  will call a subroutine foo and pass as first argument the reference  obj followed by the rest of the arguments   args   The subroutine should be defined in package  Class   If there is no subroutine foo in package  Class   a list of other packages  taken form the array  ISA in the package  Class   will be searched and the first subroutine foo found will be called

User · Answer

Short version  it s marking that hash as attached to the current package namespace  so that that package provides its class implementation

User · Answer

Short version  it s marking that hash as attached to the current package namespace  so that that package provides its class implementation

User · Answer

bless associates a reference with a package    It doesn t matter what the reference is to  it can be to a hash  most common case   to an array  not so common   to a scalar  usually this indicates an inside-out object   to a regular expression  subroutine or TYPEGLOB  see the book Object Oriented Perl  A Comprehensive Guide to Concepts and Programming Techniques by Damian Conway for useful examples  or even a reference to a file or directory handle  least common case    The effect bless-ing has is that it allows you to apply special syntax to the blessed reference    For example  if a blessed reference is stored in  obj  associated by bless with package  Class    then  obj- gt foo  args  will call a subroutine foo and pass as first argument the reference  obj followed by the rest of the arguments   args   The subroutine should be defined in package  Class   If there is no subroutine foo in package  Class   a list of other packages  taken form the array  ISA in the package  Class   will be searched and the first subroutine foo found will be called

User · Answer

I Following this thought to guide the development object-oriented Perl   Bless associate any data structure reference with a class  Given how Perl creates the inheritance structure  in a kind of tree  it is easy to take advantage of the object model to create Objects for composition   For this association we called object  to develop always have in mind that the internal state of the object and class behaviours are separated  And you can bless allow any data reference to use any package class behaviours  Since the package can understand  the emotional  state of the object

User · Answer

Short version  it s marking that hash as attached to the current package namespace  so that that package provides its class implementation

User · Answer

Along with a number of good answers  what specifically distinguishes a bless-ed reference is that the SV  for it picks up an additional FLAGS  OBJECT  and a STASH  perl -MDevel  Peek -wE      package Pack    sub func   return   a  gt 1             package Class   sub new    return bless   A  gt 10              vp    Pack  func    print Dump  vp    say ---         obj   Class- gt new    print Dump  obj    Prints  with the same  and irrelevant for this  parts suppressed   SV   IV 0x12d5530  at 0x12d5540   REFCNT   1   FLAGS    ROK    RV   0x12a5a68   SV   PVHV 0x12ab980  at 0x12a5a68     REFCNT   1     FLAGS    SHAREKEYS                SV   IV 0x12a5ce0  at 0x12a5cf0       REFCNT   1       FLAGS    IOK pIOK        IV   1 --- SV   IV 0x12cb8b8  at 0x12cb8c8   REFCNT   1   FLAGS    PADMY ROK    RV   0x12c26b0   SV   PVHV 0x12aba00  at 0x12c26b0     REFCNT   1     FLAGS    OBJECT SHAREKEYS      STASH   0x12d5300    Class                SV   IV 0x12c26b8  at 0x12c26c8       REFCNT   1       FLAGS    IOK pIOK        IV   10   With that it s known that 1  it is an object 2  what package it belongs to  and this informs its use    For example  when dereferencing on that variable is encountered   obj- gt name   a sub with that name is sought in the package  or hierarchy   the object is passed as the first argument  etc

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For example  if you can be confident that any Bug object is going to be a blessed hash  you can  finally   fill in the missing code in the Bug  print me method    package Bug   sub print me         my   self             print  ID   self- gt  id  n        print   self- gt  descr  n        print   Note  problem is fatal  n  if  self- gt  type  eq  fatal        Now  whenever the print me method is called via a reference to any hash that s been blessed into the Bug class  the  self variable extracts the reference that was passed as the first argument and then the print statements access the various entries of the blessed hash

User · Answer

I ll provide an answer here since the ones here didn t quite click for me   Perl s bless function associates any reference to all functions inside a package   Why would we need this   Let s begin by expressing an example in JavaScript         gt         use strict        class Animal           constructor args                this name   args name              this sound   args sound                           WRONG   global scope corruption         var animal   Animal               name    Jeff               sound    bark                     console log animal name          animal sound      seems good        console log window name      my window s name is Jeff                  new is important      var animal   new Animal           name    Jeff               sound    bark              console log animal name          animal sound      still fine      console log window name      undefined         Now lets strip away the class construct and make do without it         gt         use strict        var Animal   function args            this name   args name          this sound   args sound          return this     implicit context hashmap                the  new  causes the Animal to be unbound from global context  and         rebinds it to an empty hash map before being constructed  The state is        now bound to animal  not the global scope      var animal   new Animal            name    Jeff            sound    bark              console log animal sound               The function takes a hash table of unordered properties since it makes no sense to have to write properties in a specific order in dynamic languages in 2016  and returns a hash table with those properties  or if you forgot to put the new keyword  it will return the whole global context eg window in browser or global in nodejs    Perl has no  this  nor  new  nor  class   but it can still have a function that behaves similarly  We won t have a constructor nor a prototype  but we will be able to create new animals at will and modify their individual properties     self contained scope   sub       my  Animal    sub           return                name    gt     0   name                 sound    gt     0   sound                           my  animal    Animal- gt             name    gt   Jeff            sound    gt   bark               print  animal- gt  sound     - gt       Now  we have a problem  What if we want the animal to perform the sounds by themselves instead of us printing what their voice is  That is  we want a function performSound that prints the animal s own sound   One way to do this is by teaching each individual Animal how to do it s sound  This means that each Cat has its own duplicate function to performSound     self contained scope   sub       my  Animal    sub            name      0   name             sound      0   sound             return                name    gt   name               sound    gt   sound               performSound    gt  sub                   print  sound     n                                         my  animal    Animal- gt             name    gt   Jeff            sound    gt   bark                animal- gt   performSound        - gt       This is bad because performSound is put as a completely new function object each time an animal is constructed  10000 animals means 10000 performSounds  We want to have a single function performSound that is used by all animals that looks up their own sound and prints it         gt         use strict           a function that creates an Animal constructor which can be used to create animals        var Animal         gt               function is important  as fat arrow does not have  this  and will not be bound to Animal             var InnerAnimal   function args                this name   args name              this sound   args sound                        defined once and all animals use the same single function call            InnerAnimal prototype performSound   function                 console log this name                       return InnerAnimal                    we re gonna create an animal with arguments in different order        because we want to be edgy         var animal   new Animal            sound    bark            name    Jeff              animal performSound       Jeff         Here is where the parallel to Perl kinda stops   JavaScript s new operator is not optional  without it   this  inside object methods corrupts global scope         gt            use strict      uncommenting this prevents corruption and raises an error instead       var Person   function             this name    Sam                var wrong   Person       oops  we have overwritten window name or global main        console log window name      my window s name is Sam      var correct   new Person     person s name is actually stored in the person now           We want to have one function for each Animal that looks up that animal s own sound rather than hardcoding it at construction   Blessing lets us use a package as the prototype of objects  This way  the object is aware of the  package  it is  referenced to   and in turn can have the functions in the package  reach into  the specific instances that were created from the constructor of that  package object    package Animal  sub new       my  packageRef      0       my  name      1 - gt   name        my  sound      1 - gt   sound         my  this              name    gt   name           sound    gt   sound                bless  this   packageRef       return  this       all animals use the same performSound to look up their sound  sub performSound       my  this   shift      my  sound    this- gt   sound        print  sound     n      package main  my  animal   Animal- gt new        name    gt   Cat        sound    gt   meow       animal- gt performSound      Summary TL DR   Perl has no  this    class   nor  new   blessing an object to a package gives that object a reference to the package  and when it calls functions in the package  their arguments will be offset by 1 slot  and the first argument    0  or shift  will be equivalent to javascript s  this   In turn  you can somewhat simulate JavaScript s prototype model    Unfortunately it makes it impossible to my understanding  to create  new classes  at runtime  as you need each  class  to have its own package  whereas in javascript  you don t need packages at all  as  new  keyword makes up an anonymous hashmap for you to use as a package at runtime to which you can add new functions and remove functions on the fly    There are some Perl libraries creating their own ways of bridging this limitation in expressiveness  such as Moose   Why the confusion    Because of packages  Our intuition tells us to bind the object to a hashmap containing its  prototype  This lets us create  packages  at runtime like JavaScript can  Perl does not have such flexibility at least not built in  you have to invent it or get it from other modules   and in turn your runtime expressiveness is hindered  Calling it  bless  doesn t do it much favors neither   What we want to do   Something like this  but have binding to the prototype map recursive  and be implicitly bound to the prototype rather than having to explicitly do it    Here is a naive attempt at it  the issue is that  call  does not know  what called it   so it may as well be a universal perl function  objectInvokeMethod object  method   which checks whether the object has the method  or its prototype has it  or its prototype has it  until it reaches the end and finds it or not  prototypical inheritence   Perl has nice eval magic to do it but I ll leave that for something I can try doing later   Anyway here is the idea    sub        my  Animal    sub           my  AnimalPrototype                  performSound    gt  sub                   return    0 - gt   sound                                      my  call   sub               my  this      0               my  proc      1                if  exists  this- gt   proc                     return  this- gt   proc - gt                   else                   return  this- gt  prototype - gt   proc - gt   this   proc                                     return sub               my  name      0 - gt  name               my  sound      0 - gt  sound                my  this                       this    gt   this                   name    gt   name                   sound    gt   sound                   prototype    gt   AnimalPrototype                   call    gt   call                                                 - gt          my  animal    Animal- gt             name    gt   Jeff            sound   gt   bark              print  animal- gt  call   animal   performSound       - gt       Anyway hopefully somebody will find this post useful

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This function tells the entity referenced by REF that it is now an object in the CLASSNAME package  or the current package if CLASSNAME is omitted  Use of the two-argument form of bless is recommended   Example    bless REF  CLASSNAME bless REF   Return Value  This function returns the reference to an object blessed into CLASSNAME   Example   Following is the example code showing its basic usage  the object reference is created by blessing a reference to the package s class -     usr bin perl  package Person  sub new       my  class   shift      my  self              firstName   gt  shift           lastName    gt  shift           ssn         gt  shift               Print all the values just for clarification      print  First Name is  self- gt   firstName  n       print  Last Name is  self- gt   lastName  n       print  SSN is  self- gt   ssn  n       bless  self   class      return  self

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In general  bless associates an object with a class   package MyClass  my  object        bless  object   MyClass     Now when you invoke a method on  object  Perl know which package to search for the method   If the second argument is omitted  as in your example  the current package class is used   For the sake of clarity  your example might be written as follows   sub new      my  class   shift     my  self           bless  self   class        EDIT  See kixx s good answer for a little more detail

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In general  bless associates an object with a class   package MyClass  my  object        bless  object   MyClass     Now when you invoke a method on  object  Perl know which package to search for the method   If the second argument is omitted  as in your example  the current package class is used   For the sake of clarity  your example might be written as follows   sub new      my  class   shift     my  self           bless  self   class        EDIT  See kixx s good answer for a little more detail

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For example  if you can be confident that any Bug object is going to be a blessed hash  you can  finally   fill in the missing code in the Bug  print me method    package Bug   sub print me         my   self             print  ID   self- gt  id  n        print   self- gt  descr  n        print   Note  problem is fatal  n  if  self- gt  type  eq  fatal        Now  whenever the print me method is called via a reference to any hash that s been blessed into the Bug class  the  self variable extracts the reference that was passed as the first argument and then the print statements access the various entries of the blessed hash

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In general  bless associates an object with a class   package MyClass  my  object        bless  object   MyClass     Now when you invoke a method on  object  Perl know which package to search for the method   If the second argument is omitted  as in your example  the current package class is used   For the sake of clarity  your example might be written as follows   sub new      my  class   shift     my  self           bless  self   class        EDIT  See kixx s good answer for a little more detail

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In general  bless associates an object with a class   package MyClass  my  object        bless  object   MyClass     Now when you invoke a method on  object  Perl know which package to search for the method   If the second argument is omitted  as in your example  the current package class is used   For the sake of clarity  your example might be written as follows   sub new      my  class   shift     my  self           bless  self   class        EDIT  See kixx s good answer for a little more detail

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bless associates a reference with a package    It doesn t matter what the reference is to  it can be to a hash  most common case   to an array  not so common   to a scalar  usually this indicates an inside-out object   to a regular expression  subroutine or TYPEGLOB  see the book Object Oriented Perl  A Comprehensive Guide to Concepts and Programming Techniques by Damian Conway for useful examples  or even a reference to a file or directory handle  least common case    The effect bless-ing has is that it allows you to apply special syntax to the blessed reference    For example  if a blessed reference is stored in  obj  associated by bless with package  Class    then  obj- gt foo  args  will call a subroutine foo and pass as first argument the reference  obj followed by the rest of the arguments   args   The subroutine should be defined in package  Class   If there is no subroutine foo in package  Class   a list of other packages  taken form the array  ISA in the package  Class   will be searched and the first subroutine foo found will be called

User · Answer

I Following this thought to guide the development object-oriented Perl   Bless associate any data structure reference with a class  Given how Perl creates the inheritance structure  in a kind of tree  it is easy to take advantage of the object model to create Objects for composition   For this association we called object  to develop always have in mind that the internal state of the object and class behaviours are separated  And you can bless allow any data reference to use any package class behaviours  Since the package can understand  the emotional  state of the object

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This function tells the entity referenced by REF that it is now an object in the CLASSNAME package  or the current package if CLASSNAME is omitted  Use of the two-argument form of bless is recommended   Example    bless REF  CLASSNAME bless REF   Return Value  This function returns the reference to an object blessed into CLASSNAME   Example   Following is the example code showing its basic usage  the object reference is created by blessing a reference to the package s class -     usr bin perl  package Person  sub new       my  class   shift      my  self              firstName   gt  shift           lastName    gt  shift           ssn         gt  shift               Print all the values just for clarification      print  First Name is  self- gt   firstName  n       print  Last Name is  self- gt   lastName  n       print  SSN is  self- gt   ssn  n       bless  self   class      return  self

[perl] What exactly does Perl's "bless" do?

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