What is the difference between my and our in Perl

Question

I know what my is in Perl  It defines a variable that exists only in the scope of the block in which it is defined  What does our do   How does our differ from my

User · Answer

This is only somewhat related to the question, but I've just discovered a (to me) obscure bit of perl syntax that you can use with "our" (package) variables that you can't use with "my" (local) variables.

#!/usr/bin/perl

our $foo = "BAR";

print $foo . "\n";
${"foo"} = "BAZ";
print $foo . "\n";

Output:

BAR
BAZ

This won't work if you change 'our' to 'my'.

User · Answer

The PerlMonks and PerlDoc links from cartman and Olafur are a great reference - below is my crack at a summary   my variables are lexically scoped within a single block defined by    or within the  same file if not in   s   They are not accessible from packages subroutines defined outside of the same lexical scope   block   our variables are scoped within a package file and accessible from any code that use or require that package file - name conflicts are resolved between packages by prepending the appropriate namespace   Just to round it out  local variables are  dynamically  scoped  differing from my variables in that they are also accessible from subroutines called within the same block

User · Answer

Coping with Scoping is a good overview of Perl scoping rules   It s old enough that our is not discussed in the body of the text   It is addressed in the Notes section at the end   The article talks about package variables and dynamic scope and how that differs from lexical variables and lexical scope

User · Answer

The perldoc has a good definition of our      Unlike my  which both allocates storage for a variable and associates a simple name with that storage for use within the current scope  our associates a simple name with a package variable in the current package  for use within the current scope  In other words  our has the same scoping rules as my  but does not necessarily create a variable

User · Answer

print  package is        PACKAGE       n   our  test   1  print  trying to print global var from main package   test n    package Changed             my  test   10          my  test1   11          print  trying to print local vars from a closed block   test   test1 n       amp Check global   sub Check global           print  trying to print global var from a function   test n     print  package is        PACKAGE       n   print  trying to print global var outside the func and from   Changed   package       test n   print  trying to print local var outside the block  test1 n     Will Output this   package is  main trying to print global var from main package  1 trying to print local vars from a closed block  10  11 trying to print global var from a function  1 package is  Changed trying to print global var outside the func and from  Changed  package  1 trying to print local var outside the block    In case using  use strict  will get this failure while attempting to run the script   Global symbol   test1  requires explicit package name at   check global pl line 24  Execution of   check global pl aborted due to compilation errors

User · Answer

Great question  How does our differ from my and what does our do   In Summary    Available since Perl 5  my is a way to declare non-package variables  that are    private new non-global  separate from any package  so that the variable cannot be accessed in the form of  package name  variable      On the other hand  our variables are package variables  and thus automatically    global variables definitely not private not necessarily new can be accessed outside the package  or lexical scope  with the qualified namespace  as  package name  variable      Declaring a variable with our allows you to predeclare variables in order to use them under use strict without getting typo warnings or compile-time errors  Since Perl 5 6  it has replaced the obsolete use vars  which was only file-scoped  and not lexically scoped as is our   For example  the formal  qualified name for variable  x inside package main is  main  x  Declaring our  x allows you to use the bare  x variable without penalty  i e   without a resulting error   in the scope of the declaration  when the script uses use strict or use strict  vars   The scope might be one  or two  or more packages  or one small block

User · Answer

my is used for local variables  whereas our is used for global variables   More reading over at Variable Scoping in Perl  the basics

User · Answer

I ever met some pitfalls about lexical declarations in Perl that messed me up  which are also related to this question  so I just add my summary here   1  Definition or declaration   local  var   42  print  var   var n     The output is var  42  However we couldn t tell if local  var   42  is a definition or declaration  But how about this   use strict  use warnings   local  var   42  print  var   var n     The second program will throw an error   Global symbol   var  requires explicit package name     var is not defined  which means local  var  is just a declaration  Before using local to declare a variable  make sure that it is defined as a global variable previously   But why this won t fail   use strict  use warnings   local  a   42  print  var   a n     The output is  var  42   That s because  a  as well as  b  is a global variable pre-defined in Perl  Remember the sort function   2  Lexical or global   I was a C programmer before starting using Perl  so the concept of lexical and global variables seems straightforward to me  it just corresponds to auto and external variables in C  But there re small differences   In C  an external variable is a variable defined outside any function block  On the other hand  an automatic variable is a variable defined inside a function block  Like this   int global   int main void        int local      While in Perl  things are subtle   sub main        var   42      amp main   print  var   var n     The output is var  42   var is a global variable even if it s defined in a function block  Actually in Perl  any variable is declared as global by default   The lesson is to always add use strict  use warnings  at the beginning of a Perl program  which will force the programmer to declare the lexical variable explicitly  so that we don t get messed up by some mistakes taken for granted

User · Answer

Let us think what an interpreter actually is  it s a piece of code that stores values in memory and lets the instructions in a program that it interprets access those values by their names  which are specified inside these instructions  So  the big job of an interpreter is to shape the rules of how we should use the names in those instructions to access the values that the interpreter stores   On encountering  my   the interpreter creates a lexical variable  a named value that the interpreter can access only while it executes a block  and only from within that syntactic block  On encountering  our   the interpreter makes a lexical alias of a package variable  it binds a name  which the interpreter is supposed from then on to process as a lexical variable s name  until the block is finished  to the value of the package variable with the same name   The effect is that you can then pretend that you re using a lexical variable and bypass the rules of  use strict  on full qualification of package variables  Since the interpreter automatically creates package variables when they are first used  the side effect of using  our  may also be that the interpreter creates a package variable as well  In this case  two things are created  a package variable  which the interpreter can access from everywhere  provided it s properly designated as requested by  use strict   prepended with the name of its package and two colons   and its lexical alias   Sources   http   perldoc perl org functions our html http   perldoc perl org perlsub html Private-Variables-via-my

User · Answer

An example   use strict   for  1    2         Both variables are lexically scoped to the block      our   o      Belongs to  main  package      my    m      Does not belong to a package         The variables differ with respect to newness       o          m         print   PACKAGE       gt  gt  o  o m  m n       m is always 1         The package has changed  but we still have direct        unqualified access to both variables  because the       lexical scope has not changed      package Fubb      print   PACKAGE       gt  gt  o  o m  m n        The our   and my   variables differ with respect to privacy    We can still access the variable declared with our    provided   that we fully qualify its name  but the variable declared   with my   is unavailable  print   PACKAGE       gt  gt  main  o  main  o n      2 print   PACKAGE       gt  gt  main  m  main  m n      Undefined     Attempts to access the variables directly won t compile    print   PACKAGE       gt  gt  o  o n     print   PACKAGE       gt  gt  m  m n      Variables declared with use vars   are like those declared   with our    belong to a package  not private  and not new    However  their scoping is package-based rather than lexical  for  1    9       use vars qw  uv        uv          Even though we are outside the lexical scope where the   use vars   variable was declared  we have direct access   because the package has not changed  print   PACKAGE       gt  gt  uv  uv n      And we can access it from another package  package Bubb  print   PACKAGE       gt  gt  main  uv  main  uv n

User · Answer

usr bin perl -l  use strict     if string below commented out  prints  lol    if the string enabled  prints  eeeeeeeee   my  lol    eeeeeeeeeee      no errors or warnings at any case  despite of  strict   our  lol   eval   lol      lol     print  lol

User · Answer

Just try to use the following program       usr local bin perl use feature   5 10    use warnings  package a    my  b   100  our  a   10    print   a  n   print   b  n      package b    my  b   200   our  a   20    print  in package b value of  my b  a  b  n   print  in package b value of our a   a  a  n

[perl] What is the difference between 'my' and 'our' in Perl?

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