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";
}
&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.
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.
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;
}
&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.
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 $a::a \n";
An example:
use strict;
for (1 .. 2){
# Both variables are lexically scoped to the block.
our ($o); # Belongs to 'main' package.
my ($m); # Does not belong to a package.
# The variables differ with respect to newness.
$o ++;
$m ++;
print __PACKAGE__, " >> o=$o m=$m\n"; # $m is always 1.
# The package has changed, but we still have direct,
# unqualified access to both variables, because the
# lexical scope has not changed.
package Fubb;
print __PACKAGE__, " >> o=$o m=$m\n";
}
# The our() and my() variables differ with respect to privacy.
# We can still access the variable declared with our(), provided
# that we fully qualify its name, but the variable declared
# with my() is unavailable.
print __PACKAGE__, " >> main::o=$main::o\n"; # 2
print __PACKAGE__, " >> main::m=$main::m\n"; # Undefined.
# Attempts to access the variables directly won't compile.
# print __PACKAGE__, " >> o=$o\n";
# print __PACKAGE__, " >> 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);
$uv ++;
}
# Even though we are outside the lexical scope where the
# use vars() variable was declared, we have direct access
# because the package has not changed.
print __PACKAGE__, " >> uv=$uv\n";
# And we can access it from another package.
package Bubb;
print __PACKAGE__, " >> main::uv=$main::uv\n";
my is used for local variables, whereas our is used for global variables.
More reading over at Variable Scoping in Perl: the basics.
#!/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;
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.
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.
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'.
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:
Source: Stackoverflow.com