Closures
An anonymous subroutine remembers the variables that were in scope when it was created.
In this page:
Closures
When an anonymous subroutine uses a my variable from the surrounding scope, Perl keeps that variable alive for as long as the subroutine exists. Each call to a function that creates such a subroutine gets its own fresh copy of the variable. This lets you build counters, generators and other small objects without any package machinery.
Note:
Perl frees data when the last reference to it disappears, so a closure keeps its captured variables alive.
Example: Closures
use strict;
use warnings;
sub make_counter {
my $count = shift;
return sub { return $count++; };
}
my $c1 = make_counter(1);
my $c2 = make_counter(100);
print "c1: ", $c1->(), " ", $c1->(), " ", $c1->(), "\n";
print "c2: ", $c2->(), " ", $c2->(), "\n";
print "c1 again: ", $c1->(), "\n";
my @subs;
for my $i (1 .. 3) {
push @subs, sub { return $i * 10; };
}
print "loop closures: ", join(", ", map { $_->() } @subs), "\n";
# Output:
# c1: 1 2 3
# c2: 100 101
# c1 again: 4
# loop closures: 10, 20, 30Login to try C/C++/Java/PHP code in the editor
Common Mistakes
- Expecting every closure created in a loop to share one variable
- Forgetting that each call to the factory makes a new independent counter
- Using a global variable when a captured my variable would be safer
Chapter Summary
- Closures capture surrounding my variables
- Each creation gets its own copy
- Captured variables stay alive with the closure
- Closures make counters and generators
🔒
Chapter Quiz — Complete all 7 topics to unlock
0/7 topics done
Complete these topics first: