Headline

Reference counting is a Memory management technique that keeps a counter of how many times an object is referenced. When the count reaches zero, the memory is freed.

Illustration

The mechanism can easily be illustrated by the following Language:Perl script, which peeks at the internal reference count of a variable:

#!/usr/bin/perl
use feature     'say';
use Devel::Peek 'SvREFCNT'; # access to reference count

my $thing = 'stuff';        # create a thing
say SvREFCNT($thing);       # refcount = 1

my $ref1  = \$thing;        # create reference to thing
say SvREFCNT($thing);       # refcount = 2

{                           # begin new block
    my $ref2 = \$thing;     # create another reference
    say SvREFCNT($thing);   # refcount = 3
}                           # code block ends, $ref2 goes out of scope
say SvREFCNT($thing);       # refcount = 2

$ref1 = 'something else';   # remove a reference
say SvREFCNT($thing);       # refcount = 1

This approach is simple to implement (almost trivial even when using a language with RAII) and destruction happens deterministically. However, circular references can still lead to Memory leaks, which must either be avoided or explicitly marked weak references.

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