How should I compare Perl references?

perl reference compare
David B picture David B · Oct 31, 2010 · Viewed 7.8k times · Source

I want to check if two references point to the same object. It seems I can simply use 

if ($ref1 == $ref2) { 
 # cheap numeric compare of references
 print "refs 1 and 2 refer to the same thing\n";
}

as mentioned in perlref, but I vaguely remember seeing the use of some function for the same purpose. Is there any reason I shouldn't use the simple numerical equality test?

Note I only want to know whether the references point to the exact same object. I don't look for a way to compare the content of the object(s).

Answer

rafl picture rafl · Oct 31, 2010

References, by default, numify to their addresses. Those reference addresses are unique for every reference, so it can often be used in equality checks.

However, in the snippet you showed, you'd first have to make sure that both $ref1 and $ref2 are actually references. Otherwise you might get incorrect results due to regular scalars containing reference addresses.

Also, nothing guarantees references to numify to their address. Objects, for example, can use overloading to influence the value they return in different contexts.

A more solid way than comparing references directly for numeric equality would be to use the refaddr function as provided by Scalar::Util, after making sure both sides actually are references.

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