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Answer: Is it possible to do pass by reference in Perl?

by Russ (Deacon)
on Mar 05, 2002 at 19:32 UTC ( #149478=categorized answer: print w/replies, xml ) Need Help??

Q&A > subroutines > Is it possible to do pass by reference in Perl? - Answer contributed by Russ

One might argue that Perl always does Pass-By-Reference, but protects us from ourselves.

@_ holds the arguments passed to a subroutine, and it is common idiom to see something like:

sub mySub{ my $Arg = shift; } sub mySub2{ my ($Arg) = @_; }
Why is that? Why don't we just use the @_ array directly?

First, there is the laudable goal of more readable code, which is sufficient reason, in itself, to rename variables away from cryptic things like $_[3]. But really, we copy values out of @_ because (from the man page) "its elements are aliases for the actual scalar parameters."

In short, this means that if you modify an element of @_ in your subroutine, you are also modifying the original argument. This is almost never the expected behavior! Further, if the argument is not updatable (like a literal value, or a constant), your program will die with an error like "Modification of a read-only value attempted."


sub test{ $_[0] = 'New Value'; } my $Var = 'Hi there'; print "$Var\n"; test ($Var); print "$Var\n";
will print out:
Hi there New Value
So, yes, you can do pass-by-reference in Perl, even without backslashes; but it is almost always better (some would leave out the "almost" in this statement) to make your caller explicitly pass you a reference if you intend to modify a value.

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