Beefy Boxes and Bandwidth Generously Provided by pair Networks
Just another Perl shrine

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
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.

In reply to Re: Is it possible to do pass by reference in Perl? by Russ
in thread Is it possible to do pass by reference in Perl? by Anonymous Monk

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or or How to display code and escape characters are good places to start.
Log In?

What's my password?
Create A New User
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others about the Monastery: (5)
As of 2021-06-24 08:58 GMT
Find Nodes?
    Voting Booth?
    What does the "s" stand for in "perls"? (Whence perls)

    Results (124 votes). Check out past polls.