How to tell if I have a string, or a ref to a hash

mshaw has asked for the wisdom of the Perl Monks concerning the following question:

A variable I'm passing into a subroutine may be a string, or it may be a hash reference. I need to take one action or the other as a result of a test for this. Is there a better way to perform that test than I've constructed in this code?

My Perl version is 5.8.5, running under Redhat.

#!/usr/bin/perl -w

use strict;
use Carp;

my %h = (
    'one' => 'uno',
    'two' => 'dos',
    'three' => 'tres',
);

my $s = 'foo bar';

sub xyz() {
    my $arg;
    my $argNum=0;
    my $stringOrRef;

    while(defined($arg = shift)) {
        if($argNum == 0) {
            $stringOrRef = $arg;
        } else {
            carp "one arg only please";
        }
        $argNum++;
    }

    # here is where I test the argument
    ######################################
    if($stringOrRef =~ /^HASH\(/) {
        print "it's a hash:\n";
        foreach my $k (keys %{ $stringOrRef }) {
            print "    $k : $$stringOrRef{$k}\n";
        }
    } else {
        print "it's a string:\n";
        print "    $stringOrRef\n";
    }

}

&xyz($s);
&xyz(\%h);
[download]

Comment on How to tell if I have a string, or a ref to a hash Download Code

Replies are listed 'Best First'.
Re: How to tell if I have a string, or a ref to a hash by moritz (Cardinal) on Oct 19, 2010 at 16:40 UTC
It's safer to use Scalar::Util::reftype -- if you just use the string representation, you'll have trouble if the string happens to look like `HASH(0x88bd48)`. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l]
Re: How to tell if I have a string, or a ref to a hash by Corion (Patriarch) on Oct 19, 2010 at 16:38 UTC
See ref	[reply]
Re: How to tell if I have a string, or a ref to a hash by ikegami (Patriarch) on Oct 19, 2010 at 17:36 UTC
The tools have been provided already, but I wanted to add a comment: You can't do it perfectly unless you disallow certain special cases related to overloading. Perl is designed around of the concept of implicit coercion, so trying to use type-based polymorphism in Perl is inherently flawed. By the way, your use of a prototype is not only unnecessary, it's wrong. You tell Perl that the sub takes no arguments ("`sub xyz()`"), but the sub clearly does take arguments ("`while(defined($arg = shift))`") and you have to tell Perl to ignore the fact that you told it the sub takes no arguments ("`&xyz`"). You should have `sub xyz { ... } xyz($s); xyz(\%h);` [download]	[reply] [d/l] [select]
Re^2: How to tell if I have a string, or a ref to a hash by mshaw (Initiate) on Oct 19, 2010 at 18:06 UTC
Thanks very much, all. It's interesting that not only had I never heard of 'ref', but that my searches through the index of the Camel Book didn't turn it up. I've decided to use Scalar::Util::reftype. Thanks also for the note on prototypes.	[reply]
Re^3: How to tell if I have a string, or a ref to a hash by jakeease (Friar) on Oct 19, 2010 at 18:44 UTC
The ref function is in the index of the camel book in both the 2nd and 3rd editions. The original pink one is in a box in my garage, so I don't know about that one.	[reply]
Re: How to tell if I have a string, or a ref to a hash by chromatic (Archbishop) on Oct 19, 2010 at 18:42 UTC
`sub xyz_from_string { my $arg_string = shift; ... } sub xyz_from_hashref { my $arg_href = shift; ... }` [download]	[reply] [d/l]
Re: How to tell if I have a string, or a ref to a hash by locked_user sundialsvc4 (Abbot) on Oct 19, 2010 at 18:17 UTC
`perldoc -f ref` Returns a non-empty string if EXPR is a reference, the empty string otherwise. If EXPR is not specified, `$_` will be used. The value returned depends on the type of thing the reference is a reference to. Builtin types include: SCALAR ARRAY HASH CODE REF GLOB LVALUE FORMAT IO VSTRING Regexp If the referenced object has been `bless`ed into a package, then that package name is returned instead. You can think of “ref” as a “typeof” operator.