Programming Perl, 4th Edition

ref

ref EXPR
ref

The ref operator returns a true value if EXPR is a reference, and false otherwise. The value returned depends on the type of thing the reference refers to. Built-in types include:

SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp

The return value LVALUE indicates a reference to an lvalue that is not a variable. You get this from taking the reference of function calls like pos or substr. VSTRING is returned if the reference points to a version string.

The result Regexp indicates that the argument is a regular expression resulting from qr//.

If the referenced object has been blessed into a package, then that package name is returned instead. You can think of ref as a “typeof” operator.

if (ref($r) eq "HASH") {
    say "r is a reference to a hash.";
}
elsif (ref($r) eq "Hump") {    # Naughty—see below
    say "r is a reference to a Hump object.";
}
elsif (not ref $r) {
    say "r is not a reference at all.";
}

It’s considered bad OO style to test your object’s class for equality to any particular class name, since a derived class will have a different name but should be allowed access to the base class’s methods, according to the Liskov Substitution Principle. It’s better to use the UNIVERSAL method isa, as follows:

if ($r–>isa("Hump")) {
    say "r is a reference to a Hump object, || subclass.";
}

It’s usually best not to test at all, since the OO mechanism won’t send the object ...