Programming Perl, 4th Edition

Smartmatching of Objects

To avoid relying on an object’s underlying representation, if the smartmatch’s right operand is an object that doesn’t overload ~~, it raises the exception, “Smart matching a non–overloaded object breaks encapsulation”. That’s because one has no business digging around to see whether something is “in” an object. These are all illegal on objects without a ~~ overload:

 %hash ~~ $object
    42 ~~ $object
"fred" ~~ $object

However, you can change the way an object is smartmatched by overloading the ~~ operator. This is allowed to extend the usual smartmatch semantics. For objects that do have an ~~ overload, see Chapter 13.

Using an object as the left operand is allowed, although it’s not very useful. Smartmatching rules take precedence over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored. A left operand that is a nonoverloaded object falls back on a string or numeric comparison of whatever the ref operator returns. Meaning:

$object ~~ X

does not invoke the overload method with X as an argument. Instead, the above table is consulted as normal, and based on the type of X, overloading may or may not be invoked. For simple strings or numbers, it becomes equivalent to this:

$object ~~ $number          "$object" == $number
$object ~~ $string          "$object" eq $string

For example, this reports that the handle smells IOish:

use IO::Handle;
my $fh = IO::Handle–>new();
if ($fh ~~ /\bIO\b/) {
    say "handle smells IOish";
}

That’s because it ...