The Copy Constructor (=) (Programming Perl)

13.4. The Copy Constructor (=)

Although it looks like a regular operator, = has a special and slightly subintuitive meaning as an overload key. It does not overload the Perl assignment operator. It can't, because that operator has to be reserved for assigning references, or everything breaks.

The handler for = is used in situations where a mutator (such as ++, --, or any of the assignment operators) is applied to a reference that shares its object with another reference. The = handler lets you intercept the mutator and copy the object yourself so that the copy alone is mutated. Otherwise, you'd clobber the original.

$copy = $original;   # copies only the reference
++$copy;             # changes underlying shared object

Now, bear with us. Suppose that $original is a reference to an object. To make ++$copy modify only $copy and not $original, a copy of $copy is first made, and $copy is assigned a reference to this new object. This operation is not performed until ++$copy is executed, so $copy coincides with $original before the increment--but not afterward. In other words, it's the ++ that recognizes the need for the copy and calls out to your copy constructor.

The need for copying is recognized only by mutators such as ++ or +=, or by nomethod, which is described later. If the operation is autogenerated via +, as in:

$copy = $original;
$copy = $copy + 1;

then no copying occurs, because + doesn't know it's being used as a mutator.

If the copy constructor is required during the execution of some mutator, but a handler for = was not specified, it can be autogenerated as a string copy provided the object is a plain scalar and not something fancier.

For example, the code actually executed for the sequence:

$copy = $original;
...
++$copy;

might end up as something like this:

$copy = $original;
...
$copy = $copy->clone(undef, "");
$copy->incr(undef, "");

This assumes $original points to an overloaded object, ++ was overloaded with \&incr, and = was overloaded with \&clone.

Similar behavior is triggered by $copy = $original++, which is interpreted as $copy = $original; ++$original.