I Bless You in the Name of the Stringified Object


Originally posted as: I Bless You in the Name of the Stringified Object on blogs.perl.org.

A co-worker came to me today with a curious error message:

use DateTime;
my $date = DateTime->new( year => 2013, month => 4, day => 15 );
print $date->today;'

This code gives the error Can't locate object method "_normalize_nanoseconds" via package "2013-04-15T00:00:00" at /usr2/local/perlbrew/perls/perl-5.16.3/lib/site_perl/5.16.3/x86_64-linux-thread-multi/DateTime.pm line 252.

The package "2013-04-15T00:00:00" is the curious part: It looks like a stringified DateTime, but who could possibly be stringifying a DateTime object and then using that as a package name?

It turns out the problem is at $date->today. today() is a constructor, constructors are class methods, constructors inevitably call bless on the class they are invoked with. But, we called this constructor with an object instance (a blessed reference), not a class.

In an object without overloads, this fails (as expected) with an error message: Attempt to bless into a reference. But DateTime overloads stringification. So instead of trying to use an object as a package name, the object gets stringified and that string gets used as a package name.

This problem could be mitigated by checking for refs in the constructor, dying as a result. I'm not sure if it would be a good idea to disallow stringification during a call to bless, for historical reasons (it's not a bug if it's working as expected, it is a bug if a change breaks code).

Here's some sample code to play around with the problem. Enable/disable the overload (comment out the use overload (...)) and see what changes:

use strict;
use warnings;

package Foo;

use overload (
    q{""} => sub { return "Bar" },

sub new {
    my ( $class ) = @_;
    return bless {}, $class;

sub greet {
    print "Hello, World!\n";

package main;

my $obj = Foo->new;

my $clone = $obj->new;