How would you prevent someone from calling new() directly, in this case skipping your argument checks?

Actually, this is possible. Your valid constructor methods just create a sufficiently secure random token and pass this to new. Here's a simple example where you can create objects with calls to the valid_constructor function, but not by directly calling new:

use 5.37.9;
use feature 'class';
no warnings 'experimental';
class No::New {
    use Carp;

    field $token :param;
    my %valid_tokens;

    ADJUST {
        delete $valid_tokens{$token}
          or  croak "Directly calling ", __PACKAGE__,
          "->new is forbidden";
    }

    sub valid_constructor {
    my $token = rand;
        $valid_tokens{$token} = 1;
    __PACKAGE__->new(token => $token);
    }

    method text {
    "Congratulations!";
    }
}

say No::New->valid_constructor->text;

[download]

Such a technique, as already mentioned by tobyink, is entirely impossible for bless. Since all "traditional" Perl OO frameworks including Object::Pad are based on blessed references, only core OO can prevent objects like bless \q"Arbitrary junk", Your::Class from happening!

In reply to Re^4: Thoughts on new 'class' OO in upcoming perl by haj
in thread Thoughts on new 'class' OO in upcoming perl by cavac

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