How? How do you prevent your OO system from throwing in case a mandatory parameter is missing and convince it to return undef instead?

Good catch! I mixed up things here. It is definitely not possible to coerce a new method to return undef on failure. If you really want that, you need to write a custom constructor. I often write custom constructors, though mostly for other reasons.

The new core OO system didn't invent this behavior, though. It is quite common with many OO systems on CPAN: They write a constructor method new for you, according to some declarative syntax. Therefore, new is no longer a "user defined constructor method": It is rather a part of the language on the same level as bless (disregarding the specifics of its medication for now). In none of these OO systems you get a chance to call the parent constructor. They all claim that you don't need to call the parent constructor, and they all have some mechanism to deal with inheritance during construction. For the new core OO system, ADJUST blocks are that mechanism.

There are differences between the new core OO system and the popular CPAN OO systems, and these affect the construction of objects, in particular in an inheritance hierarchy:

• Moose (also Moo and Object::Pad) offer a BUILDARGS method to mangle the parameter list provided to new before object creation starts. The new core OO system (like e.g. Dios) does not have this, it only allows a hash-style parameter list. So, you could keep arbitrary constructor APIs when moving from bless-style OO to Moose, but you can not achieve that with core OO. This is the main reason why I write custom constructors these days: I want to have control over my constructor API.
• Child classes have no access to parent attributes unless there are (public) accessor methods for them. In Moose, you could always access the hash containing all fields, this is now gone. Also, Moose allowed to change a field declaration in a subclass with the has '+field' syntax, neither Object::Pad nor core OO allow this. This is sometimes an obstacle during object construction, and the discussion is still ongoing.

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