Operator overloading, nomethod, and Introspection

renodino has asked for the wisdom of the Perl Monks concerning the following question:

In the process of finishing up Thread::Apartment (see also Apartment Threading in Perl), I realized I had completely overlooked support for operator overloading. Based on my research, I suspect it may be possible to expose operator overloading in T::A objects, using the

use overload nomethod => \&operAutoLoad;
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in the client stub, but the docs are a bit sketchy about what to return if the nomethod coderef doesn't implement the specified operator; should it just return undef, or something else ? Do I need to use 'fallback => 1' with nomethod ?

Also, how do I go about introspecting a class to find its overloads ? (I need the latter so that POPOs can be handled by Thread::Apartment::Server's default introspection, which would return a list of operators which the class overloads).

Any help much appreciated.

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Re: Operator overloading, nomethod, and Introspection by diotalevi (Canon) on Sep 13, 2005 at 17:30 UTC
The `nomethod' function does not have the ability to decline to handle a request. If you have implemented some of the methods, the value of `fallback' becomes relevant. The `undef' value is reasonably sane - if an unimplemented method is called, it attempts to create one from what is already implemented. If that fails, it calls your `nomethod' function. If you didn't have a `nomethod' function, it would throw an exception. Since you promised to handle everything by specifying a `nomethod' function, no exceptions will be thrown. If you really want to decline to handle a method, throw the exception yourself from inside your `nomethod' function. The `nomethod' function will not handle ${}, @{}, %{}, &{}, or {} overloading. You will have to overload those manually. For Data::Postponed, I explicitly overloaded everything. Everything.* The idea is, when I promised that the value would appear to act like a normal value, I took that and applied it to everything available. This had the side effect of not allowing me to actually store anything inside the object because now I couldn't dereference it either. I used an inside-out implementation to avoid that problem. The following snippet produces a list of overloaded methods. `grep overload::Method( $some_object, $_ ), values %overload::ops` [download]	[reply] [d/l]
Re: Operator overloading, nomethod, and Introspection by ikegami (Patriarch) on Sep 13, 2005 at 17:26 UTC
If nomethod doesn't implement a specified operator, that means an operator was used on the object, but the object doesn't support it. I'd expect `perl` to `die` in that case. Here's an example of what happens on non-objects: `>perl -e "%hash++"; Can't modify hash dereference in postincrement (++) at -e line 1, near + "%hash++" Execution of -e aborted due to compilation errors.` [download] Here's what would happen nomethod wasn't there: `>perl -le "$r=bless({},''); print $r; printf '%x', ($r+1)-1" main=HASH(0x1abefa0) 1abefa0` [download]	[reply] [d/l] [select]