Test::MockObject doesn't fool UNIVERSAL

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

Perhaps I'm wanting too much, but I think I'm seeing that an object mocked by Test::MockObject doesn't fool UNIVERSAL::isa and UNIVERSAL::can. Here's some code giving the idea:

use strict;
use warnings FATAL => 'all';
use Test::MockObject;
use Test::More tests => 6;

my $c = Test::MockObject->new;
$c->set_isa('Foo');
$c->mock('id', sub {return -1;});
 
ok($c->can('id'),  "c can id (via 'can' method)");
ok($c->isa('Foo'), "c isa Foo (via 'isa' method)");
isa_ok($c, 'Foo', 'c isa_ok Foo');
is($c->id, -1, 'mocked accessor');

# FAILS!
ok(UNIVERSAL::isa($c, 'Foo'), 'c isa Foo (via UNVERSAL::isa)');

# FAILS!
ok(UNIVERSAL::can($c, 'id'), 'c can id (via UNIVERSAL::can');
[download]

Is this because UNIVERSAL is the root of the object hierarchy, and thus can't be fooled?

If so, it would seem that methods which need to check the type of their arguments (asserting if the wrong param type sent, for example), should use $self->isa rather than UNIVERSAL::isa to do so, as otherwise you can't use Mock Objects to test the methods.

(In contrast, this nice perl OO style guide suggests UNIVERSAL:: over the method call approach. Which I think isn't great advice if you'll be testing with Test::MockObject.)

This isn't a SOPW post per se, but I end with the following question: are the observations above correct, or did I misuse UNIVERSAL?

Thanks!

water

Retitled by davido from 'Class::MockObject doesn't fool UNIVERSAL'.

Comment on Test::MockObject doesn't fool UNIVERSAL Select or Download Code

Replies are listed 'Best First'.
Re: Test::MockObject doesn't fool UNIVERSAL by itub (Priest) on Jul 10, 2005 at 22:25 UTC
There are two modules on CPAN, curiously called UNIVERSAL::isa and UNIVERSAL::can which fix this problem. Read the documentation for a detailed discussion about this issue, including the problems with Test::MockObject.	[reply]
Re: Test::MockObject doesn't fool UNIVERSAL by ysth (Canon) on Jul 11, 2005 at 01:01 UTC
Yes, the default UNIVERSAL:: methods don't do what the overriding methods do. The guide you refer to prefers UNIVERSAL::isa (etc.) over $obj->isa because $foo->method croaks when $foo is a reference that isn't an object. This has the cost of breaking anything that overrides the UNIVERSAL method. I've started preferring always using the method call, wrapped in eval: `if (eval { $obj->isa("Foo") }) { print "It's a foo!"; }` [download] The only drawback that I see with this so far is that $@ is altered and would need to be localized if it's being otherwise used, such as in a DESTROY routine.	[reply] [d/l]
Re: Test::MockObject doesn't fool UNIVERSAL by chromatic (Archbishop) on Jul 11, 2005 at 01:04 UTC
Test::MockObject provides `isa()` and `can()` methods that do the right things. If you don't call them, though, you won't get the right answers. Calling a base class method directly on a derived class that may have overrdden the method is a bad idea. Any code that does that needs a very good reason to do so, or it's broken. (The only good reason I can think of is in UNIVERSAL::isa, which I'll probably use in the next version of Test::MockObject.) I have the temptation to create a module called `UNIVERSAL::new` and tell everyone to create their objects with `UNIVERSAL::new( 'ClassName', %arguments );`.	[reply] [d/l] [select]
Re: Test::MockObject doesn't fool UNIVERSAL by etcshadow (Priest) on Jul 11, 2005 at 01:30 UTC
Well, the problem is that there is a difference between: `$x->method(...)` [download] and `class::method($x, ...)` [download] even if "class" is the class in which $x is blessed, or one of the ancestors of that class. When you say `$x->method(...)`, you are invoking what's called "method dispatch", a (breadth-first) search for a method in the class in which the object is blessed and its ancestors. When you say `class::method($x, ...)` (basically equivalent to `$x->class::method(...)`), you are foregoing the search and telling perl which class's method to use. Now, in this example, you are going straight to UNIVERSAL for its can and isa methods, rather than allowing method dispatch to find the can and isa methods in the (nearer than UNIVERSAL) ancestors of your class (or the class itself). Thus, what you're getting is UNIVERSAL::can, rather than Class::MockObject::can (which is what the method dispatch would have found). The last thing, though, is that perl almost forces you to do this. If you've got an object, and you don't know what it is, then calling `$x->can('foo')` could be a really bad idea. Well, that's not true... if you know that $x is, in fact, an "object" then you're cool. The problem is: what if you don't know whether $x is an object (a blessed reference), or if it's just a plain, unblessed reference, or not even a reference at all. Then, if it can foo, then it'll return true, but if it cannot foo, then it might die! Anyway, the UNIVERSAL::can and UNIVERSAL::isa CPAN modules mentioned by itub are pretty cool and basically deal with this, in a way (but they warn if you do it). `------------ :Wq Not an editor command: Wq` [download]	[reply] [d/l] [select]


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