blocks|key|1716355|text|为了直接回答您的问题，答案位于您链接到的页面的底部，即如果包定义了isa方法，则直接调用UNIVERSAL::isa不会调用包的isa方法。从面向对象的角度来看，这是非常不直观的行为。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1716356|这篇文章的其余部分只是关于你为什么要这样做的更多问题。|1716357|在类似上面的代码中，在什么情况下特定的isa测试会失败？也就是说，如果它是一个方法，在哪种情况下，第一个参数不会是package类或它的实例？|1716358|我之所以这样问，是因为我想知道是否有合法的理由，让你一开始就想测试第一个参数是不是对象。也就是说，你只是想抓住人们说FooBar::method而不是FooBar->method或$foobar->method吗？我猜Perl不是为这种纵容而设计的，如果人们错误地使用了FooBar::method，他们很快就会发现的。|1716359|您的里程可能会有所不同。|1716360|entityMap^0|X|3|18|E|1S|3|0|0|J|3|0|1M|E|23|E|2I|F|3R|E|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|L|3|M|5|6|7|19|8|@]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|O|$]]

To directly answer your question, the answer is at the bottom of the page you linked to, namely that if a package defines an <code>isa</code> method, then calling <code>UNIVERSAL::isa</code> directly will not call the package <code>isa</code> method. This is very unintuitive behaviour from an object-orientation point of view.

The rest of this post is just more questions about why you're doing this in the first place.

In code like the above, in what cases would that specific <code>isa</code> test fail? i.e., if it's a method, in which case would the first argument not be the package class or an instance thereof?

I ask this because I wonder if there is a legitimate reason why you would want to test whether the first argument is an object in the first place. i.e., are you just trying to catch people saying <code>FooBar::method</code> instead of <code>FooBar-&gt;method</code> or <code>$foobar-&gt;method</code>? I guess Perl isn't designed for that sort of coddling, and if people mistakenly use <code>FooBar::method</code> they'll find out soon enough.

Your mileage may vary.

blocks|key|1716425|text|请参阅UNIVERSAL::isa和UNIVERSAL::can的文档，了解为什么不应该这样做。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1716426|简而言之，有一些重要的模块确实需要覆盖'isa‘(比如Test::MockObject)，如果你把它作为一个函数调用，你就破坏了这一点。|1716427|我不得不说，my+$self+=+shift+if+UNIVERSAL::isa($_[0],+__PACKAGE__)在我看来并不是很干净--反Perl的倡导者会抱怨线路噪音。:)|style|CODE|1716428|entityMap|0|LINK|mutability|MUTABLE|url|http://search.cpan.org/dist/UNIVERSAL-isa/lib/UNIVERSAL/isa.pm|1|http://search.cpan.org/dist/UNIVERSAL-can/lib/UNIVERSAL/can.pm|2|http://search.cpan.org/dist/Test-MockObject/^0|3|E|0|I|E|1|0|R|G|2|0|6|1I|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@$A|W|B|X|1|Y]|$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|F|3|G|5|6|7|16|8|@$A|17|B|18|H|I]]|9|@]|C|$]]|$1|J|3|-4|5|6|7|19|8|@]|9|@]|C|$]]]|K|$L|$5|M|N|O|C|$P|Q]]|R|$5|M|N|O|C|$P|S]]|T|$5|M|N|O|C|$P|U]]]]

See the docs for <a href="http://search.cpan.org/dist/UNIVERSAL-isa/lib/UNIVERSAL/isa.pm" rel="nofollow noreferrer">UNIVERSAL::isa</a> and <a href="http://search.cpan.org/dist/UNIVERSAL-can/lib/UNIVERSAL/can.pm" rel="nofollow noreferrer">UNIVERSAL::can</a> for why you shouldn't do it.

In a nutshell, there are important modules with a genuine need to override 'isa' (such as <a href="http://search.cpan.org/dist/Test-MockObject/" rel="nofollow noreferrer">Test::MockObject</a>), and if you call it as a function, you break this.

I have to say, <code>my $self = shift if UNIVERSAL::isa($_[0], __PACKAGE__)</code> doesn't look terribly clean to me - anti-Perl advocates would be complaining about line noise. :)

blocks|key|1501371|text|主要问题是，如果直接调用UNIVERSAL::isa，就会绕过任何重载了isa的类。如果这些类依赖于重载行为(它们可能会这样做，否则就不会覆盖它)，那么这就是一个问题。如果您直接在受保护的对象上调用isa，那么在任何一种情况下都将调用正确的isa方法(如果存在，则重载；如果不存在，则调用UNIVERSAL:：)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1501372|第二个问题是，就像isa的其他用法一样，UNIVERSAL::isa只会在有福的引用上执行您想要的测试。它对非祝福引用和简单标量有不同的行为。所以你不检查$ref是否有福的例子是不正确的，你忽略了一个错误条件，并使用了UNIVERSAL的替代行为。在某些情况下，这可能会导致细微的错误(例如，如果你的变量包含一个类名)。|1501373|考虑一下：|1501374|use+CGI;

my+$a+=+CGI->new();

my+$b+=+"CGI";

print+UNIVERSAL::isa($a,"CGI");++#+prints+1,+$a+is+a+CGI+object.
print+UNIVERSAL::isa($b,"CGI");++#+Also+prints+1!!+Uh-oh!!|code-block|syntax|javascript|1501375|所以，总之，不要使用UNIVERSAL::isa...执行额外的错误检查，并直接在对象上调用isa。|1501376|entityMap^0|C|E|10|3|2R|3|3C|3|0|9|3|K|E|25|4|31|9|0|0|0|A|E|1A|3|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1A|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1B|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|R|$]]

The primary problem is that if you call <code>UNIVERSAL::isa</code> directly, you are bypassing any classes that have overloaded <code>isa</code>. If those classes rely on the overloaded behavior (which they probably do or else they would not have overridden it), then this is a problem. If you invoke <code>isa</code> directly on your blessed object, then the correct <code>isa</code> method will be called in either case (overloaded if it exists, UNIVERSAL:: if not).

The second problem is that <code>UNIVERSAL::isa</code> will only perform the test you want on a blessed reference just like every other use of <code>isa</code>. It has different behavior for non-blessed references and simple scalars. So your example that doesn't check whether <code>$ref</code> is blessed is not doing the right thing, you're ignoring an error condition and using <code>UNIVERSAL</code>'s alternate behavior. In certain circumstances this can cause subtle errors (for example, if your variable contains the name of a class).

Consider:

<pre><code>use CGI;

my $a = CGI-&gt;new();

my $b = "CGI";

print UNIVERSAL::isa($a,"CGI"); # prints 1, $a is a CGI object.
print UNIVERSAL::isa($b,"CGI"); # Also prints 1!! Uh-oh!!
</code></pre>

So, in summary, don't use <code>UNIVERSAL::isa</code>... Do the extra error check and invoke <code>isa</code> on your object directly.

blocks|key|1501437|text|正确的。它对重载isa的类做了错误的事情。只需使用以下成语：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1501438|if+(eval+{+$obj->isa($class)+})+{|code-block|syntax|javascript|1501439|它很容易理解并被普遍接受。|1501440|entityMap^0|8|3|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

Right. It does a wrong thing for classes that overload <code>isa</code>. Just use the following idiom:

<pre><code>if (eval { $obj-&gt;isa($class) }) {
</code></pre>

It is easily understood and commonly accepted.

blocks|key|1716612|text|2020年的更新:Perlv5.32具有class+infix+operator、isa，它可以处理左手边的任何事情。如果$something不是一个对象，您将返回false，而不会放大。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1716613|use+v5.32;

if(+$something+isa+'Animal'+)+{+...+}|code-block|syntax|javascript|1716614|entityMap|0|LINK|mutability|MUTABLE|url|https://www.effectiveperlprogramming.com/2020/01/use-the-infix-class-instance-operator/^0|15|3|1P|A|K|K|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

Update for 2020: Perl v5.32 has the <a href="https://www.effectiveperlprogramming.com/2020/01/use-the-infix-class-instance-operator/" rel="nofollow noreferrer">class infix operator</a>, <code>isa</code>, which handles any sort of thing on the lefthand side. If the <code>$something</code> is not an object, you get back false with no blowup.
<pre><code>use v5.32;

if( $something isa 'Animal' ) { ... }
</code></pre>

According to this

<a href="http://perldoc.perl.org/UNIVERSAL.html" rel="noreferrer">http://perldoc.perl.org/UNIVERSAL.html</a>

I shouldn't use UNIVERSAL::isa() and should instead use $obj->isa() or CLASS->isa().

This means that to find out if something is a reference in the first place and then is reference to this class I have to do

<pre><code>eval { $poss-&gt;isa("Class") }
</code></pre>

and check $@ and all that gumph, or else

<pre><code>use Scalar::Util 'blessed';
blessed $ref &amp;&amp; $ref-&gt;isa($class);
</code></pre>

My question is why? What's wrong with UNIVERSAL::isa called like that? It's much cleaner for things like:

<pre><code>my $self = shift if UNIVERSAL::isa($_[0], __PACKAGE__)
</code></pre>

To see whether this function is being called on the object or not. And is there a nice clean alternative that doesn't get cumbersome with ampersands and potentially long lines?

Why shouldn't I use UNIVERSAL::isa?

Perl

根据这个我不应该使用UNIVERSAL::isa()，而应该使用$obj->isa()或CLASS->isa()。这意味着首先要找出某个东西是否是引用，然后是对这个类的引用，我必须这样做eval { $poss->isa("Class") }检查一下$@和所有的口香糖，否则use Scalar::Util 'bless...

问为什么我不能使用UNIVERSAL::isa呢？
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