什么！！(双感叹号)是什么意思? [英] What does !! (double exclamation point) mean?

问题描述

在下面的代码中，来自 Alias 的 博文，我注意到使用了双感叹号 !!.我想知道这意味着什么，以及将来我可以去哪里寻找这样的 Perl 语法的解释.(是的，我已经在 perlsyn 搜索了 !!.

In the code below, from a blog post by Alias, I noticed the use of the double exclamation point !!. I was wondering what it meant and where I could go in the future to find explanations for Perl syntax like this. (Yes, I already searched for !! at perlsyn).

package Foo;

use vars qw{$DEBUG};
BEGIN {
    $DEBUG = 0 unless defined $DEBUG;
}
use constant DEBUG => !! $DEBUG;

sub foo {
    debug('In sub foo') if DEBUG;

    ...
}

更新
感谢您的所有回答.

UPDATE
Thanks for all of your answers.

这是我刚刚发现的其他相关列表壁球运算符x!!

Here is something else I just found that is related The List Squash Operator x!!

推荐答案

这只是两个 ! 布尔而不是相邻的运算符.

It is just two ! boolean not operators sitting next to each other.

使用这个成语的原因是确保你收到一个1 或一个0.实际上它返回一个空字符串，它 numifys 为 0.不过它通常只用于数字或布尔上下文.

The reason to use this idiom is to make sure that you receive a 1 or a 0. Actually it returns an empty string which numifys to 0. It's usually only used in numeric, or boolean context though.

您会经常在 Code Golf 比赛，因为它比使用三元 更短?: 运算符与 1 和 0 _{($test ? 1 : 0)}.

You will often see this in Code Golf competitions, because it is shorter than using the ternary ? : operator with 1 and 0 _{($test ? 1 : 0)}.

!! undef  == 0
!! 0      == 0
!! 1      == 1
!! $obj   == 1
!! 100    == 1

undef ? 1 : 0  == 0
0     ? 1 : 0  == 0
1     ? 1 : 0  == 1
$obj  ? 1 : 0  == 1
100   ? 1 : 0  == 1

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