为什么在这种情况下 Perl 会自动激活? [英] Why does Perl autovivify in this case?

问题描述

为什么 $a 会变成一个数组引用?我没有强迫它.

Why does $a become an arrayref? I'm not pushing anything to it.

perl -MData::Dumper -e 'use strict; 1 for @$a; print Dumper $a'
$VAR1 = [];

推荐答案

这是因为 for 循环将 @$a 的内容视为左值——你可以赋值给.请记住，for 将数组的内容别名为 $_.看起来，在 @$a 中寻找可别名内容的行为足以导致自动激活，即使没有要别名的内容也是如此.

It is because the for loop treats contents of @$a as lvalues--something that you can assign to. Remember that for aliases the contents of the array to $_. It appears that the act of looking for aliasable contents in @$a, is sufficient to cause autovivification, even when there are no contents to alias.

这种混叠效果也是一致的.以下内容也会导致自动激活:

This effect of aliasing is consistent, too. The following also lead to autovivification:

• map {stuff} @$a;
• grep {stuff} @$a;
• a_subroutine(@$a);

如果你想管理自动激活，你可以使用同名pragma来实现词法控制.

If you want to manage autovivification, you can use the eponymous pragma to effect lexical controls.

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