为什么undef值在Perl中成为有效的数组引用？

Question

在perl 5.8.5中，如果我执行以下操作，则不会出现错误：

use strict; 

my $a = undef; 
foreach my $el (@$a) { 
    ...whatever 
} 

这里发生了什么？打印出ref($a)的输出表明$a在某些时候变为有效的数组引用。但我从来没有明确地将$a设置为任何东西。

似乎很奇怪，如果没有我做任何事情，变量的内容可能会改变。

想，有人吗？

编辑：是的，我知道所有关于自动生存的知识。我一直认为必须在某个地方有一个任务来触发它，而不仅仅是一个参考。

Answer 1

读Uri Guttman's article on autovivification被禁用。

一旦你知道它并且节省很多尴尬，它就没有什么奇怪的了。

Perl first evaluates a dereference expression and sees that the current reference value is undefined. It notes the type of dereference (scalar, array or hash) and allocates an anonymous reference of that type. Perl then stores that new reference value where the undefined value was stored. Then the dereference operation in progress is continued. If you do a nested dereference expression, then each level from top to bottom can cause its own autovivication.

Answer 2

Auto-vivification是这个词。从链接：

Autovivification is a distinguishing feature of the Perl programming language involving the dynamic creation of data structures. Autovivification is the automatic creation of a variable reference when an undefined value is dereferenced. In other words, Perl autovivification allows a programmer to refer to a structured variable, and arbitrary sub-elements of that structured variable, without expressly declaring the existence of the variable and its complete structure beforehand.

In contrast, other programming languages either: 1) require a programmer to expressly declare an entire variable structure before using or referring to any part of it; or 2) require a programmer to declare a part of a variable structure before referring to any part of it; or 3) create an assignment to a part of a variable before referring, assigning to or composing an expression that refers to any part of it.

Perl autovivication can be contrasted against languages such as Python, PHP, Ruby, JavaScript and all the C style languages.

自动vivification可以no autovivification;