爲什麼undef值在Perl中成爲有效的數組引用？

Question

在perl 5.8.5中，如果我執行以下操作，則不會出現錯誤：

use strict; 

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

這裏發生了什麼？打印出ref($a)的輸出表明$a在某些時候變爲有效的數組引用。但我從來沒有明確地將$a設置爲任何東西。

似乎很奇怪，如果沒有我做任何事情，變量的內容可能會改變。

想，有人嗎？

編輯：是的，我知道所有關於自動生存的知識。我一直認爲必須在某個地方有一個任務來觸發它，而不僅僅是一個參考。

Answer 1

讀Uri Guttman's article on autovivification被禁用。

一旦你知道它並且節省很多尷尬，它就沒有什麼奇怪的了。

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是這個詞。從鏈接：

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.

自動vivification可以no autovivification;