为什么Perl的foreach不需要用我的变量声明？ [英] Why doesn't Perl's foreach require its variable to be declared with my?

问题描述

今天，我偶然发现了Perl中的一些东西，我并没有意识到：它本地化列表元素被分配给的变量。

当然，这是在Perl文档中记录的 - 但是我没有记住或读取它。

以下脚本演示了我的意思：

 使用警告; 
使用strict; 
 
 my $ g = 99; 
 
 foreach $ g（1..5）{
 p（$ g）; 
} 
 
 sub p {
 my $ l = shift; 
 printf（％2d％2d \ n，$ g，$ l）; 
 
 
 
 $ b 
脚本打印
 
 
  99 1 
 99 2 
 99 3 
 99 4 
 99 5 
  
因为 $ g 是本地化到 foreach  loop。
 
 
据我所知，如果我已经将 my 添加到<$在foreach循环中，c $ c> $ g ：
 
 $ p $ foreach $ g 5）{
  

其实，我最终这样做是因为我觉得这个变量更清晰本地的循环。

我的问题是现在：是否有一个情况下，我使用我确实有所作为（因为全局已经声明了 $ g ）。 调查行为记录在 perlsyn中的Foreach循环

forea ch 循环迭代正常的列表值，并将标量变量VAR设置为列表中的每个元素。如果变量前面有关键字 my ，那么它在词汇上是有作用域的，因此只在循环中可见。

继续解释

否则，该变量对于循环隐式地是局部的并且在退出循环时重新获得其以前的值。如果变量之前是用 my 声明的，它使用该变量而不是全局变量，但是它仍然是本地化的。

因此，将它与 my 进行本地化或保留到 foreach 。

有点好奇的是

lockquote >

这个隐式的本地化只发生在 foreach 循环中。

所有这些在私有变量通过 my（） from perlsub

foreach 循环默认以 local 的方式动态地定位它的索引变量。但是，如果索引变量的前缀是关键字 my ，或者如果在范围内已经有一个由该名称组成的词汇，则会创建一个新的词汇。

由于在这两种情况下创建了一个新的词法 ，因此不会有任何实际的区别。
$ b

我绝对支持并建议在那里放置 my 。

Today, I stumbled over something in Perl I was not aware of: it "localizes" the variable that the elements of the list iterated over is assigned to.

This, of course, is documented in the Perl documentation - however I failed to remember or read it.

The following script demonstrates what I mean:

use warnings;
use strict;

my $g = 99;

foreach $g (1..5) {
  p($g);
}

sub p {
  my $l = shift;
  printf ("%2d %2d\n", $g, $l);
}

The script prints

99  1
99  2
99  3
99  4
99  5

because $g is "localized" to the foreach loop.

As far as I can tell there is no difference if I had added my to $g in the foreach loop:

foreach my $g (1..5) {

Actually, I ended up doing it because I feel it makes it clearer that the variable is local to the loop.

My question is now: is there a scenario where my using my does make a difference (given that $g is already declared globally).

解决方案

The investigated behavior is documented in Foreach Loops in perlsyn

The foreach loop iterates over a normal list value and sets the scalar variable VAR to be each element of the list in turn. If the variable is preceded with the keyword my, then it is lexically scoped, and is therefore visible only within the loop.

which continues to the explanation

Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared with my, it uses that variable instead of the global one, but it's still localized to the loop.

Thus there should be no difference between localizing it with my or leaving that to foreach.

A little curiosity is that

This implicit localization occurs only in a foreach loop.

All this is further clarified in this snippet from Private Variables via my() from perlsub

The foreach loop defaults to scoping its index variable dynamically in the manner of local. However, if the index variable is prefixed with the keyword my, or if there is already a lexical by that name in scope, then a new lexical is created instead.

Since a new lexical is created inside in both cases there cannot be any practical difference.

I absolutely support and recommend putting a my there.

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