【求教】请教一个很奇怪的问题，真的很奇怪

wxlfh

1. #-- Test.pm
   
2. package Test;
   
3. 
   
4. sub get_row {
   
5.     my $file = $_[0];
   
6.     open my $fh, "<", $file or return;
   
7.     return sub {
   
8.         my $row = <$fh>;
   
9.         return $row;
   
10.     }
    
11. }

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1. # 这样调用输出为空
   
2. use strict;
   
3. use lib 'C:/Test';
   
4. my $row = Test::get_row($0);
   
5. while ( $row->() ) { print; }
   

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1. # 这样调用输出正常
   
2. use strict;
   
3. use lib 'C:/Test';
   
4. my $row = Test::get_row($0);
   
5. while ( my $line = $row->() ) { print $line; }
   

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百思不得其解，请高手指教，谢谢！！
忘记说了，我的Perl版本是草莓发行版 5.18.1，32位，操作系统是Windows XP sp3。

104359176

局部变量的引用，不能用 $_; 好好写代码吧，别再追求酷了，要不就该@哭@了。

wxlfh

看不懂，真心请教，能不能举个例子说明一下？

104359176 发表于 2014-04-13 09:47
局部变量的引用，不能用 $_; 好好写代码吧，别再追求酷了，要不就该@哭@了。

104359176

http://perldoc.perl.org/perl5180 ... is-now-experimental

Lexical $_ is now experimental
Since it was introduced in Perl v5.10, it has caused much confusion with no obvious solution:
Various modules (e.g., List::Util) expect callback routines to use the global $_ . use List::Util 'first'; my $_; first { $_ == 1 } @list does not work as one would expect.
A my $_ declaration earlier in the same file can cause confusing closure warnings.
The "_" subroutine prototype character allows called subroutines to access your lexical $_ , so it is not really private after all.
Nevertheless, subroutines with a "(@)" prototype and methods cannot access the caller's lexical $_ , unless they are written in XS.
But even XS routines cannot access a lexical $_ declared, not in the calling subroutine, but in an outer scope, iff that subroutine happened not to mention $_ or use any operators that default to $_ .
It is our hope that lexical $_ can be rehabilitated, but this may cause changes in its behavior. Please use it with caution until it becomes stable.

wxlfh

谢谢楼上。

ba_du_co

test this code
输出: 百思不得其解.....

1. use strict;
   
2. use lib 'C:/Test.pm';
   
3. my $row = Test::get_row($0);
   
4. $_ = "百思不得其解\n";  # for test
   
5. while ( $row->() ) { print; }

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print;    # => print $_;

while ( $row->() ) { print; }
# while $row->() true do print $_   
# $_ != $row->()
# $_ no value => 输出为空

# 这样调用输出正常

while ( $_ = $row->() ) { print; }
# $_ = $row->()
# if $_ true then print $_
# print $_;

104359176

回复 6# ba_du_co


    This is new "特性" of Version 5.018;

zhlong8

回复 4# 104359176


    你贴这个不太合适，不建议的是 my $_ ，这个我都没见人用过。我们平时用的都是全局变量 $_ 即 $main::_ 用这个没什么问题，你严格要求自己不用 $_ 也行。

    他的问题是认为 while 会把条件判断的值赋值给 $_，事实并不是这样，while 只是特殊处理 while (<sth>){} 或 ... while <sth> 这个固定模式，会自动插入代码变成 while (defined($_ = <sth>)){}  见 perop #I/O-Operators

wxlfh

版主的建议是中肯的，我以前就是这样认为的。

104359176

回复 8# zhlong8


    你说的很对，我的帖子岂止是不适合，简直就是误导。我个人基本不用这个变量。怕过段时间，自己看不懂。