如何从 Perl 的 STDOUT 取消别名? [英] How do I unalias from Perl's STDOUT?

问题描述

当我运行这个时:

open FP, ">xyz";

my $file = *FP;

printf $file "first\n";

$file = *STDOUT;

printf $file "second\n";

open $file, ">abc";

print $file "third\n";

print STDOUT "fourth\n";

print FP "fifth\n";

第四个"打印不会转到 STDOUT，而是转到abc".

The "fourth" print does not go to STDOUT, rather to "abc".

STDOUT 与 FP 不同，其行为符合预期.

STDOUT is different from FP which behaves as expected.

我做错了什么?我有什么不明白的?

What am I doing wrong? What am I not understanding?

推荐答案

好吧，首先，您错误地使用了打开".

Well, for starters, you're using 'open' wrongly.

open my $fp , '>', 'xyz' ;

是推荐的语法.

强烈建议您不要使用裸露的FP"，因为它不是词法.

the bare 'FP' you have there is strongly recommended against, as it is not a lexical.

其次，您将文件指针作为新事物重新打开.这不是一个很好的做法，它不应该是一个问题，但它只是一个坏主意.您应该关闭文件指针或让它超出范围(通过词法).

Secondly, you're re-opening file-pointers as new things. This is not very good practice, it shouldn't be a problem, but its just a bad idea. You should close the file pointer or let it run out of scope ( via a lexical ).

第三，'*STDOUT' 是一个引用.

Thirdly, '*STDOUT' is a reference.

my $fh = *STDOUT; 
print "$fh\n";   #prints  '*main::STDOUT';

所以当你这样做时:

open $fh, '>abc'; 

你在做什么

open *STDOUT, '>abc'; 

如果你之后立即做

print "$fh\n"; 

你会注意到它仍然打印 *main::STDOUT;

you will note it still prints *main::STDOUT;

一些有趣的代码片段可以解决这个问题:

Some interesting code snippets that clear this up:

my $fh = *STDOUT;
open $fh, '<', "foo.txt"; 
print $fh "hello";
# Filehandle STDOUT opened only for input at (eval 288) line 6.

my $fh = *STDIN;
open $fh, '<', "foo.txt"; 
print <>; 
# contents of foo.txt here 

以下是使用 open 的推荐方法:

Here's a recommended way to use open:

sub foo { 
    my $fh;
    open $fh , '<', 'file.txt' or Carp::croak('Cannot Open File.txt'); 
    # do stuff with $fh; 
    close $fh or Carp::carp('Something annoying in close :S '); 
}

请注意，如果您省略关闭，则一旦 $fh 不可见，该文件将立即关闭.

Note that if you omit close, the file will be closed as soon as $fh goes out of visibility.

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