Perl 内部变量.下一个表达式是否相同? [英] Perl internal variables. Are next expressions same?
问题描述
我有两个表达:
${ 'main::' }{ "_<$filename" }
和
${ "::_<$filename" }
这两个表达式一样吗?
推荐答案
否，但以下内容是等效的:
No, but the following are equivalent:
${"_<$filename"} # When this expression is found in package main
${"::_<$filename"}
${"main::_<$filename"}
${ $::{"_<$filename"} }
${ ${'::'}{"_<$filename"} }
${ $main::{"_<$filename"} }
${ ${'main::'}{"_<$filename"} }
这些是指根/主命名空间中的变量，其名称由 "_<$filename" 生成.
These are refer to the variable in the root/main namespace whose name is produced by "_<$filename".
举个更清楚的例子，以下都是等价的(假设前两个代码是在Foo::Bar包中编译的):
As a clearer example, the following are all equivalent (assuming the code is compiled in package Foo::Bar for the first two):
@moo # Via hardcoded, unqualified name
@{'moo'} # Via symref using unqualified name
@Foo::Bar::moo # Via hardcoded, qualified name
@{'Foo::Bar::moo'} # Via symref using qualified name
@{ *Foo::Bar::moo } # Via hardcoded glob
@{ *{'Foo::Bar::moo'} } # Via symref to glob
@{ $Foo::Bar::{moo} } # Via glob via hardcoded package
@{ ${'Foo::Bar::'}{moo} } # Via glob via symref to package
@{ $Foo::{'Bar::'}{moo} } # Via glob via hardcoded parent package
@{ ${'Foo'}::{'Bar::'}{moo} } # Via glob via symref to parent package
@{ $::{'Foo::'}{'Bar::'}{moo} } # Via glob via root package
@{ $main::{'Foo::'}{'Bar::'}{moo} } # Same
<小时>
Perl 允许使用符号名称作为引用.
Perl allows symbol names to be used as references.
$ perl -e'%h = ( a => 4, b => 5 ); my $ref = "h"; CORE::say $ref->{a};'
4
这些被称为符号引用"(并且在使用 use strict; 时是不允许的).以下使用 main:: 作为符号引用:
These are called "symbolic references" (and they're not allowed when using use strict;). The following uses main:: as a symbolic reference:
${ 'main::' }{ "_<$filename" }
然而，这样做没有意义.由于变量的名称无论如何都是硬编码的，我们不妨使用以下内容:
There's no point to doing that, however. Since the name of the variable is hardcoded anyway, we might as well use the following:
$main::{ "_<$filename" }
这不仅更简单，在使用use strict;时也是允许的.
Not only is this simpler, it's allowed when using use strict;.
但是什么是%main::?嗯，这就是 main 包的符号表.%main:: 的键是 main 包中存在的符号(变量)的名称(没有任何符号).
But what is %main::? Well, that's the symbol table for the package main. The keys of %main:: are the names of the symbols (variables) that exist in the package main (without any sigil).
%main:: 的值是符号表条目.我们称这些为typeglobs"，或简称为globs".取消引用一个 glob 就好像它是一个引用一样访问适合取消引用的类型的变量.例如，
The values of %main:: are symbol table entries. We call these "typeglobs", or "globs" for short. Dereferencing a glob as if it was a reference access the variable of the type appropriate for the dereference. For example,
$main::{a}
返回
*main::a # A glob
和
${ $main::{a} }
返回
$main::a
这意味着下面的代码返回一个 glob 到 main 命名空间中的变量，其名称由 "_<$filename"
This means that the following returns a glob to the variable in the main namespace whose name is produced by "_<$filename"
$main::{ "_<$filename" }
<小时>
主命名空间也是根命名空间.这意味着
The main namespace is also the root namespace. This means that
$main::a
和
$::a
两者都指向同一个变量.这也意味着
both refer to the same variable. It also means that
%main::
和
%::
两者都指向同一个变量.这也意味着
both refer to the same variable. It also means that
$main::{ "_<$filename" }
和
$::{ "_<$filename" }
两者都返回相同的 glob.
both return the same glob.
我们已经确定，下面的代码会返回一个指向根/主命名空间中名称由 "_<$filename"
We've established that the following returns a glob to the symbol in the root/main namespace whose name is produced by "_<$filename"
$::{ "_<$filename" }
我们还确定以下引用根目录/主目录中的标量，其名称由 "_<$filename" 生成:
We've also established that the following references the scalar in the root/main whose name is produced by "_<$filename":
${ $::{ "_<$filename" } }
最后，我们还确定以下是引用相同标量的符号引用:
And finally, we've also established that following is a symbolic reference that references the same scalar:
${ "::_<$filename" }
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