如何在Perl中访问名称包含在变量中的常量? [英] How do I access a constant in Perl whose name is contained in a variable?

问题描述

我在Perl中声明了一组常量:

I have a set of constants declared in Perl:

   use constant C1 => 111;
   use constant C2 => 222;
   ..
   use constant C9 => 999;
   my $which_constant = "C2";

我如何构造一个基于$which_constant的Perl表达式，该表达式派生以该变量的值命名的常量的值-例如"222".

How do I construct a Perl expression which, based on $which_constant, derives the value of a constant named with the value of this variable - e.g. "222".

请注意，我不能更改上述任何条件-它们是实际情况的简化:我有一个模块(我无法控制)，可以从中导入这些常量.常量之一的名称由用户从命令行提供.我需要访问适当的常量的值.

Please note that I can not change any of the conditions above - they are a simplification of a real scenario: I have a module (which I have no control over) from which these constants are imported. The `name of one of the constants is supplied by the user from command line. I need to access the appropriate constants' value.

我一直在撞墙(主要是在各种各样的怪异的glob构造周围)，但没有一个起作用.

I've been beating my head against the wall (mostly around all sorts of weird glob constructs) but none of them work.

P.S.如果解决方案访问其本机模块中的常量-说My::Constants::C2(无需导入)，则更好，但不是必须的-我可以使用My::Constants->import($which_constant)轻松地将正确的常量导入到main::中.是的，最重要的是，默认情况下不会导出te常量，因此需要显式import()调用.

P.S. If the solution accesses the constants inside their native module - say My::Constants::C2 (without needing to import them), even better, but not necessary - I can import the correct constants into main:: easily using My::Constants->import($which_constant). and yes, to top it off, te constants are NOT exported by default thus needing the explicit import() call.

我尝试过的一些事情:

• main::$which_constant-语法错误

main::${which_constant}-语法错误

${*$which_constant}-返回空值

*$which_constant-返回"* main :: C2"

*$which_constant - returns "*main::C2"

${*${*which_constant}}-空

推荐答案

由 constant.pm定义的常量只是子例程.如果字符串中包含常量的名称，则可以使用方法调用语法:

Constants defined by constant.pm are just subroutines. You can use method invocation syntax if you have the name of the constant in a string:

#!/usr/bin/perl -l

use strict; use warnings;
use constant C1 => 111;
use constant C2 => 222;

print __PACKAGE__->$_ for qw( C1 C2 );
# or print main->$_ for qw( C1 C2 );

这样，如果您尝试使用未定义的常量，则会收到错误消息.

This way, if you try to use a constant which is not defined, you will get an error.

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