Perl是否有类似于PHP常量()的内容?

你怎么了 Readonly

如果它太慢,你可以补充它 Readonly:XS . 但是如果你不喜欢 只读 ,总是有老的 constant

use constant PI => 3.14159265;

记住

1. 它们像潜艇一样工作,因此不需要做功就不会插值。

2. 如果要在一条语句中创建多个常量,则需要传递哈希引用。

   use constant { PI => 3.14159265
                , E  => 2.71828183
                };
   

从你的例子来看:

从您的示例判断,没有理由使用只读 搞砸 不能做同样的事情。

Readonly::Hash my %field_example => { L => 25, O => 345 };

然后你可以在任何你想填充常数的地方使用它:

print "The example is $field_example{$var}\n";

Readonly::Hash my %field 
    => { example => { L => 25,     O => 345 }
       , name    => { L => 'Lion', O => 'ocelot' }
       };

这样说吧:

$field{$var}{L};

如果你不想让一门语言以另一种方式去做它有更好支持的事情,你会得到很多好处。

与PHP同源 常数

但是,如果您想这样做,那么我的建议是,下面的sub也是这样做的(并且避免 eval

sub read_constant { 
    use Symbol qw<qualify_to_ref>;
    my $name = join( '', @_ ); # no need to concatenate before passing
    my $constant;
    # use the first that works: calling package and then "" (main)
    for my $pkg ( scalar( caller ), "" ) { 
        # get symbol reference
        my $symb_ref = qualify_to_ref( $name, $pkg );
        # get the code slot
        $constant    = *{$symb_ref}{CODE};
        last if $constant;
    }
    return unless $constant;
    # call the sub named
    return $constant->();
}

你可以这样称呼它:

$value = read_constant( 'FIELD_', $var, 'L' );

Carp::croak "Invalid constant name '$name'" if $name =~ /[^\p{UpperCase}_]/;

你可以用 constant .

use constant PI    => 4 * atan2(1, 1);
use constant DEBUG => 0;

print "Pi equals ", PI, "...\n" if DEBUG;

use constant {
    SEC   => 0,
    MIN   => 1,
    HOUR  => 2,
    MDAY  => 3,
    MON   => 4,
    YEAR  => 5,
    WDAY  => 6,
    YDAY  => 7,
    ISDST => 8,
};

use constant WEEKDAYS => qw(
    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
);

print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";

或者你可以用 Readonly .

use Readonly;

# Read-only scalar
Readonly::Scalar     $sca => $initial_value;
Readonly::Scalar  my $sca => $initial_value;

# Read-only array
Readonly::Array      @arr => @values;
Readonly::Array   my @arr => @values;

# Read-only hash
Readonly::Hash       %has => (key => value, key => value, ...);
Readonly::Hash    my %has => (key => value, key => value, ...);
# or:
Readonly::Hash       %has => {key => value, key => value, ...};

# You can use the read-only variables like any regular variables:
print $sca;
$something = $sca + $arr[2];
next if $has{$some_key};

# But if you try to modify a value, your program will die:
$sca = 7;
push @arr, 'seven';
delete $has{key};
# The error message is "Modification of a read-only value attempted"

对看见 perldoc constant .

这是我的建议 constant 您正在寻找的功能:

sub constant
{
  no strict 'refs';
  shift->();                    # Call the supplied function by name
} # end constant

constant pragma只是子例程,按名称调用子例程很容易。

这里有一个更高级的,即使您从不同的包调用它,它仍然可以工作:

sub constant
{
  my $constant = shift;
  $constant = caller() . "::$constant" unless $constant =~ /::/;
  no strict 'refs';
  $constant->();                # Call function by name
} # end constant

您可以使用“use constant”杂注,但以这种方式定义的常量不像普通变量那样被引用。不过,它们确实让您获得了常数带来的性能提升。如果您不寻求使用常量的编译时优化,那么最好的方法可能是在编写代码时使用规程,而不是分配给不应该分配给的变量。:)

通过查看您发布的示例代码,我感觉您可能正在寻找 Hash::Util::lock_hash 各位朋友:

#!/usr/bin/perl

use strict;
use warnings;

use Hash::Util qw(lock_hash);

my %constant = (
    FIELD_EXAMPLE_O => 345,
    FIELD_EXAMPLE_L => 25,
);

lock_hash %constant;

my $var = 'EXAMPLE';

print $constant{"FIELD_${var}_L"}, "\n";

输出:

C:\Temp> xuk
25

使用以下格式:

use constant DFLT_MIN_LENGTH_PWD => 6;