Perl 的 reference

上網看了些入門文件, 自己試一陣子, 總算弄懂 Perl 的 reference, 不懂這個東西無法好好地寫函式。貼個程式備忘, 到頭來, 還是精簡的程式範例最好懂。主要是參考Perl 學習手札 - 14. 參照 (Reference)。

Perl 的參考和 C 的指標差不多, {$p} 相當於 C 的 *p 取出指標指向的變數, ${$p}{key} 寫起來太麻煩, 可改用 $p->{key} 比較好寫, 這點也和 C 的 -> 同樣。取參考用 \ 類似 C 的 &, 像是 \%h 和 \@a 各別是取出雜湊 %h 和陣列 @a 的參考。另外要注意的是 Perl 會看 @、% 決定變數的意義, 等號左邊用什麼符號, 就決定它的語境。

另外 Perl 有 GC 的機制, 不用擔心從函式內傳回參考後, 資料會不見。

陣列和參考

@a = qw/x y z/;
print @a, "\n";                        # xyz

# reference of an array
$p = \@a;
print $p, "\n";                        # ARRAY(0x605050)
print "\$p[0]: ", $p[0], "\n";         # $p[0]:
print "\${\$p}[0]: ", ${$p}[0], "\n";  # ${$p}[0]: x
print "\$p->[0]: ", $p->[0], "\n";     # $p->[0]: x
unshift @{$p}, "w";
print @{$p}, "\n";                     # wxyz

# []
$p = [];
print "\$p: ", $p, "\n";               # $p: ARRAY(0x6326a0)
push @{$p}, "q";
print ${$p}[0], "\n";                  # q
print $p->[0], "\n";                   # q

$p2 = [];
print "\$p2: ", $p2, "\n";             # $p2: ARRAY(0x6326c0)

# subroutine
sub foo {
    $pp = shift;
    print @{$pp}, "\n";                # wxyz
}

foo(\@a);

雜湊和參考

# two ways to initialize a hash.
%h = qw(k1 v1 k2 v2);
print %h, "\n";          # k2v2k1v1
print $h{k1}, "\n";      # v1

%h2 = (
    k1 => 'v1',
    k2 => 'v2',
);
print "--------------\n";
print %h2, "\n";         # k2v2k1v1
print $h2{k1}, "\n";     # v1

# {} generates a reference of hash.
$h3 = {
    k1 => 'v1',
    k2 => 'v1',
};
print "--------------\n";
print $h3, "\n";        # HASH(0x6327c0)
print %{$h3}, "\n";     # k2v2k1v1
print $h3->{k1}, "\n";  # v1

# subroutine.
sub foo {
    $p = shift;
    print "--------------\n";
    print $p, "\n";
    print %{$p}, "\n";
}

# get the reference of %h.
foo(\%h);               # HASH(0x605050)
                        # k2v2k1v1
# #h3 is already a reference.
foo($h3);               # HASH(0x6327c0)
                        # k2v2k1v1