Perl中的符號 ->、=> 和 :: 分別表示什麼意思？

What do the ->, => and :: symbols mean?

　　The -> is the "infix dereference operator". In other words it is the means by which one calls a sub with a pass by reference (among other things you can do with ->). As stated above most things in calls to perl/Tk routines are passed by reference. The -> is used in perl just as in C or C++. (Most of the widget primitives are elements of the Tk:: "perl class".) A simple example of dereferencing would be: $x = { def => bar }; # $x is a reference to an anon. hash print $x->{def},"/n"; # prints ``bar''

　　Note that in the case of calling perl/Tk subs there may be more than one way to call by reference. Compare my($top) = MainWindow->new;

　　with my($top) = new MainWindow;

　　But in general you will be making extensive use of calls like: $top -> Widge-type;

　　There is a clear and succint discussion of references, dereferences, and even closures in man perlref(1) or see the perl 5 info page at: http://www.metronet.com/perlinfo/perl5.html

　　The use of the => operator is quite common in perl/Tk scripts. Quoting from man perlop(1):

　　The => digraph is simply a synonym for the comma operator. It's useful for documenting arguments that come in pairs.

　　You could say that => is used for aesthetic or organizational reasons. Note in the following how hard it is to keep track of whether or not every -option has an argument: $query -> Button(-in,/$reply,-side,'left',-padx,2m,-pady, 2m,-ipadx,2m,-ipady,1m)->pack(-side,'bottom');

　　As opposed to: $query ->Button( -in => /$reply, -side => 'left', -padx => 2m, -pady => 2m, -ipadx => 2m, -ipady => 1m )->pack(-side => 'bottom');

　　By the way if you wanted the numeric "greater than or equal" you would use >= not =>.

　　While the :: symbol can be thought of as similar to the period in a C struct, it is much more akin to the :: class scope operator in C++: a.b.c; /* something in C */ a::b::c(); // function in C++ $a::b::c; # a scalar in Perl 5 @a::b::c; # a list in Perl 5 %a::b::c; # an associative array or "hash" in Perl 5 &a::b::c; # a function in Perl 5

　　It is also analogous to the single forward quotation mark in perl 4: $main'foo; # a $foo scalar in perl 4 $main::foo; # a $foo scalar in Perl 5

　　For backward compatibility perl 5 allows you to refer to $main'foo but $main::foo is recommended.

　　譯文：

　　符號->，=>和::分別表示什麼意思？

　　‘- >’符號是“插入式解引用操作符”（infix dereference operator）。換句話說，它是調用由引用傳遞參數的子程序的方法（當然，還有其它的作用）。正如我們上面所提到的，在調用Perl/Tk的函數的時候，大部分參數都是通過引用傳遞的。Perl中的‘->’功能就和它們在C或C++中一樣。（大部分原始的組件都是Tk中的Perl類的元素。）下面是一個簡單的解引用的例子：

　　$x = { def => bar }; # $x是指向一個匿名hash的引用

　　print $x->{def},"/n"; # 輸出``bar''

　　注意，在調用Perl/Tk的子程序時有多種不同的方法進行引用。我們可以比較一下：

　　my($top) = MainWindow->new;

　　和

　　my($top) = new MainWindow;

　　兩種方法的不同。

　　但是，一般來說我們通常都使用這樣的方法調用：

　　$top -> Widge-type;

　　在perlref的手冊頁中有詳盡的關於引用、解引用、和閉包的討論，或者也可以在下面的網頁上查看Perl5的信息頁：

　　http://www.metronet.com/perlinfo/perl5.html

　　在Perl/Tk的腳本中‘=>’操作符時很常見的。perlop手冊頁中說：關係操作符=>只是逗號操作符的替代物，它在顯示成對的參數時非常有用。

　　你可以認爲=>只是爲了程序的美觀和易維護而被使用的。請看，在下面的例子中，要想監測是否每個選項都有對應的值，是多麼的困難：

　　$query -> Button(-in,/$reply,-side,'left',-padx,2m,-pady,

　　2m,-ipadx,2m,-ipady,1m)->pack(-side,'bottom');

　　而下面的這個則相反：

　　$query ->Button( -in => /$reply,

　　-side => 'left',

　　-padx => 2m,

　　-pady => 2m,

　　-ipadx => 2m,

　　-ipady => 1m

　　)->pack(-side => 'bottom');

　　順便說一下，如果你需要用數字“大於等於”的符號，你應該用“>=”而不是“=>”。

　　“::”符號可以認爲是與C語言中的“.”相似的，而它更像C++中的::類範圍操作符。

　　a.b.c; /* C語言中的 */

　　a::b::c(); // C++ 中的函數

　　$a::b::c; # Perl 5中的標量

　　@a::b::c; # Perl 5中的列表

　　%a::b::c; # Perl 5中的關聯數組（或叫hash）

　　&a::b::c; # Perl 5中的函數

　　另外，Perl4中的單撇號也具有相同的功能：

　　$main'foo; # Perl 4中的標量$foo

　　$main::foo; # Perl 5中的標量$foo

　　出於向後兼容的考慮，Perl5也運行使用$main'foo，但是仍推薦使用$main::foo。