Perl中的符号 ->;、=>; 和 :: 分别表示什么意思？

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.

　　译文：

　　符号->，=>和::分别表示什么意思？

　　‘- >'符号是“插入式解引用操作符”（infix dereference operator）。换句话说，它是调用由引用传递参数的子程序的方法（当然，还有其它的作用）。正如我们上面所提到的，在调用perl/tk的函数的时候，大部分参数都是通过引用传递的。perl中的‘->'功能就和它们在c或c++中一样。（大部分原始的组件都是tk中的perl类的元素。）下面是一个简单的解引用的例子：

　　$x = { def => bar }; # $x是指向一个匿名hash的引用

　　print $x->{def},"/n"; # 输出``bar''

　　注意，在调用perl/tk的子程序时有多种不同的方法进行引用。我们可以比较一下：

　　my($top) = mainwindow->new;

　　和

　　my($top) = new mainwindow;

　　两种方法的不同。

　　但是，一般来说我们通常都使用这样的方法调用：

　　$top -> widge-type;

　　在perlref的手册页中有详尽的关于引用、解引用、和闭包的讨论，或者也可以在下面的网页上查看perl5的信息页：

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

　　在perl/tk的脚本中‘=>'操作符时很常见的。perlop手册页中说：关系操作符=>只是逗号操作符的替代物，它在显示成对的参数时非常有用。

　　你可以认为=>只是为了程序的美观和易维护而被使用的。请看，在下面的例子中，要想监测是否每个选项都有对应的值，是多么的困难：

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

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

　　而下面的这个则相反：

　　$query ->button( -in => /$reply,

　　-side => 'left',

　　-padx => 2m,

　　-pady => 2m,

　　-ipadx => 2m,

　　-ipady => 1m

　　)->pack(-side => 'bottom');

　　顺便说一下，如果你需要用数字“大于等于”的符号，你应该用“>=”而不是“=>”。

　　“::”符号可以认为是与c语言中的“.”相似的，而它更像c++中的::类范围操作符。

　　a.b.c; /* c语言中的 */

　　a::b::c(); // c++ 中的函数

　　$a::b::c; # perl 5中的标量

　　@a::b::c; # perl 5中的列表

　　%a::b::c; # perl 5中的关联数组（或叫hash）

　　&a::b::c; # perl 5中的函数

　　另外，perl4中的单撇号也具有相同的功能：

　　$main'foo; # perl 4中的标量$foo

　　$main::foo; # perl 5中的标量$foo

　　出于向后兼容的考虑，perl5也运行使用$main'foo，但是仍推荐使用$main::foo。