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。
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