Perl中的符号 -;、=; 和 :: 分别表示什么意思？

技术  /  管理员 发布于 6年前   358

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。