Perl中的符号 ->;、=>; 和 :: 分别表示什么意思?

转载  2017-10-09   投稿:mdxy-dxy   我要评论

这篇文章主要介绍了Perl中的符号 ->;、=>; 和 :: 分别表示什么意思,需要的朋友可以参考下

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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