和::分别表示什么意思？

王朝other·作者佚名 2008-05-18

原文：

12.1. 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：

12.1. 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。