全局变量

Global variables

全局变量

A global variable has a name beginning with $.

全局变量的名字以$开头。

It can be referred to from anywhere in a program. Before initialization, a global variable has the special value nil.

你可以在程序的任何地方引用全局变量。在对其进行初始化之前，一个全局变量的值是特殊的nil值。

ruby> $foo
   nil
ruby> $foo = 5
   5
ruby> $foo
   5

Global variables should be used sparingly.

应当谨慎使用全局变量。

They are dangerous because they can be written to from anywhere.

全局变量非常危险，因为它们可以在任何地方被改写。

Overuse of globals can make isolating bugs difficult; it also tends to indicate that the design of a program has not been carefully thought out.

过度使用全局变量会使隔离Bug变得很困难;它也倾向于表明一个程序的设计并没有经过仔细的考虑。

Whenever you do find it necessary to use a global variable, be sure to give it a descriptive name that is unlikely to be inadvertently used for something else later (calling it something like $foo as above is probably a bad idea).

无论何时，当你发现确实有必要使用全局变量时，一定要给它一个描述性的名称，这个名称不太可能在以后因为其他事情而无意中使用它(将其命名为$foo很可能是一个坏主意)。

One nice feature of a global variable is that it can be traced; you can specify a procedure which is invoked whenever the value of the variable is changed.

全局变量的一个很好的特性是它可以被跟踪;你可以指定一个过程，该过程在变量的值被更改时被调用。

ruby> trace_var :$x, proc{puts "$x is now #{$x}"}
   nil
ruby> $x = 5
$x is now 5
   5

When a global variable has been rigged to work as a trigger to invoke a procedure whenever changed, we sometimes call it an active variable.

当一个全局变量被操纵，作为触发器来调用一个过程时，我们有时会把它称为一个活动变量。

For instance, it might be useful for keeping a GUI display up to date.

例如，对于保持GUI展示最新内容，全局变量可能有用。

There is a collection of special variables whose names consist of a dollar sign ($) followed by a single character.

有一组特殊的变量，它们的名字由一个美元符号($)组成，后面跟着一个字符。

For example, $$ contains the process id of the ruby interpreter, and is read-only. Here are the major system variables:

例如，$$包含Ruby解释器的进程id，并且是只读的。以下是主要的系统变量:

In the above, $_ and $~ have local scope.

上面的$_和$~都有局部作用域。

Their names suggest they should be global, but they are much more useful this way, and there are historical reasons for using these names.

它们的名称表明它们应该是全局的，但是它们在这种方式中更有用，使用这些名字有一些历史原因。

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