User Guide #20 - Ruby - global variables

global variables, Ruby, ruby lesson, ruby уроки, user guide, глобальные переменные, руководство пользователя

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

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

Global variables should be used sparingly. They are dangerous because they can be written to from anywhere. 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. 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).

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.

ruby> trace_var :$x, proc{print "$x is now ", $x, "\n"}
   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 . For instance, it is useful for keeping a GUI display up to date.

There is a collection of special variables whose names consist of a dollar sign ( $ ) followed by a single character. For example, $$ contains the process id of the ruby interpreter, and is read-only. Here are the major system variables and their meanings:

  • $! - latest error message
  • $@ - location of error
  • $_ - string last read by gets
  • $. - line number last read by interpreter
  • $& - string last matched by regexp
  • $~ - the last regexp match, as an array of subexpressions
  • $ n - the nth subexpression in the last match (same as $~[ n ])
  • $= - case-insensitivity flag
  • $/ - input record separator
  • $\ - output record separator
  • $0 - the name of the ruby script file
  • $* - the command line arguments
  • $$ - interpreter's process ID
  • $? - exit status of last executed child process

In the above, $_ and $~ have local scope. Their names suggest they should be global, but they are much more useful this way, and there are historical reasons for using these names.

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Feb. 17, 2020, 3:22 a.m.
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Добрый день. Это кастомный тег, помещается в файл, который находится в каталоге templatetags myapp/ templatetags/ myapp.py
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Feb. 16, 2020, 1:36 p.m.
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Добрый вечер! Монжно по подробней о теге get_companion? ссылка не работает.
Feb. 16, 2020, 9:35 a.m.
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Добрый день. На GitHub исходники, можете посмотреть в официальном репозитории
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Feb. 16, 2020, 9:29 a.m.
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Здравстсвуйте Евгений, непонятно мне где эти исходники найти?
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