Ruby - Global variables
| | Ruby user's guide | Global variables | |
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,
$$ contains the process id of the ruby interpreter, and is
read-only. Here are the major system variables and their meanings
(see the ruby
reference manual
for details):
$! | latest error message |
$@ | location of error |
$_ | string last read by gets |
$. | line number last read by interpreter |
$& | string last matched by regexp |
$~ | the last regexp match, as an array of subexpressions |
$n | the nth subexpression in the last match (same as $~[n]) |
$= | case-insensitivity flag |
$/ | input record separator |
$\ | output record separator |
$0 | the name of the ruby script file |
$* | the command line arguments |
$$ | 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.
