die
【文法】
die リスト
【説明】
Outside an `eval', prints the value of LIST to
`STDERR' and exits with the current value of `$!'
(errno). If `$!' is `0', exits with the value of
`($? >> 8)' (backtick `command` status). If `($?
>> 8)' is `0', exits with `255'. Inside an
`eval(),' the error message is stuffed into `$@'
and the `eval' is terminated with the undefined
value. This makes `die' the way to raise an
exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline,
the current script line number and input line
number (if any) are also printed, and a newline is
supplied. Note that the "input line number" (also
known as "chunk") is subject to whatever notion of
"line" happens to be currently in effect, and is
also available as the special variable `$.'. See
the section on "$/" in the perlvar manpage and the
section on "$." in the perlvar manpage.
Hint: sometimes appending `", stopped"' to your
message will cause it to make better sense when
the string `"at foo line 123"' is appended.
Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and `$@' already contains a value
(typically from a previous eval) that value is
reused after appending `"\t...propagated"'. This
is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If `$@' is empty then the string `"Died"' is used.
die() can also be called with a reference
argument. If this happens to be trapped within an
eval(), $@ contains the reference. This behavior
permits a more elaborate exception handling
implementation using objects that maintain
arbitary state about the nature of the exception.
Such a scheme is sometimes preferable to matching
particular string values of $@ using regular
expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception
messages before displaying them, you may want to
overload stringification operations on such custom
exception objects. See the overload manpage for
details about that.
You can arrange for a callback to be run just
before the `die' does its deed, by setting the
`$SIG{__DIE__}' hook. The associated handler will
be called with the error text and can change the
error message, if it sees fit, by calling `die'
again. See the section on "$SIG{expr}" in the
perlvar manpage for details on setting `%SIG'
entries, and the section on "eval BLOCK" for some
examples. Although this feature was meant to be
run only right before your program was to exit,
this is not currently the case--the
`$SIG{__DIE__}' hook is currently called even
inside eval()ed blocks/strings! If one wants the
hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see the section
on "$^S" in the perlvar manpage). Because this
promotes strange action at a distance, this
counterintuitive behavior may be fixed in a future
release.
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