a2p
accept
access
acct
addftinfo
addr2line
adjtime
afmtodit
after
aio_cancel
aio_error
aio_read
aio_return
aio_suspend
aio_waitcomplete
aio_write
alias
aliases
alloc
anvil
append
apply
apropos
ar
array
as
asa
asn1parse
at
atq
atrm
attemptckalloc
attemptckrealloc
authlib
authtest
autopoint
awk
b64decode
b64encode
basename
batch
bc
bdes
bell
bg
bgerror
biff
big5
binary
bind
bindkey
bindtags
bindtextdomain
bio
bitmap
blowfish
bn
bootparams
bootptab
bounce
brandelf
break
breaksw
brk
bsdiff
bsdtar
bsnmpd
bspatch
bthost
btsockstat
buffer
builtin
builtins
bunzip2
button
byacc
bzcat
bzegrep
bzfgrep
bzgrep
bzip2
c2ph
c89
c99
ca
cal
calendar
canvas
cap_mkdb
case
cat
catch
catman
cc
cd
cdcontrol
chdir
checkbutton
checknr
chflags
chfn
chgrp
chio
chkey
chmod
chown
chpass
chroot
chsh
ci
ciphers
ckalloc
ckdist
ckfree
ckrealloc
cksum
cleanup
clear
clipboard
clock
clock_getres
clock_gettime
clock_settime
close
cmp
co
col
colcrt
colldef
colors
colrm
column
comm
command
compile_et
complete
compress
concat
config
connect
console
continue
core
courierlogger
couriertcpd
cp
cpan
cpio
cpp
creat
crl
crontab
crunchgen
crunchide
crypt
crypto
csh
csplit
ctags
ctm
ctm_dequeue
ctm_rmail
ctm_smail
cu
cursor
cursors
cut
cvs
date
dbiprof
dbiproxy
dc
dcgettext
dcngettext
dd
dde
default
defer
deliverquota
des
destroy
devfs
df
dgettext
dgst
dh
dhparam
dialog
diff
diff3
dig
dir
dirent
dirname
dirs
discard
disktab
dngettext
do
domainname
done
dprofpp
dsa
dsaparam
dtmfdecode
du
dup
dup2
eaccess
ec
ecdsa
echo
echotc
ecparam
ed
edit
editrc
ee
egrep
elf
elfdump
elif
else
enc
enc2xs
encoding
end
endif
endsw
engine
enigma
entry
env
envsubst
eof
eqn
err
errno
error
errstr
esac
ethers
euc
eui64
eval
event
evp
ex
exec
execve
exit
expand
export
exports
expr
extattr
extattr_delete_fd
extattr_delete_file
extattr_get_fd
extattr_get_file
extattr_set_fd
extattr_set_file
f77
false
famm
famx
fblocked
fbtab
fc
fchdir
fchflags
fchmod
fchown
fcntl
fconfigure
fcopy
fdescfs
fdformat
fdread
fdwrite
fetch
fg
fgrep
fhopen
fhstat
fhstatfs
fi
file
file2c
fileevent
filename
filetest
find
find2perl
finger
flex
flock
flush
fmt
focus
fold
font
fontedit
for
foreach
fork
format
forward
fpathconf
frame
from
fs
fstab
fstat
fstatfs
fsync
ftp
ftpchroot
ftpusers
ftruncate
futimes
g711conv
gb2312
gb18030
gbk
gcc
gcore
gcov
gdb
gencat
gendsa
genrsa
gensnmptree
getconf
getdents
getdirentries
getdtablesize
getegid
geteuid
getfacl
getfh
getfsstat
getgid
getgroups
getitimer
getlogin
getopt
getopts
getpeername
getpgid
getpgrp
getpid
getppid
getpriority
getresgid
getresuid
getrlimit
getrusage
gets
getsid
getsockname
getsockopt
gettext
gettextize
gettimeofday
gettytab
getuid
glob
global
gmake
goto
gperf
gprof
grab
grep
grid
grn
grodvi
groff
groff_font
groff_out
groff_tmac
grog
grolbp
grolj4
grops
grotty
group
groups
gunzip
gzcat
gzexe
gzip
h2ph
h2xs
hash
hashstat
hd
head
help2man
hesinfo
hexdump
history
host
hostname
hosts
hosts_access
hosts_options
hpftodit
http
hup
i386_get_ioperm
i386_get_ldt
i386_set_ioperm
i386_set_ldt
i386_vm86
iconv
id
ident
idprio
if
ifnames253
ifnames259
image
imapd
incr
indent
indxbib
info
infokey
inode
install
instmodsh
interp
intro
introduction
ioctl
ipcrm
ipcs
ipf
ipftest
ipnat
ippool
ipresend
issetugid
jail
jail_attach
jobid
jobs
join
jot
kbdcontrol
kbdmap
kcon
kdestroy
kdump
kenv
kevent
keycap
keylogin
keylogout
keymap
keysyms
kgdb
kill
killall
killpg
kinit
kldfind
kldfirstmod
kldload
kldnext
kldstat
kldsym
kldunload
klist
kpasswd
kqueue
kse
kse_create
kse_exit
kse_release
kse_switchin
kse_thr_interrupt
kse_wakeup
ktrace
label
labelframe
lam
lappend
last
lastcomm
lastlog
lchflags
lchmod
lchown
ld
ldap
ldapadd
ldapcompare
ldapdelete
ldapmodify
ldapmodrdn
ldappasswd
ldapsearch
ldapwhoami
ldd
leave
less
lesskey
lex
lgetfh
lhash
libnetcfg
library
limit
limits
lindex
link
linprocfs
linsert
lint
lio_listio
list
listbox
listen
lj4_font
lkbib
llength
lmtp
ln
load
loadfont
local
locale
locate
lock
lockf
log
logger
login
logins
logname
logout
look
lookbib
lorder
lower
lp
lpq
lpr
lprm
lptest
lrange
lreplace
ls
lsearch
lseek
lset
lsort
lstat
lsvfs
lutimes
lynx
m4
madvise
magic
mail
maildiracl
maildirkw
maildirmake
mailq
mailx
make
makeinfo
makewhatis
man
manpath
master
mc
mcedit
mcview
md2
md4
md5
mdc2
memory
menu
menubar
menubutton
merge
mesg
message
mincore
minherit
minigzip
mkdep
mkdir
mkfifo
mkimapdcert
mklocale
mknod
mkpop3dcert
mkstr
mktemp
mlock
mlockall
mmap
mmroff
modfind
modfnext
modnext
modstat
moduli
more
motd
mount
mprotect
mptable
msdos
msdosfs
msgattrib
msgcat
msgcmp
msgcomm
msgconv
msgen
msgexec
msgfilter
msgfmt
msggrep
msginit
msgmerge
msgs
msgunfmt
msguniq
mskanji
msql2mysql
msync
mt
munlock
munlockall
munmap
mv
myisamchk
myisamlog
myisampack
mysql
mysqlaccess
mysqladmin
mysqlbinlog
mysqlcheck
mysqld
mysqldump
mysqld_multi
mysqld_safe
mysqlhotcopy
mysqlimport
mysqlshow
mysql_config
mysql_fix_privilege_tables
mysql_zap
namespace
nanosleep
nawk
nc
ncal
ncplist
ncplogin
ncplogout
neqn
netconfig
netgroup
netid
netstat
networks
newaliases
newgrp
nex
nfsstat
nfssvc
ngettext
nice
nl
nm
nmount
nohup
nologin
notify
nroff
nseq
nslookup
ntp_adjtime
ntp_gettime
nvi
nview
objcopy
objdump
objformat
ocsp
od
onintr
open
openssl
opieaccess
opieinfo
opiekey
opiekeys
opiepasswd
option
options
oqmgr
pack
package
packagens
pagesize
palette
pam_auth
panedwindow
parray
passwd
paste
patch
pathchk
pathconf
pawd
pax
pbm
pcre
pcreapi
pcrebuild
pcrecallout
pcrecompat
pcrecpp
pcregrep
pcrematching
pcrepartial
pcrepattern
pcreperform
pcreposix
pcreprecompile
pcresample
pcretest
perl
perl56delta
perl58delta
perl561delta
perl570delta
perl571delta
perl572delta
perl573delta
perl581delta
perl582delta
perl583delta
perl584delta
perl585delta
perl586delta
perl587delta
perl588delta
perl5004delta
perl5005delta
perlaix
perlamiga
perlapi
perlapio
perlapollo
perlartistic
perlbeos
perlbook
perlboot
perlbot
perlbs2000
perlbug
perlcall
perlcc
perlce
perlcheat
perlclib
perlcn
perlcompile
perlcygwin
perldata
perldbmfilter
perldebguts
perldebtut
perldebug
perldelta
perldgux
perldiag
perldoc
perldos
perldsc
perlebcdic
perlembed
perlepoc
perlfaq
perlfaq1
perlfaq2
perlfaq3
perlfaq4
perlfaq5
perlfaq6
perlfaq7
perlfaq8
perlfaq9
perlfilter
perlfork
perlform
perlfreebsd
perlfunc
perlglossary
perlgpl
perlguts
perlhack
perlhist
perlhpux
perlhurd
perlintern
perlintro
perliol
perlipc
perlirix
perlivp
perljp
perlko
perllexwarn
perllinux
perllocale
perllol
perlmachten
perlmacos
perlmacosx
perlmint
perlmod
perlmodinstall
perlmodlib
perlmodstyle
perlmpeix
perlnetware
perlnewmod
perlnumber
perlobj
perlop
perlopenbsd
perlopentut
perlos2
perlos390
perlos400
perlothrtut
perlpacktut
perlplan9
perlpod
perlpodspec
perlport
perlqnx
perlre
perlref
perlreftut
perlrequick
perlreref
perlretut
perlrun
perlsec
perlsolaris
perlstyle
perlsub
perlsyn
perlthrtut
perltie
perltoc
perltodo
perltooc
perltoot
perltrap
perltru64
perltw
perlunicode
perluniintro
perlutil
perluts
perlvar
perlvmesa
perlvms
perlvos
perlwin32
perlxs
perlxstut
perror
pfbtops
pftp
pgrep
phones
photo
pic
pickup
piconv
pid
pipe
pkcs7
pkcs8
pkcs12
pkg_add
pkg_check
pkg_create
pkg_delete
pkg_info
pkg_sign
pkg_version
pkill
pl2pm
place
pod2html
pod2latex
pod2man
pod2text
pod2usage
podchecker
podselect
poll
popd
popup
posix_madvise
postalias
postcat
postconf
postdrop
postfix
postkick
postlock
postlog
postmap
postqueue
postsuper
pr
pread
preadv
printcap
printenv
printf
proc
procfs
profil
protocols
prove
proxymap
ps
psed
psroff
pstruct
ptrace
publickey
pushd
puts
pwd
pwrite
pwritev
qmgr
qmqpd
quota
quotactl
radiobutton
raise
rand
ranlib
rcp
rcs
rcsclean
rcsdiff
rcsfile
rcsfreeze
rcsintro
rcsmerge
read
readelf
readlink
readonly
readv
realpath
reboot
recv
recvfrom
recvmsg
red
ree
refer
regexp
registry
regsub
rehash
remote
rename
repeat
replace
req
reset
resolver
resource
return
rev
revoke
rfcomm_sppd
rfork
rhosts
ripemd
ripemd160
rlog
rlogin
rm
rmd160
rmdir
rpc
rpcgen
rs
rsa
rsautl
rsh
rtld
rtprio
rup
ruptime
rusers
rwall
rwho
s2p
safe
sasl
sasldblistusers2
saslpasswd2
sbrk
scache
scale
scan
sched
sched_getparam
sched_getscheduler
sched_get_priority_max
sched_get_priority_min
sched_rr_get_interval
sched_setparam
sched_setscheduler
sched_yield
scon
scp
script
scrollbar
sdiff
sed
seek
select
selection
semctl
semget
semop
send
sendbug
sendfile
sendmail
sendmsg
sendto
services
sess_id
set
setegid
setenv
seteuid
setfacl
setgid
setgroups
setitimer
setlogin
setpgid
setpgrp
setpriority
setregid
setresgid
setresuid
setreuid
setrlimit
setsid
setsockopt
settc
settimeofday
setty
setuid
setvar
sftp
sh
sha
sha1
sha256
shar
shells
shift
shmat
shmctl
shmdt
shmget
showq
shutdown
sigaction
sigaltstack
sigblock
sigmask
sigpause
sigpending
sigprocmask
sigreturn
sigsetmask
sigstack
sigsuspend
sigvec
sigwait
size
slapadd
slapcat
slapd
slapdn
slapindex
slappasswd
slaptest
sleep
slogin
slurpd
smbutil
smime
smtp
smtpd
socket
socketpair
sockstat
soelim
sort
source
spawn
speed
spinbox
spkac
splain
split
squid
squid_ldap_auth
squid_ldap_group
squid_unix_group
sscop
ssh
sshd_config
ssh_config
stab
startslip
stat
statfs
stop
string
strings
strip
stty
su
subst
sum
suspend
swapoff
swapon
switch
symlink
sync
sysarch
syscall
sysconftool
sysconftoolcheck
systat
s_client
s_server
s_time
tabs
tail
talk
tar
tbl
tclsh
tcltest
tclvars
tcopy
tcpdump
tcpslice
tcsh
tee
tell
telltc
telnet
term
termcap
terminfo
test
texindex
texinfo
text
textdomain
tfmtodit
tftp
then
threads
time
tip
tk
tkerror
tkvars
tkwait
tlsmgr
tmac
top
toplevel
touch
tput
tr
trace
trafshow
trap
troff
true
truncate
truss
tset
tsort
tty
ttys
type
tzfile
ui
ul
ulimit
umask
unalias
uname
uncomplete
uncompress
undelete
unexpand
unhash
unifdef
unifdefall
uniq
units
unknown
unlimit
unlink
unmount
unset
unsetenv
until
unvis
update
uplevel
uptime
upvar
usbhidaction
usbhidctl
users
utf8
utimes
utmp
utrace
uudecode
uuencode
uuidgen
vacation
variable
verify
version
vfork
vgrind
vgrindefs
vi
vidcontrol
vidfont
view
virtual
vis
vt220keys
vwait
w
wait
wait3
wait4
waitpid
wall
wc
wget
what
whatis
where
whereis
which
while
who
whoami
whois
window
winfo
wish
wm
write
writev
wtmp
x509
xargs
xgettext
xmlwf
xstr
xsubpp
yacc
yes
ypcat
ypchfn
ypchpass
ypchsh
ypmatch
yppasswd
ypwhich
yyfix
zcat
zcmp
zdiff
zegrep
zfgrep
zforce
zgrep
zmore
znew
_exit
__syscall
 
FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
perlref
 
PERLREF(1)	       Perl Programmers Reference Guide 	    PERLREF(1)



NAME
       perlref - Perl references and nested data structures

NOTE
       This is complete documentation about all aspects of references.	For a
       shorter, tutorial introduction to just the essential features, see
       perlreftut.

DESCRIPTION
       Before release 5 of Perl it was difficult to represent complex data
       structures, because all references had to be symbolic--and even then it
       was difficult to refer to a variable instead of a symbol table entry.
       Perl now not only makes it easier to use symbolic references to vari-
       ables, but also lets you have "hard" references to any piece of data or
       code.  Any scalar may hold a hard reference.  Because arrays and hashes
       contain scalars, you can now easily build arrays of arrays, arrays of
       hashes, hashes of arrays, arrays of hashes of functions, and so on.

       Hard references are smart--they keep track of reference counts for you,
       automatically freeing the thing referred to when its reference count
       goes to zero.  (Reference counts for values in self-referential or
       cyclic data structures may not go to zero without a little help; see
       "Two-Phased Garbage Collection" in perlobj for a detailed explanation.)
       If that thing happens to be an object, the object is destructed.  See
       perlobj for more about objects.	(In a sense, everything in Perl is an
       object, but we usually reserve the word for references to objects that
       have been officially "blessed" into a class package.)

       Symbolic references are names of variables or other objects, just as a
       symbolic link in a Unix filesystem contains merely the name of a file.
       The *glob notation is something of a symbolic reference.  (Symbolic
       references are sometimes called "soft references", but please don't
       call them that; references are confusing enough without useless syn-
       onyms.)

       In contrast, hard references are more like hard links in a Unix file
       system: They are used to access an underlying object without concern
       for what its (other) name is.  When the word "reference" is used with-
       out an adjective, as in the following paragraph, it is usually talking
       about a hard reference.

       References are easy to use in Perl.  There is just one overriding prin-
       ciple: Perl does no implicit referencing or dereferencing.  When a
       scalar is holding a reference, it always behaves as a simple scalar.
       It doesn't magically start being an array or hash or subroutine; you
       have to tell it explicitly to do so, by dereferencing it.

       Making References

       References can be created in several ways.

       1.  By using the backslash operator on a variable, subroutine, or
	   value.  (This works much like the & (address-of) operator in C.)
	   This typically creates another reference to a variable, because
	   there's already a reference to the variable in the symbol table.
	   But the symbol table reference might go away, and you'll still have
	   the reference that the backslash returned.  Here are some examples:

	       $scalarref = \$foo;
	       $arrayref  = \@ARGV;
	       $hashref   = \%ENV;
	       $coderef   = \&handler;
	       $globref   = \*foo;

	   It isn't possible to create a true reference to an IO handle (file-
	   handle or dirhandle) using the backslash operator.  The most you
	   can get is a reference to a typeglob, which is actually a complete
	   symbol table entry.	But see the explanation of the *foo{THING}
	   syntax below.  However, you can still use type globs and globrefs
	   as though they were IO handles.

       2.  A reference to an anonymous array can be created using square
	   brackets:

	       $arrayref = [1, 2, ['a', 'b', 'c']];

	   Here we've created a reference to an anonymous array of three ele-
	   ments whose final element is itself a reference to another anony-
	   mous array of three elements.  (The multidimensional syntax
	   described later can be used to access this.	For example, after the
	   above, "$arrayref->[2][1]" would have the value "b".)

	   Taking a reference to an enumerated list is not the same as using
	   square brackets--instead it's the same as creating a list of refer-
	   ences!

	       @list = (\$a, \@b, \%c);
	       @list = \($a, @b, %c);	   # same thing!

	   As a special case, "\(@foo)" returns a list of references to the
	   contents of @foo, not a reference to @foo itself.  Likewise for
	   %foo, except that the key references are to copies (since the keys
	   are just strings rather than full-fledged scalars).

       3.  A reference to an anonymous hash can be created using curly brack-
	   ets:

	       $hashref = {
		   'Adam'  => 'Eve',
		   'Clyde' => 'Bonnie',
	       };

	   Anonymous hash and array composers like these can be intermixed
	   freely to produce as complicated a structure as you want.  The mul-
	   tidimensional syntax described below works for these too.  The val-
	   ues above are literals, but variables and expressions would work
	   just as well, because assignment operators in Perl (even within
	   local() or my()) are executable statements, not compile-time decla-
	   rations.

	   Because curly brackets (braces) are used for several other things
	   including BLOCKs, you may occasionally have to disambiguate braces
	   at the beginning of a statement by putting a "+" or a "return" in
	   front so that Perl realizes the opening brace isn't starting a
	   BLOCK.  The economy and mnemonic value of using curlies is deemed
	   worth this occasional extra hassle.

	   For example, if you wanted a function to make a new hash and return
	   a reference to it, you have these options:

	       sub hashem {	   { @_ } }   # silently wrong
	       sub hashem {	  +{ @_ } }   # ok
	       sub hashem { return { @_ } }   # ok

	   On the other hand, if you want the other meaning, you can do this:

	       sub showem {	   { @_ } }   # ambiguous (currently ok, but may change)
	       sub showem {	  {; @_ } }   # ok
	       sub showem { { return @_ } }   # ok

	   The leading "+{" and "{;" always serve to disambiguate the expres-
	   sion to mean either the HASH reference, or the BLOCK.

       4.  A reference to an anonymous subroutine can be created by using
	   "sub" without a subname:

	       $coderef = sub { print "Boink!\n" };

	   Note the semicolon.	Except for the code inside not being immedi-
	   ately executed, a "sub {}" is not so much a declaration as it is an
	   operator, like "do{}" or "eval{}".  (However, no matter how many
	   times you execute that particular line (unless you're in an
	   "eval("...")"), $coderef will still have a reference to the same
	   anonymous subroutine.)

	   Anonymous subroutines act as closures with respect to my() vari-
	   ables, that is, variables lexically visible within the current
	   scope.  Closure is a notion out of the Lisp world that says if you
	   define an anonymous function in a particular lexical context, it
	   pretends to run in that context even when it's called outside the
	   context.

	   In human terms, it's a funny way of passing arguments to a subrou-
	   tine when you define it as well as when you call it.  It's useful
	   for setting up little bits of code to run later, such as callbacks.
	   You can even do object-oriented stuff with it, though Perl already
	   provides a different mechanism to do that--see perlobj.

	   You might also think of closure as a way to write a subroutine tem-
	   plate without using eval().	Here's a small example of how closures
	   work:

	       sub newprint {
		   my $x = shift;
		   return sub { my $y = shift; print "$x, $y!\n"; };
	       }
	       $h = newprint("Howdy");
	       $g = newprint("Greetings");

	       # Time passes...

	       &$h("world");
	       &$g("earthlings");

	   This prints

	       Howdy, world!
	       Greetings, earthlings!

	   Note particularly that $x continues to refer to the value passed
	   into newprint() despite "my $x" having gone out of scope by the
	   time the anonymous subroutine runs.	That's what a closure is all
	   about.

	   This applies only to lexical variables, by the way.	Dynamic vari-
	   ables continue to work as they have always worked.  Closure is not
	   something that most Perl programmers need trouble themselves about
	   to begin with.

       5.  References are often returned by special subroutines called con-
	   structors.  Perl objects are just references to a special type of
	   object that happens to know which package it's associated with.
	   Constructors are just special subroutines that know how to create
	   that association.  They do so by starting with an ordinary refer-
	   ence, and it remains an ordinary reference even while it's also
	   being an object.  Constructors are often named new() and called
	   indirectly:

	       $objref = new Doggie (Tail => 'short', Ears => 'long');

	   But don't have to be:

	       $objref	 = Doggie->new(Tail => 'short', Ears => 'long');

	       use Term::Cap;
	       $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });

	       use Tk;
	       $main	= MainWindow->new();
	       $menubar = $main->Frame(-relief		    => "raised",
				       -borderwidth	    => 2)

       6.  References of the appropriate type can spring into existence if you
	   dereference them in a context that assumes they exist.  Because we
	   haven't talked about dereferencing yet, we can't show you any exam-
	   ples yet.

       7.  A reference can be created by using a special syntax, lovingly
	   known as the *foo{THING} syntax.  *foo{THING} returns a reference
	   to the THING slot in *foo (which is the symbol table entry which
	   holds everything known as foo).

	       $scalarref = *foo{SCALAR};
	       $arrayref  = *ARGV{ARRAY};
	       $hashref   = *ENV{HASH};
	       $coderef   = *handler{CODE};
	       $ioref	  = *STDIN{IO};
	       $globref   = *foo{GLOB};
	       $formatref = *foo{FORMAT};

	   All of these are self-explanatory except for *foo{IO}.  It returns
	   the IO handle, used for file handles ("open" in perlfunc), sockets
	   ("socket" in perlfunc and "socketpair" in perlfunc), and directory
	   handles ("opendir" in perlfunc).  For compatibility with previous
	   versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO},
	   though it is deprecated as of 5.8.0.  If deprecation warnings are
	   in effect, it will warn of its use.

	   *foo{THING} returns undef if that particular THING hasn't been used
	   yet, except in the case of scalars.	*foo{SCALAR} returns a refer-
	   ence to an anonymous scalar if $foo hasn't been used yet.  This
	   might change in a future release.

	   *foo{IO} is an alternative to the *HANDLE mechanism given in "Type-
	   globs and Filehandles" in perldata for passing filehandles into or
	   out of subroutines, or storing into larger data structures.	Its
	   disadvantage is that it won't create a new filehandle for you.  Its
	   advantage is that you have less risk of clobbering more than you
	   want to with a typeglob assignment.	(It still conflates file and
	   directory handles, though.)	However, if you assign the incoming
	   value to a scalar instead of a typeglob as we do in the examples
	   below, there's no risk of that happening.

	       splutter(*STDOUT);	   # pass the whole glob
	       splutter(*STDOUT{IO});	   # pass both file and dir handles

	       sub splutter {
		   my $fh = shift;
		   print $fh "her um well a hmmm\n";
	       }

	       $rec = get_rec(*STDIN);	   # pass the whole glob
	       $rec = get_rec(*STDIN{IO}); # pass both file and dir handles

	       sub get_rec {
		   my $fh = shift;
		   return scalar <$fh>;
	       }

       Using References

       That's it for creating references.  By now you're probably dying to
       know how to use references to get back to your long-lost data.  There
       are several basic methods.

       1.  Anywhere you'd put an identifier (or chain of identifiers) as part
	   of a variable or subroutine name, you can replace the identifier
	   with a simple scalar variable containing a reference of the correct
	   type:

	       $bar = $$scalarref;
	       push(@$arrayref, $filename);
	       $$arrayref[0] = "January";
	       $$hashref{"KEY"} = "VALUE";
	       &$coderef(1,2,3);
	       print $globref "output\n";

	   It's important to understand that we are specifically not derefer-
	   encing $arrayref[0] or $hashref{"KEY"} there.  The dereference of
	   the scalar variable happens before it does any key lookups.	Any-
	   thing more complicated than a simple scalar variable must use meth-
	   ods 2 or 3 below.  However, a "simple scalar" includes an identi-
	   fier that itself uses method 1 recursively.	Therefore, the follow-
	   ing prints "howdy".

	       $refrefref = \\\"howdy";
	       print $$$$refrefref;

       2.  Anywhere you'd put an identifier (or chain of identifiers) as part
	   of a variable or subroutine name, you can replace the identifier
	   with a BLOCK returning a reference of the correct type.  In other
	   words, the previous examples could be written like this:

	       $bar = ${$scalarref};
	       push(@{$arrayref}, $filename);
	       ${$arrayref}[0] = "January";
	       ${$hashref}{"KEY"} = "VALUE";
	       &{$coderef}(1,2,3);
	       $globref->print("output\n");  # iff IO::Handle is loaded

	   Admittedly, it's a little silly to use the curlies in this case,
	   but the BLOCK can contain any arbitrary expression, in particular,
	   subscripted expressions:

	       &{ $dispatch{$index} }(1,2,3);	   # call correct routine

	   Because of being able to omit the curlies for the simple case of
	   $$x, people often make the mistake of viewing the dereferencing
	   symbols as proper operators, and wonder about their precedence.  If
	   they were, though, you could use parentheses instead of braces.
	   That's not the case.  Consider the difference below; case 0 is a
	   short-hand version of case 1, not case 2:

	       $$hashref{"KEY"}   = "VALUE";	   # CASE 0
	       ${$hashref}{"KEY"} = "VALUE";	   # CASE 1
	       ${$hashref{"KEY"}} = "VALUE";	   # CASE 2
	       ${$hashref->{"KEY"}} = "VALUE";	   # CASE 3

	   Case 2 is also deceptive in that you're accessing a variable called
	   %hashref, not dereferencing through $hashref to the hash it's pre-
	   sumably referencing.  That would be case 3.

       3.  Subroutine calls and lookups of individual array elements arise
	   often enough that it gets cumbersome to use method 2.  As a form of
	   syntactic sugar, the examples for method 2 may be written:

	       $arrayref->[0] = "January";   # Array element
	       $hashref->{"KEY"} = "VALUE";  # Hash element
	       $coderef->(1,2,3);	     # Subroutine call

	   The left side of the arrow can be any expression returning a refer-
	   ence, including a previous dereference.  Note that $array[$x] is
	   not the same thing as "$array->[$x]" here:

	       $array[$x]->{"foo"}->[0] = "January";

	   This is one of the cases we mentioned earlier in which references
	   could spring into existence when in an lvalue context.  Before this
	   statement, $array[$x] may have been undefined.  If so, it's auto-
	   matically defined with a hash reference so that we can look up
	   "{"foo"}" in it.  Likewise "$array[$x]->{"foo"}" will automatically
	   get defined with an array reference so that we can look up "[0]" in
	   it.	This process is called autovivification.

	   One more thing here.  The arrow is optional between brackets sub-
	   scripts, so you can shrink the above down to

	       $array[$x]{"foo"}[0] = "January";

	   Which, in the degenerate case of using only ordinary arrays, gives
	   you multidimensional arrays just like C's:

	       $score[$x][$y][$z] += 42;

	   Well, okay, not entirely like C's arrays, actually.	C doesn't know
	   how to grow its arrays on demand.  Perl does.

       4.  If a reference happens to be a reference to an object, then there
	   are probably methods to access the things referred to, and you
	   should probably stick to those methods unless you're in the class
	   package that defines the object's methods.  In other words, be
	   nice, and don't violate the object's encapsulation without a very
	   good reason.  Perl does not enforce encapsulation.  We are not
	   totalitarians here.	We do expect some basic civility though.

       Using a string or number as a reference produces a symbolic reference,
       as explained above.  Using a reference as a number produces an integer
       representing its storage location in memory.  The only useful thing to
       be done with this is to compare two references numerically to see
       whether they refer to the same location.

	   if ($ref1 == $ref2) {  # cheap numeric compare of references
	       print "refs 1 and 2 refer to the same thing\n";
	   }

       Using a reference as a string produces both its referent's type,
       including any package blessing as described in perlobj, as well as the
       numeric address expressed in hex.  The ref() operator returns just the
       type of thing the reference is pointing to, without the address.  See
       "ref" in perlfunc for details and examples of its use.

       The bless() operator may be used to associate the object a reference
       points to with a package functioning as an object class.  See perlobj.

       A typeglob may be dereferenced the same way a reference can, because
       the dereference syntax always indicates the type of reference desired.
       So "${*foo}" and "${\$foo}" both indicate the same scalar variable.

       Here's a trick for interpolating a subroutine call into a string:

	   print "My sub returned @{[mysub(1,2,3)]} that time.\n";

       The way it works is that when the "@{...}" is seen in the double-quoted
       string, it's evaluated as a block.  The block creates a reference to an
       anonymous array containing the results of the call to "mysub(1,2,3)".
       So the whole block returns a reference to an array, which is then
       dereferenced by "@{...}" and stuck into the double-quoted string. This
       chicanery is also useful for arbitrary expressions:

	   print "That yields @{[$n + 5]} widgets\n";

       Symbolic references

       We said that references spring into existence as necessary if they are
       undefined, but we didn't say what happens if a value used as a refer-
       ence is already defined, but isn't a hard reference.  If you use it as
       a reference, it'll be treated as a symbolic reference.  That is, the
       value of the scalar is taken to be the name of a variable, rather than
       a direct link to a (possibly) anonymous value.

       People frequently expect it to work like this.  So it does.

	   $name = "foo";
	   $$name = 1;		       # Sets $foo
	   ${$name} = 2;	       # Sets $foo
	   ${$name x 2} = 3;	       # Sets $foofoo
	   $name->[0] = 4;	       # Sets $foo[0]
	   @$name = (); 	       # Clears @foo
	   &$name();		       # Calls &foo() (as in Perl 4)
	   $pack = "THAT";
	   ${"${pack}::$name"} = 5;    # Sets $THAT::foo without eval

       This is powerful, and slightly dangerous, in that it's possible to
       intend (with the utmost sincerity) to use a hard reference, and acci-
       dentally use a symbolic reference instead.  To protect against that,
       you can say

	   use strict 'refs';

       and then only hard references will be allowed for the rest of the
       enclosing block.  An inner block may countermand that with

	   no strict 'refs';

       Only package variables (globals, even if localized) are visible to sym-
       bolic references.  Lexical variables (declared with my()) aren't in a
       symbol table, and thus are invisible to this mechanism.	For example:

	   local $value = 10;
	   $ref = "value";
	   {
	       my $value = 20;
	       print $$ref;
	   }

       This will still print 10, not 20.  Remember that local() affects pack-
       age variables, which are all "global" to the package.

       Not-so-symbolic references

       A new feature contributing to readability in perl version 5.001 is that
       the brackets around a symbolic reference behave more like quotes, just
       as they always have within a string.  That is,

	   $push = "pop on ";
	   print "${push}over";

       has always meant to print "pop on over", even though push is a reserved
       word.  This has been generalized to work the same outside of quotes, so
       that

	   print ${push} . "over";

       and even

	   print ${ push } . "over";

       will have the same effect.  (This would have been a syntax error in
       Perl 5.000, though Perl 4 allowed it in the spaceless form.)  This con-
       struct is not considered to be a symbolic reference when you're using
       strict refs:

	   use strict 'refs';
	   ${ bareword };      # Okay, means $bareword.
	   ${ "bareword" };    # Error, symbolic reference.

       Similarly, because of all the subscripting that is done using single
       words, we've applied the same rule to any bareword that is used for
       subscripting a hash.  So now, instead of writing

	   $array{ "aaa" }{ "bbb" }{ "ccc" }

       you can write just

	   $array{ aaa }{ bbb }{ ccc }

       and not worry about whether the subscripts are reserved words.  In the
       rare event that you do wish to do something like

	   $array{ shift }

       you can force interpretation as a reserved word by adding anything that
       makes it more than a bareword:

	   $array{ shift() }
	   $array{ +shift }
	   $array{ shift @_ }

       The "use warnings" pragma or the -w switch will warn you if it inter-
       prets a reserved word as a string.  But it will no longer warn you
       about using lowercase words, because the string is effectively quoted.

       Pseudo-hashes: Using an array as a hash

       WARNING:  This section describes an experimental feature.  Details may
       change without notice in future versions.

       NOTE: The current user-visible implementation of pseudo-hashes (the
       weird use of the first array element) is deprecated starting from Perl
       5.8.0 and will be removed in Perl 5.10.0, and the feature will be
       implemented differently.  Not only is the current interface rather
       ugly, but the current implementation slows down normal array and hash
       use quite noticeably.  The 'fields' pragma interface will remain avail-
       able.

       Beginning with release 5.005 of Perl, you may use an array reference in
       some contexts that would normally require a hash reference.  This
       allows you to access array elements using symbolic names, as if they
       were fields in a structure.

       For this to work, the array must contain extra information.  The first
       element of the array has to be a hash reference that maps field names
       to array indices.  Here is an example:

	   $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];

	   $struct->{foo};  # same as $struct->[1], i.e. "FOO"
	   $struct->{bar};  # same as $struct->[2], i.e. "BAR"

	   keys %$struct;   # will return ("foo", "bar") in some order
	   values %$struct; # will return ("FOO", "BAR") in same some order

	   while (my($k,$v) = each %$struct) {
	      print "$k => $v\n";
	   }

       Perl will raise an exception if you try to access nonexistent fields.
       To avoid inconsistencies, always use the fields::phash() function pro-
       vided by the "fields" pragma.

	   use fields;
	   $pseudohash = fields::phash(foo => "FOO", bar => "BAR");

       For better performance, Perl can also do the translation from field
       names to array indices at compile time for typed object references.
       See fields.

       There are two ways to check for the existence of a key in a
       pseudo-hash.  The first is to use exists().  This checks to see if the
       given field has ever been set.  It acts this way to match the behavior
       of a regular hash.  For instance:

	   use fields;
	   $phash = fields::phash([qw(foo bar pants)], ['FOO']);
	   $phash->{pants} = undef;

	   print exists $phash->{foo};	  # true, 'foo' was set in the declaration
	   print exists $phash->{bar};	  # false, 'bar' has not been used.
	   print exists $phash->{pants};  # true, your 'pants' have been touched

       The second is to use exists() on the hash reference sitting in the
       first array element.  This checks to see if the given key is a valid
       field in the pseudo-hash.

	   print exists $phash->[0]{bar};      # true, 'bar' is a valid field
	   print exists $phash->[0]{shoes};# false, 'shoes' can't be used

       delete() on a pseudo-hash element only deletes the value corresponding
       to the key, not the key itself.	To delete the key, you'll have to
       explicitly delete it from the first hash element.

	   print delete $phash->{foo};	   # prints $phash->[1], "FOO"
	   print exists $phash->{foo};	   # false
	   print exists $phash->[0]{foo};  # true, key still exists
	   print delete $phash->[0]{foo};  # now key is gone
	   print $phash->{foo}; 	   # runtime exception

       Function Templates

       As explained above, an anonymous function with access to the lexical
       variables visible when that function was compiled, creates a closure.
       It retains access to those variables even though it doesn't get run
       until later, such as in a signal handler or a Tk callback.

       Using a closure as a function template allows us to generate many func-
       tions that act similarly.  Suppose you wanted functions named after the
       colors that generated HTML font changes for the various colors:

	   print "Be ", red("careful"), "with that ", green("light");

       The red() and green() functions would be similar.  To create these,
       we'll assign a closure to a typeglob of the name of the function we're
       trying to build.

	   @colors = qw(red blue green yellow orange purple violet);
	   for my $name (@colors) {
	       no strict 'refs';       # allow symbol table manipulation
	       *$name = *{uc $name} = sub { "@_" };
	   }

       Now all those different functions appear to exist independently.  You
       can call red(), RED(), blue(), BLUE(), green(), etc.  This technique
       saves on both compile time and memory use, and is less error-prone as
       well, since syntax checks happen at compile time.  It's critical that
       any variables in the anonymous subroutine be lexicals in order to cre-
       ate a proper closure.  That's the reasons for the "my" on the loop
       iteration variable.

       This is one of the only places where giving a prototype to a closure
       makes much sense.  If you wanted to impose scalar context on the argu-
       ments of these functions (probably not a wise idea for this particular
       example), you could have written it this way instead:

	   *$name = sub ($) { "$_[0]" };

       However, since prototype checking happens at compile time, the assign-
       ment above happens too late to be of much use.  You could address this
       by putting the whole loop of assignments within a BEGIN block, forcing
       it to occur during compilation.

       Access to lexicals that change over type--like those in the "for" loop
       above--only works with closures, not general subroutines.  In the gen-
       eral case, then, named subroutines do not nest properly, although
       anonymous ones do. Thus is because named subroutines are created (and
       capture any outer lexicals) only once at compile time, whereas anony-
       mous subroutines get to capture each time you execute the 'sub' opera-
       tor.  If you are accustomed to using nested subroutines in other pro-
       gramming languages with their own private variables, you'll have to
       work at it a bit in Perl.  The intuitive coding of this type of thing
       incurs mysterious warnings about "will not stay shared".  For example,
       this won't work:

	   sub outer {
	       my $x = $_[0] + 35;
	       sub inner { return $x * 19 }   # WRONG
	       return $x + inner();
	   }

       A work-around is the following:

	   sub outer {
	       my $x = $_[0] + 35;
	       local *inner = sub { return $x * 19 };
	       return $x + inner();
	   }

       Now inner() can only be called from within outer(), because of the tem-
       porary assignments of the closure (anonymous subroutine).  But when it
       does, it has normal access to the lexical variable $x from the scope of
       outer().

       This has the interesting effect of creating a function local to another
       function, something not normally supported in Perl.

WARNING
       You may not (usefully) use a reference as the key to a hash.  It will
       be converted into a string:

	   $x{ \$a } = $a;

       If you try to dereference the key, it won't do a hard dereference, and
       you won't accomplish what you're attempting.  You might want to do
       something more like

	   $r = \@a;
	   $x{ $r } = $r;

       And then at least you can use the values(), which will be real refs,
       instead of the keys(), which won't.

       The standard Tie::RefHash module provides a convenient workaround to
       this.

SEE ALSO
       Besides the obvious documents, source code can be instructive.  Some
       pathological examples of the use of references can be found in the
       t/op/ref.t regression test in the Perl source directory.

       See also perldsc and perllol for how to use references to create com-
       plex data structures, and perltoot, perlobj, and perlbot for how to use
       them to create objects.



perl v5.8.8			  2006-01-07			    PERLREF(1)
=18872
+190
(68)