Skunkware 5/Skunkware 5.iso/man/cat.1/perlref.1

Text File | 1995-07-25 | 20KB | 463 lines

     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



     NNNNAAAAMMMMEEEE
          perlref - Perl references and nested data structures

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          In Perl 4 it was difficult to represent complex data
          structures, because all references had to be symbolic, and
          even that was difficult to do when you wanted to refer to a
          variable rather than a symbol table entry.  Perl 5 not only
          makes it easier to use symbolic references to variables, but
          lets you have "hard" references to any piece of data.  Any
          scalar may hold a hard reference.  Since 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.  If that thing
          happens to be an object, the object is destructed.  See the
          _p_e_r_l_o_b_j manpage 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.)

          A symbolic reference contains the name of a variable, just
          as a symbolic link in the filesystem merely contains the
          name of a file. The *glob notation is a kind of symbolic
          reference.  Hard references are more like hard links in the
          file system: merely another way at getting at the same
          underlying object, irrespective of its name.

          "Hard" references are easy to use in Perl.  There is just
          one overriding principle:  Perl does no implicit referencing
          or dereferencing.  When a scalar is holding a reference, it
          always behaves as a scalar.  It doesn't magically start
          being an array or a hash unless you tell it so explicitly by
          dereferencing it.

          References can be constructed several ways.

          1.  By using the backslash operator on a variable,
              subroutine, or value.  (This works much like the &
              (address-of) operator works in C.)  Note that this
              typically creates _A_N_O_T_H_E_R reference to a variable, since
              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:







     Page 1                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



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


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

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

              Here we've constructed a reference to an anonymous array
              of three elements whose final element is itself
              reference to another anonymous 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".)

          3.  A reference to an anonymous hash can be constructed
              using curly brackets:

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

              Anonymous hash and array constructors can be intermixed
              freely to produce as complicated a structure as you
              want.  The multidimensional syntax described below works
              for these too.  The values above are literals, but
              variables and expressions would work just as well,
              because assignment operators in Perl (even within
              _l_o_c_a_l() or _m_y()) are executable statements, not
              compile-time declarations.

              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






     Page 2                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



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

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

              Note the presence of the semicolon.  Except for the fact
              that the code inside isn't executed immediately, 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 line (unless you're in an eval("...")),
              $coderef will still have a reference to the _S_A_M_E
              anonymous subroutine.)

              For those who worry about these things, the current
              implementation uses shallow binding of _l_o_c_a_l()
              variables; _m_y() variables are not accessible.  This
              precludes true closures.  However, you can work around
              this with a run-time (rather than a compile-time)
              _e_v_a_l():

                  {
                      my $x = time;
                      $coderef = eval "sub { \$x }";
                  }

              Normally--if you'd used just sub{} or even eval{}--your
              unew sub would only have been able to access the global
              $x.  But because you've used a run-time _e_v_a_l(), this
              will not only generate a brand new subroutine reference
              each time called, it will all grant access to the _m_y()
              variable lexically above it rather than the global one.
              The particular $x accessed will be different for each
              new sub you create.  This mechanism yields deep binding
              of variables.  (If you don't know what closures, deep
              binding, or shallow binding are, don't worry too much
              about it.)

          5.  References are often returned by special subroutines
              called constructors.  Perl objects are just reference a
              special kind 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
              reference, and it remains an ordinary reference even
              while it's also being an object.  Constructors are
              customarily named _n_e_w(), but don't have to be:

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


          6.  References of the appropriate type can spring into
              existence if you dereference them in a context that



     Page 3                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



              assumes they exist.  Since we haven't talked about
              dereferencing yet, we can't show you any examples yet.

          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 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);

              It's important to understand that we are specifically
              _N_O_T dereferencing $arrayref[0] or $hashref{"KEY"} there.
              The dereference of the scalar variable happens _B_E_F_O_R_E it
              does any key lookups.  Anything more complicated than a
              simple scalar variable must use methods 2 or 3 below.
              However, a "simple scalar" includes an identifier that
              itself uses method 1 recursively.  Therefore, the
              following prints "howdy".

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


          2.  Anywhere you'd put an identifier 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);

              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



     Page 4                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



              wonder about their precedence.  If they were, though,
              you could use parens instead of braces.  That's not the
              case.  Consider the difference below; case 0 is a
              short-hand version of case 1, _N_O_T 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 presumably referencing.  That
              would be case 3.

          3.  The case of individual array elements arises often
              enough that it gets cumbersome to use method 2.  As a
              form of syntactic sugar, the two lines like that above
              can be written:

                  $arrayref->[0] = "January";
                  $hashref->{"KEY} = "VALUE";

              The left side of the array can be any expression
              returning a reference, including a previous dereference.
              Note that $array[$x] is _N_O_T 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 automatically 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.

              One more thing here.  The arrow is optional _B_E_T_W_E_E_N
              brackets subscripts, 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.



     Page 5                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



          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.

          The _r_e_f() operator may be used to determine what type of
          thing the reference is pointing to.  See the _p_e_r_l_f_u_n_c
          manpage.

          The _b_l_e_s_s() operator may be used to associate a reference
          with a package functioning as an object class.  See the
          _p_e_r_l_o_b_j manpage.

          A type glob may be dereferenced the same way a reference
          can, since the dereference syntax always indicates the kind
          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)}\n";

          The way it works is that when the ${...} is seen in the
          double-quoted string, it's evaluated as a block.  The block
          executes the call to mysub(1,2,3), and then takes a
          reference to that.  So the whole block returns a reference
          to a scalar, which is then dereferenced by ${...} and stuck
          into the double-quoted string.

          SSSSyyyymmmmbbbboooolllliiiicccc rrrreeeeffffeeeerrrreeeennnncccceeeessss

          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 reference is already defined, but _I_S_N'_T a
          hard reference.  If you use it as a reference in this case,
          it'll be treated as a symbolic reference.  That is, the
          value of the scalar is taken to be the _N_A_M_E of a variable,
          rather than a direct link to a (possibly) anonymous value.

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









     Page 6                                          (printed 6/30/95)






     PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))  UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))  PPPPEEEERRRRLLLLRRRREEEEFFFF((((1111))))



              $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 very powerful, and slightly dangerous, in that it's
          possible to intend (with the utmost sincerity) to use a hard
          reference, and accidentally 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 are visible to symbolic references.
          Lexical variables (declared with _m_y()) 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 _l_o_c_a_l()
          affects package variables, which are all "global" to the
          package.

          FFFFuuuurrrrtttthhhheeeerrrr RRRReeeeaaaaddddiiiinnnngggg

          Besides the obvious documents, source code can be
          instructive.  Some rather pathological examples of the use
          of references can be found in the _t/_o_p/_r_e_f._t regression test
          in the Perl source directory.










     Page 7                                          (printed 6/30/95)