RE: Need some tips on pointers

[Robert Friberg]

> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED]]On Behalf
> Of Olivier CLERE
> Sent: Thursday, May 18, 2000 11:23 AM
> To: Perl-Win32-Users Mailing List
> Subject: Need some tips on pointers
> 
> 
> Here is my Perl code:
> 
> %berval=  (
>   bv_len=>3,
>   bv_val=>"123",
>   );
> 
> $ldapctrl =    {
>  ldctl_oid=>"2.16.840.1.113730.3.4.2",
>  ldctl_iscritical=>1,
>  ldctl_value=> %berval,
>                     };
> 
> I would like to define the Perl equivalent of
> what we would call in C:
> "an array of pointers to the structure ldapctrl",

All array elements are scalars, you can't define
any specific type for an array. A scalar is simply
a single value of any type. The equivalent of a
pointer in C is a reference, which is a scalar
value. The reference can refer to just about anything,
both named scalars and structures as well as anonymous
data.

Your scalar variable $ldapctrl is actually a reference to
a hash.

   #define and init to empty
   my @arr = ();

Now just push your reference on to the array

   push @arr, $ldapctrl;

Be aware of the fact that you have 2 references to the same
data. You could have pushed the reference directly
onto the array. Notice how I added a \ in front of the
%berval hash, this is like C's adressoperator, &.


   #add an anonymous reference 
   push @arr, { ldctl_oid => "2.16.840.1.113730.3.4.2",
                ldctl_iscritical => 1,
                ldctl_value => \%berval
              }


Some expressions to access the data, first arrow notation

  $arr[0]->{ldctl_oid}
  $arr[0]->{ldctl_value}->{bv_val}

and second, by adding a type character at the left to 
"dereference" the "pointer"

  ${$arr[0]}{ldctl_oid}
  ${$arr[0]->{ldctl_value}}{bv_val}
  ${${$arr[0]}{ldctl_value}}{bv_val} # same as previous line

the similarities with C are probably not coincidental :) Notice the
extra brackets, which are needed to get the precedence right.

Being a C programmer you might instinctively iterate the
array in this manner

  for($i = 0; $i < $#arr; $i++)
  {
     # do something with $arr[$i]
  }

This is valid perl but not idiomatic. Try this instead

 for (@arr)
  {
    # use $_ instead of $arr[n]
  }


regards,

--
robert friberg, ensofus ab
+46(0)708 98 57 01


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