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Programming Perl

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Functions
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3.2.171 tie

tie VARIABLE, CLASSNAME, LIST

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be tied. CLASSNAME is the name of a class implementing objects of an appropriate type. Any additional arguments are passed to the "new" method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH). Typically these are arguments such as might be passed to the dbm_open(3) function of C, but this is package dependent. The object returned by the "new" method is also returned by the tie function, which can be useful if you want to access other methods in CLASSNAME. (The object can also be accessed through the tied function.) So, a class for tying a hash to an ISAM implementation might provide an extra method to traverse a set of keys sequentially (the "S" of ISAM), since your typical DBM implementation can't do that.

Note that functions such as keys and values may return huge list values when used on large objects like DBM files. You may prefer to use the each function to iterate over such. For example:

use NDBM_File;
tie %ALIASES, "NDBM_File", "/etc/aliases", 1, 0
    or die "Can't open aliases: $!\n";
while (($key,$val) = each %ALIASES) {
    print $key, ' = ', $val, "\n";
}
untie %ALIASES;

A class implementing a hash should provide the following methods:

TIEHASH $class, LIST
DESTROY $self
FETCH $self, $key
STORE $self, $key, $value
DELETE $self, $key
EXISTS $self, $key
FIRSTKEY $self
NEXTKEY $self, $lastkey

A class implementing an ordinary array should provide the following methods:

TIEARRAY $classname, LIST
DESTROY $self
FETCH $self, $subscript
STORE $self, $subscript, $value

(As of this writing, other methods are still being designed. Check the online documentation for additions.)

A class implementing a scalar should provide the following methods:

TIESCALAR $classname, LIST
DESTROY $self
FETCH $self, 
STORE $self, $value

See "Using Tied Variables" in Chapter 5 for detailed discussion of all these methods. Unlike dbmopen, the tie function will not use or require a module for you - you need to do that explicitly yourself. See the DB_File and Config modules for interesting tie implementations.


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