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Perl: The Complete Reference
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The tie system uses objects and classes to associate a variable with the underlying object methods that support the interface between the Perl data type and the external data source The base function is tie:
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tie VARIABLE, CLASSNAME, LIST
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The VARIABLE is just a normal variable that you will use to access the information in the tied resource CLASSNAME defines the name of the package that supports the required class methods to tie the variable Note that CLASSNAME is a string, not a bareword or other value It can come from a variable, but that variable must contain the string to a valid class package The package that contains the class definition must have been imported via use or require The tie function doesn t do this for you The LIST is passed on directly to the class constructor at the point when the variable is tied Also note that the underlying class never gets passed the variable The tie function creates the association, not the method class The actual constructor name is the only way a package can determine what type of variable is being tied If you tie a scalar, then it calls TIESCALAR as the constructor With an array, it s TIEARRAY, and TIEHASH is used if it s a hash variable In addition, in each case, the constructors are identical in all other respects to a typical object constructor They just return a reference to the correct object; the constructor has no way of determining whether it was called from tie If you want to determine the name of the class to which a variable is tied, you use the tied function:
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tied VARIABLE
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This returns a reference to the object underlying the tied VARIABLE Once you have finished with the variable, you need to disassociate the variable from the underlying class methods, and for this you use untie:
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This breaks the binding between a variable and a package, undoing the association created by the tie function It calls the DESTROY method, if it has been defined
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In this section we ll look at the creation of new base classes for tying different variable types In each case, the base class must define a number of methods in order for the tie operation to work Three methods are constant across all three variable types: FETCH,
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10:
Complex Data Structures
for reading the value of a tied variable; STORE, for assigning a value to a tied variable; and DESTROY, which deletes the tied variable when you call untie The FETCH and STORE methods are used to provide an interface to the variable that has been tied, not the underlying object Accessing the object directly doesn t invoke these methods The object reference is available either by using tied or by capturing the value returned by tie, which will be a reference to the underlying object being used
Tying Scalars
We ll use the methods for tying scalars as our base reference, examining the entire process from start to finish For this we ll use the example of file ownership, supported by the FileOwner package When you tie a scalar using the methods in the FileOwner class, the variable tied contains the name of the file Accessing the tied variable returns the owner (name, or user ID if the name cannot be resolved) Assigning a value to the tied variable sets the file s ownership, accepting a user ID or name accordingly For example, consider this script:
use FileOwner; tie $profile, 'FileOwner', 'bash_profile'; print "Current owner is: $profile\n"; $profile = 'mcslp'; print "New owner is: $profile\n"; PROGRAMMING WITH PERL
When the tie function is called, what actually happens is that the TIESCALAR method from FileOwner is called, passing 'bash_profile' as the argument to the method This returns an object, which is associated by tie to the $profile variable When $profile is used in the print statements, the FETCH method is called When you assign a value to $profile, the STORE method is called, with 'mcslp' as the argument to the method If you can follow this, then you can create tied scalars, arrays, and hashes, since they all follow the same basic model Now let s examine the details of our new FileOwner class, starting with the TIESCALAR method:
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