Tie::Scalar

Section: Perl Programmers Reference Guide (3pm)
Updated: 2021-03-31
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NAME

Tie::Scalar, Tie::StdScalar - base class definitions for tied scalars

SYNOPSIS

    package NewScalar;
    require Tie::Scalar;

    @ISA = qw(Tie::Scalar);

    sub FETCH { ... }           # Provide a needed method
    sub TIESCALAR { ... }       # Overrides inherited method


    package NewStdScalar;
    require Tie::Scalar;

    @ISA = qw(Tie::StdScalar);

    # All methods provided by default, so define
    # only what needs be overridden
    sub FETCH { ... }


    package main;

    tie $new_scalar, 'NewScalar';
    tie $new_std_scalar, 'NewStdScalar';

DESCRIPTION

This module provides some skeletal methods for scalar-tying classes. See perltie for a list of the functions required in tying a scalar to a package. The basic Tie::Scalar package provides a "new" method, as well as methods "TIESCALAR", "FETCH" and "STORE". The Tie::StdScalar package provides all the methods specified in perltie. It inherits from Tie::Scalar and causes scalars tied to it to behave exactly like the built-in scalars, allowing for selective overloading of methods. The "new" method is provided as a means of grandfathering, for classes that forget to provide their own "TIESCALAR" method.

For developers wishing to write their own tied-scalar classes, the methods are summarized below. The perltie section not only documents these, but has sample code as well:

TIESCALAR classname, LIST: The method invoked by the command "tie $scalar, classname". Associates a new scalar instance with the specified class. "LIST" would represent additional arguments (along the lines of AnyDBM_File and compatriots) needed to complete the association.
FETCH this: Retrieve the value of the tied scalar referenced by this.
STORE this, value: Store data value in the tied scalar referenced by this.
DESTROY this: Free the storage associated with the tied scalar referenced by this. This is rarely needed, as Perl manages its memory quite well. But the option exists, should a class wish to perform specific actions upon the destruction of an instance.

Tie::Scalar vs Tie::StdScalar

"Tie::Scalar" provides all the necessary methods, but one should realize they do not do anything useful. Calling "Tie::Scalar::FETCH" or "Tie::Scalar::STORE" results in a (trappable) croak. And if you inherit from "Tie::Scalar", you must provide either a "new" or a "TIESCALAR" method.

If you are looking for a class that does everything for you that you don't define yourself, use the "Tie::StdScalar" class, not the "Tie::Scalar" one.

MORE INFORMATION

The perltie section uses a good example of tying scalars by associating process IDs with priority.