Brian McCauley > Tie-OneOff > Tie::OneOff

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Module Version: 1.03   Source  

NAME ^

Tie::OneOff - create tied variables without defining a separate package

SYNOPSIS ^

    require Tie::OneOff;
    
    tie my %REV, 'Tie::OneOff' => sub {
        reverse shift;
    };

    print "$REV{olleH}\n"; # Hello

    sub make_counter {
        my $step = shift;
        my $i = 0;
        Tie::OneOff->scalar({
            BASE => \$i, # Implies: STORE => sub { $i = shift }
            FETCH => sub { $i += $step },
        });
    }

    my $c1 = make_counter(1);
    my $c2 = make_counter(2);
    $$c2 = 10;
    print "$$c1 $$c2 $$c2 $$c2 $$c1 $$c1\n"; # 1 12 14 16 2 3

    sub foo : lvalue {
        +Tie::OneOff->lvalue({
            STORE => sub { print "foo()=$_[0]\n" },
            FETCH => sub { "wibble" },
        });
    }

    foo='wobble';              # foo()=wobble
    print "foo()=", foo, "\n"; # foo()=wibble

DESCRIPTION ^

The Perl tie mechanism ties a Perl variable to a Perl object. This means that, conventionally, for each distinct set of tied variable semantics one needs to create a new package. The package symbol table then acts as a dispatch table for the intrinsic actions (such as FETCH, STORE, FETCHSIZE) that can be performed on Perl variables.

Sometimes it would seem more natural to associate a dispatch table hash directly with the variable and pretend as if the intermediate object did not exist. This is what Tie::OneOff does.

It is important to note that in this model there is no object to hold the instance data for the tied variable. The callbacks in the dispatch table are called not as object methods but as simple subroutines. If there is to be any instance information for a variable tied using Tie::OneOff it must be in lexical variables that are referenced by the callback closures.

Tie::OneOff does not itself provide any default callbacks. This can make defining a full featured hash interface rather tedious. To simplify matters the element BASE in the dispatch table can be used to specify a "base object" whose methods provide the default callbacks. If a reference to an unblessed Perl variable is specified as the BASE then the variable is blessed into the appropriate Tie::StdXXXX package. In this case the unblessed variable used as the base must, of course, be of the same type as the variable that is being tied.

In make_counter() in the synopsis above, the variable $i gets blessed into Tie::StdScalar. Since there is no explict STORE in the dispatch table, an attempt to store into a counter is implemented by calling (\$i)->STORE(@_) which in turn is resolved as Tie::StdScalar::STORE(\$i,@_) which in turn is equivalent to $i=shift.

Since many tied variables need only a FETCH method Tie::OneOff ties can also be specified by giving a simple code reference that is taken to be the variable's FETCH callback.

For convience the class methods scalar, hash and array take the same arguments as the tie inferface and return a reference to an anonymous tied variable. The class method lvalue is like scalar but returns an lvalue rather than a reference.

Relationship to other modules ^

This module's original working title was Tie::Simple however it was eventually released as Tie::OneOff. Some time later another, substancially identical, module was developed independantly and released as Tie::Simple.

This module can be used as a trick to make functions that interpolate into strings but if that's all you want you may want to use Interpolation instead.

XXX Want XXX

SEE ALSO ^

perltie, Tie::Scalar, Tie::Hash, Tie::Array, Interpolation, Tie::Simple.

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