Overloading assignment

[v_o_i_d]

Hi i'm new here..and am humbly seeking wisdom from those within...

Can anyone tell me if it is possible to overload operators such as addition(+) AND overload assignment as well. i have searched but found no hard answer-not even vague suggestions.

Specifically, ...

In my own experiments I have tried to overload and tiescalar in the same class, but FETCH seems to interfere and the tie function causes a loss of context because the overloded operators never work after this. Using the returned object from tie() still works however but then you have 2 individual variables each with separate functionality.

...Any ideas on how to make custom assignment code like FETCH AND overload operators work in the same class?
Thanks perl monks!

*UPDATE* Just so you know I posted some code in a new reply!

[Zaxo]

Assignment can't really be overloaded. If you say use overload '=' . . .;, you are arranging for a copy constructor for use by mutators like ++, += and so on.

To intercept and modify the workings of the assignment operator, you need to use a tied interface, as you seem to be getting at. If you are having trouble with that code, please show what you've tried and what errors you get.

[v_o_i_d]

I have done what you said. Here is the code...
Simple class example for demonstration purposes:


#---Sample.pm---
package Sample;

use warnings;
use strict;

use overload( '+'    => \&add,
        fallback => 1, );

sub TIESCALAR
{
    my $class = shift;
    my $self = { data => [@_], };
    bless( $self, $class );
    return $self;    
}

sub add
{
    $_ .= $_[1] foreach( @{$_[0]->{data}} );
}

sub STORE
{
    @{$_[0]->{data}} = (ref( $_[1] ) eq 'ARRAY') ? @{$_[1]}: ($_[1]);
}

sub FETCH
{
    return $_[0];
}

1;
#---------------

Next is the program..

#----code.pl----
use warnings;
use strict;

use Sample;

my $var1;
my $var2 = tie( $var1, 'Sample', 'black', 'white' );

# The assigment test...
#$var1 = 'green';
#$var2 = ['green','red'];

# The overload test...
#$var1 + ' bishop';
#$var2 + ' bishop';

#print ref($var2),"\n";
print join( "\n", @{$var2->{data}} );

exit 0;
#---------------

Compiler warnings/errors: (addition operation on the tie() variable $var1)


Useless use of addition (+) in void context at code.pl line 28.
Argument " bishop" isn't numeric in addition (+) at code.pl line 28.

* The returned variable $var2 however, will cause the 1st warning but successfully calls the 'addition' Sample::add.

The opposite works with assignment:
$var2 = ['green','red'] destroys the class instance of Sample and becomes an array reference. $var1 = ['green','red'] works as it should because of tie()!

Both operations don't work on the same variable, $var1 can't do the addition + and $var2 can't do the assignment!

[Zaxo]

Both your tie class and the overloaded add are not quite correct.

The FETCH routine does not return a scalar of the expected kind. You have it returning a reference to the underlying hash, rather than some scalar data stored in it.

An overridden operator like '+' is written like a constructor. It should take data from its arguments and return a new object of the correct type constructed from the result. What you have written for add is a mutator which behaves more like '.=' than '+'. It is a good idea to make sure overridden operators are as much like the builtins as possible.

I'm not sure there is a way to make a tied variable honor the underlying classes' operator overrides. I wasn't aware of that, but my attempts to make it happen have failed.

[v_o_i_d]

w00t!

Combining use overload and tie() can be done, but it requires overloading one of the scalar dereference operators (or any of the others). ie. ${}  @{}  %{} etc.

The basic idea is that you tie() self inbetween the dereferencing operation.

The only drawback is you must dereference to tie() the object (in ribbons ;). You will use it like this:


$var + 1;   # use overload '+'...

$$var = 'custom STORE';   # STORE() called...

Cheers buds!

Disclaimer: Use at your own risk. May not work for you like it did for me.