Overloading multiplication involving BigInt

grondilu has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I recently wanted to overload '*' with one of my class which can accept external multiplication with bigints.

In other words, I wanted to do something like:

my $obj = new My::Custom::Object;
use bigint;
my $mult = 2**100 *  $obj;
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I realised that overloading '*' inside My::Custom::Object would not be enough as the class resolution would first process '*' inside Math::BigInt if the number is the first operand.

So I thought I should directly overload '*' inside Math::BigInt. I guess I can do that even if this class is not "mine", right?

package Math::BigInt;
use overload '*' =>
sub {
    if (ref $_[1] eq 'My::Custom::Object') {
         $_[1]->MultiplyByInt($_[0]);
    }
    else {
         $_[0]->bmult($_[1]);
    }
}
[download]

I'm not sure this is a good solution, though. My concern is about using 'bmult' instead of '*'. I doubt bmult will behave correctly if something else than a BigInt is provided as a second operand.

I thought about creating a child class of Math::BigInt so '*' can fallback to parent '*' when necessary but then I'll have to explicitely bless my integers which would be annoying.

So, how should I do this right?

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Re: Overloading multiplication involving BigInt by pileofrogs (Priest) on Jan 13, 2012 at 21:43 UTC
No one else is commenting, so I'll take a wild stab. Why not add more elsif branches to your sub? You say you're worried about bmul behaving badly if it's handed something other than a BigInt, so why not just check for it? You could also run some tests or read the source to find out what bmul really does if the 2nd operand isn't a BigInt. Why would you have to explicitly bless your integers if you made a subclass of Math::BigInt? And to step back a little: are you sure you want to do overloading? Will future users (or yourself) be flummoxed that $a*$b isn't doing what they expect? Is typing '$obj->mult($other_thing)' really that bad? At least that way you know exactly what you just did. Okay, that's about all I can think of. I hope I was helpful. --Pileofrogs	[reply]
Re^2: Overloading multiplication involving BigInt by grondilu (Friar) on Jan 14, 2012 at 15:17 UTC
Well, I had a look at Math::BigInt source code and it seemed to me that the '' overloading had already a lot of elsif branches. I just don't want to reinvent the wheel and redo what has already been done. Basically I whish I could just say "if the second operand is a My::Custom::Object then do this, otherwise just do whatever '' does in the Math::BigInt class". About explicit blessing, I just don't see how I could do it otherwise, unless I use a constructor anyway. But now that you make me think of it , I wonder if the magic of bigint wouldn't be inherited. I have to check this out, indeed. You are right about using overloading not being necessary. But it would definitely be cool.	[reply]


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