Math::BigInt new

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

Math::BigInt new does something funny when creating a new object from an existing Math::BigInt object:

#!perl
use strict;
use warnings;

use Math::BigInt;

my $x = new Math::BigInt 27; 
my $y = $x->new(44); # this is the funny bit
my $z = Math::BigInt->new(99);
exit;
[download]

From the debugger, this is what $y is:

  DB<1> x $y
0  27=HASH(0x14f15ac)
   'sign' => '+'
   'value' => ARRAY(0xbc9cd4)
      0  44
[download]

Why is the class now '27'? And a print on $y gives:

  DB<2> p $y
27=HASH(0x14f15ac)
[download]

While the 'value' is right, any further operations with $y are wrong.

Update: Bug reported here.

-QM
--
Quantum Mechanics: The dreams stuff is made of

Comment on Math::BigInt new Select or Download Code

Replies are listed 'Best First'.
Re: Math::BigInt new by tobyink (Canon) on Sep 27, 2012 at 15:30 UTC
`$x` overloads stringification, stringifying as "27". The `new` method presumably does something like: `sub new { my ($class, $value) = @_; bless { value => $value } => "$class"; }` [download] Thus you end up with a reference blessed into package "27". Generally speaking, constructors are designed to be called as class methods not object methods. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re^2: Math::BigInt new by QM (Parson) on Sep 27, 2012 at 15:46 UTC
Agreed. However, here is the first part of `new`: sub new { # create a new BigInt object from a string or another BigInt object. + # see hash keys documented at top # the argument could be an object, so avoid \|\|, && etc on it, this w +ould # cause costly overloaded code to be called. The only allowed ops ar +e # ref() and defined. my ($class,$wanted,$a,$p,$r) = @_; # avoid numify-calls by not using \|\| on $wanted! return $class->bzero($a,$p) if !defined $wanted; # default to 0 return $class->copy($wanted,$a,$p,$r) if ref($wanted) && $wanted->isa($class); # MBI or subclass $class->import() if $IMPORT == 0; # make require work my $self = bless {}, $class; [download] If it gets to the last line, and `$class == 27`, then `$class` must have been changed somewhere? And generally speaking, if I want a new object from an existing object, shouldn't the existing object be able to create one for me? Or do I have to go around doing this? `my $y = {ref $x}->new();` [download] (or whatever the syntax is for that) -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]
Re^3: Math::BigInt new by tobyink (Canon) on Sep 27, 2012 at 16:13 UTC
`$class` is the same as `$x` (i.e. the bigint 27) the whole way through `new`, and `bless` implicitly stringifies its second argument (like `print` implicitly stringifies its arguments). `ref($x)->new(48)` should do what you want. Another reasonably common idiom is: `my $new = (ref($existing) // $existing)->new(...)` [download] This allows `$existing` to be either an existing object or a class name. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re^3: Math::BigInt new by remiah (Hermit) on Sep 27, 2012 at 23:12 UTC
I also think this is strange. There is no checking for ref($class). I guess something like ... `if ( ref($class) ){ my $t = $wanted; $t =~ s/^[+-]//; return bless { sign => $class->{sign}, value => $CALC->_new($t), }, ref($class); }` [download] should be in new constructor, doesn't it?	[reply] [d/l]