The + overload constructs a new object. Perl uses that overload, and then overwrites the stored value. This means every += op creates a new object
Thanks for the explanation - helps me understand what's going on, and also helps me understand the overload documentation.
Not that it needed to be checked, but with the help of Devel::Peek::Dump() I was able to verify that, with Math::GMP, the address of the object does indeed change with every +=
And I was also able to verify that the same is not true of Math::GMPz.
I find this overloading of '=' to be a bit of a headache, in that it provides a lot more rope than I would like.
WRT Math::GMP, if you have a Math::GMP object ($x) to which you wish to add, say, 4 then there's 2 ways you can do that:
$x += 4;
or
$x->add_ui_gmp(4);
They both do the same thing in that they increase the value held in $x by 4.
But they don't do the same thing if they were preceded by $x = $orig; where $orig is a Math::GMP object:
$x = $orig;
$x += 4; # $orig remains unchanged
# $x has been incremented by 4
VERSUS:
$x = $orig;
$x->add_ui_gmp(4); # Both $x and $orig have
# been incremented by 4
AFAIK, the same sort of trap occurs in every math module that overloads '=', including my own Math modules, along with Math::BigInt and Math::BigFloat. (This is not limited to Math::GMP, and I'm not singling it out.)
Although it doesn't really fit with the title I've given this thread, a second question is "Should the documentation of the module (eg Math::GMP) draw attention to this trap ?".
Or is it acceptable to say nothing and put the onus of dodging the trap upon the user ?
BTW, this all started with this Math::MPFR bug report, which is about this same trap in a different module. I keep changing my mind about what, if anything, should be done ....
Cheers,
Rob
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There's no denying that being able to do $clone = $orig is very convenient if you both understand the limitations, and make sure you work around them.
It's quicker to type, and also easier to remember, than $clone = $orig->gmp_copy()
But, as an author who has enabled this convenience in his modules, is it acceptable that I don't document the traps that users might fall into when they avail themselves of this option ?
With Math::GMP (which I've neither authored or maintained), I initially considered it smart of them to not explicitly overload '=', and to also not mention that this option exists.
I thought that, in doing this, they were taking advantage of the fact that there's no onus upon them to document anything at all about this option.
And I also thought that I might take the same path with my modules.
However, I'm no longer so sure about the validity of that thinking - and I think, in my modules, I will make some brief mention of the traps, along with a recommendation to read the overload documentation carefully.
It occurs to me that, if one wanted to code defensively, doing $clone = $orig + 0 is safer than doing $clone = $orig.
Cheers,
Rob
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