The "no gc" crowd

[H. S. Teoh]

On Thu, Oct 10, 2013 at 08:39:52PM +0200, Joseph Rushton Wakeling wrote:
> On 10/10/13 20:28, Sean Kelly wrote:
> >Isn't BigInt a struct?  I'd expect it to work via copying just like
> >concrete types.
> 
> Yes, it's a struct, but somewhere inside its internals I think it
> contains arrays.  I'm not sure how that affects copying etc., but
> suffice to say that if you try the following:
> 
>     BigInt a = 2;
>     BigInt b = a;
>     b = 3;
>     assert(a != b);
>     assert(a !is b);
> 
> ... then it passes.  So it behaves at least in this extent like a
> value type.

I took a glance over the BigInt code, and it appears to have some kind
of copy-on-write semantics. For example, in your code above, when you
wrote b=a, b actually *aliases* a, but when you assign 3 to b, a new
data array is created and b is updated to point to the new data instead.

So it's not really a true value type, but more like a COW reference
type.


> But suffice to say that it was an unpleasant surprise that I
> couldn't just take it and pass to a function accepting an
> unqualified BigInt argument.

That only works with true value types, but BigInt isn't really one of
them. :)


T

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