Growing a Language (applicable to @attribute design)

[Joseph Rushton Wakeling]

On 11/15/2012 11:54 AM, Walter Bright wrote:
> size_t x = 1;

Complete misunderstanding there -- I'd interpreted Simen's remark as saying that 
e.g. auto x = 1; would automatically assign the correct type where builtins were 
concerned, and I was pointing out that this wouldn't cover all builtins.  Though 
I guess auto x = 1UL; would work.

I wasn't asking how to create a size_t per se, which I do know how to do ... :-)

I once came an awful cropper due to lack of UL in an integer assignment.  I'd 
got a bit of C(++) code like this:

      size_t p = 1 << m;

where m was chosen such that p would be the largest power of 2 on the system 
that could (i) be multiplied by 2 without integer wraparound and (ii) was within 
the range of the uniform integer random number generator in use.

And that worked fine ... until I installed a 64-bit OS, and suddenly, all the 
numbers were coming out different.

They shouldn't have been, because the RNG in use was still based around int32_t 
and so the same constraints on m and p should have been in place ... and then I 
discovered what was happening: 1 << m; wasn't taking a size_t (unsigned long) 
and bitshifting it by m places, it was taking a regular int and bitshifting it 
by m places ... which given the value of m, was causing integer wraparound, the 
result of which was then converted to a size_t.

It just so happened that on 32-bit this was taking the value back to where it 
was supposed to be anyway.  But on 64-bit the wraparound made 1 << m a negative 
number which in turn corresponded to a far too _large_ value when converted into 
a size_t.

And so I learned that I had to use 1UL << m; instead ... :-P