assumeSafeAppend and purity
Jonathan M Davis
jmdavisProg at gmx.com
Mon Feb 6 21:35:14 PST 2012
On Monday, February 06, 2012 21:40:55 Steven Schveighoffer wrote:
> I thought of a better solution:
> pure T pureSafeShrink(T)(ref T arr, size_t maxLength)
> if(maxLength < arr.length)
> bool safeToShrink = (arr.capacity == arr.length);
> arr = arr[0..maxLength];
> if(safeToShrink) arr.assumeSafeAppend(); // must workaround purity
> return arr;
> This guarantees that you only affect data you were passed.
Does it really? What if I did this:
auto arr = new int(63);
auto saved = arr;
assert(arr.capacity == 63);
assert(saved.capacity == 63);
This happens to pass on my computer, though the exact value required for the
length will probably vary. So, a slice of the data which is now supposed to be
no longer part of any array still exists.
Also, given that allocating a new array and then immediately trying to shrink
it with pureSafeShrink will only use assumeSafeAppend if you just so happen to
have picked a length that lines up with the block size allocated makes it
pretty much useless IMHO. I'm only going to use assumeSafe append if I _know_
that it's safe. pureSafeShrink is therefore trying to protect me when I don't
need it and is ruining the guarantees that assumeSafeAppend gives me, since
it's only better than arr = arr[0 .. maxLength]; if the array just so happens
to have the same length as its capacity.
So, I don't think that this function really buys us anything. I'm inclined to
just make assumeSafeAppend pure.
- Jonathan M Davis
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