On Fri, 09 Mar 2012 01:59:34 -0500, Jonathan M Davis <jmdavisp...@gmx.com>
wrote:
On Thursday, March 08, 2012 22:05:57 H. S. Teoh wrote:
On Thu, Mar 08, 2012 at 09:13:13PM -0800, H. S. Teoh wrote:
> So, I'm plodding along with my AA implementation that *hopefully* will
> eventually reach the point where it's usable enough to be dropped into
> druntime. I'm writing .keys and .values right now, and wondering
what's
> the best way to construct the returned array.
>
> Obviously, using =~ repeatedly is a bad idea, since we already know
> the resulting array size. Would this be the best way to do it?
>
> Key[] keys;
> keys.length = num_keys;
> for (size_t i=0; i < num_keys; i++) {
>
> keys[i] = ...;
>
> }
>
> Looking at aaA.d, I see that _aaKeys calls gc_malloc directly and sets
> BlkAttr.NO_SCAN. Should I just copy this code?
[...]
Another problem: if Key is a const/immutable type, then keys[i] is
immutable, so the keys can't be copied into the resulting array. I tried
inout but it doesn't seem to be good enough, because individual array
elements need to be assigned to, which violates const/immutable, even
though we're really just copying const/immutable data here.
I see two options. Allocate the entire array, set the length to 0, and
use
assumeSafeAppend (or maybe the function that it uses, since
assumeSafeAppend
is in _object.d) so that appending doesn't cause reallocations, or
create the
array as mutable and then cast it to the appropriate type.
1.
auto keys = new Key[](num_keys);
keys.length = 0;
assumeSafeAppend(keys);
for(i; 0 .. num_keys)
keys ~= ...;
2.
auto keys = new (Unqual!Key)[](num_keys);
foreach(ref key; keys)
key = ...;
auto actualKeys = cast(Key[])keys;
Use the second method. This is low-level runtime code, it should be as
fast as possible. Casting is OK as long as it's justified.
-Steve
