Creating dynamic arrays of known size

[Jonathan M Davis]

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;

Now, you couldn't use Unqual, since that's in Phobos, but you could copy it 
from Phobos and have a version internal to the AA stuff in druntime. However, 
the first choice is probably better, since it doesn't require any casting.

- Jonathan M Davis