new T[size] vs .reserve - alloca

[monarch_dodra]

On Tuesday, 5 February 2013 at 20:47:46 UTC, Nick Treleaven wrote:
> On 05/02/2013 16:47, monarch_dodra wrote:
>> On Tuesday, 5 February 2013 at 16:37:41 UTC, Nick Treleaven 
>> wrote:
>>> I've just realized this doesn't work for variable-length 
>>> allocation:
>>>
>>> T[] stack(T)(size_t N, void* m = alloca(T.sizeof * N))
>>>
>>> Error: undefined identifier N, did you mean alias T?
>>>
>>> N is not visible in the caller's scope.
>>
>> It does, just alias it.
>>
>> //----
>> import std.stdio;
>> import core.stdc.stdlib:alloca;
>>
>> T* stack(T)(void* m = alloca(T.sizeof))
>> {
>>     return cast(T*)m;
>> }
>> T[] stack(T, alias N)(void* m = alloca(T.sizeof * N))
>> {
>>     return (cast(T*)m)[0 .. N];
>> }
>
> This works if you know N at compile-time. But there doesn't 
> seem to be a way to wrap alloca to accept a runtime-only value, 
> e.g.:
>
> // allocate as many ints as command-line parameters
> int[] arr = stack!int(args.length);

I don't have access to my compiler, but that *should*work. Did 
you try it? BTW, the syntax would be:

int[] arr = stack!(int, args.length)();