RE: Access to parameterized type
- From: William Adams <william_a_adams@xxxxxxx>
- To: "luajit@xxxxxxxxxxxxx" <luajit@xxxxxxxxxxxxx>
- Date: Sun, 2 Sep 2012 16:12:56 +0000
OK, maybe I spoke too soon.
Here's the full situation. I'm trying to construct a Vector class. I have
this function:
local function makevec(ct) local tp = ffi.typeof("struct { int n; int
Capacity; $ *Data; }", ct); tp = ffi.metatype(tp, kvec_mt); return
tp;end
some of the the metatype looks like this:
local kvec_mt = { __new = function(ct, capacity) local obj =
ffi.new(ct)
-- I would like to allocate obj.Data here --
obj.Data = ffi.new(ffi.typeof(self.Data[0]), capacity); -- but
it doesn't quite work out
return obj; end,}
Now, it's right here in the constructor of a new instance of an object of the
type where I have the problem, or rather in the allocation of the .Data
element, no matter where it occurs.
I can not say:
obj.Data = ffi.new(obj.Data[0], capacity)
because at this moment in time, obj.Data doesn't point to anything, so when I
try to dereference, I just get a crash.
If I try the following:
local newdata = ffi.new(ffi.typeof(self.Data[0]),
nelems);
I'll get: "cannot convert 'number' to 'struct 102 &'",
which makes sense if it is trying to use the number (nelems) as an initializer
of a single instance of the structure, rather than as a count of the number of
elements to allocate. Kind of confusing as both are optional parameters.
The test case looks like this:
function test_pointvec() local point3 = ffi.typeof("struct {float x;
float y; float z;}"); local Point3Vector = makevec(point3);
local vertices = Point3Vector(); for i=1,10000 do
vertices:Push(point3(i,i*2, i*3)); endend
Might the answer here be to switch to a structure that looks more like this:
local function makevec(ct) local tp = ffi.typeof("struct { int n; int
Capacity; $ Data[?]; }", ct); tp = ffi.metatype(tp, kvec_mt);
return tp;end
Where I would use a VLA instead of a pointer?
Or should the pointer case work as well?
-- William=============================== - Shaping clay is easier than digging
it out of the ground.
From: william_a_adams@xxxxxxx
Subject: RE: Access to parameterized type
Date: Sat, 1 Sep 2012 16:19:22 -0700
To: luajit@xxxxxxxxxxxxx
Perfect. Thanks.
From:
Mike Pall
Sent:
9/1/2012 3:15 PM
To:
luajit@xxxxxxxxxxxxx
Subject:
Re: Access to parameterized type
William Adams wrote:
> I have need to access the type of the parameter used to construct a
> parameterized structure:
> local Vector = ffi.typeof("struct { int n; int Capacity; $ *Data; }",
> ct);
> Is there any way to get at what 'ct' was from within a metamethod?
> I can find out how large it is by doing ffi.sizeof(instance.Data[0])
> but, all I get out of ffi.typeof(instance.Data) is "struct 102".
> Alternative, could I use "struct 102" in any meaningful way, like
> ffi.new("struct 102", 100);
ffi.new() takes a cdecl (a string), a ctype object or a cdata
object. So just use ffi.new(instance.Data[0], ...).
--Mike
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