Re: Feature request: userdata slice

[Daurnimator <quae@...>]

On 19 August 2015 at 14:07, 云风 Cloud Wu <cloudwu@gmail.com> wrote:
>
>
> Daurnimator <quae@daurnimator.com>于2015年8月19日周三 上午11:50写道：
>>
>>
>> You're saying the slice offset would be stored in the userdata itself?
>> This would imply that setting a slice would mutate the userdata object?
>>
>> If not, your slice still needs to create an extra object,
>> in which case, how is this better than the uservalue option?
>> i.e.
>>
>> void makeslice(lua_State *L, int idx, ssize_t offset) {
>>     idx = lua_absindex(L, idx);
>>     ssize_t* slice = lua_newuserdata(L, sizeof(ssize_t));
>>     *slice = offset;
>>     lua_pushvalue(L, idx);
>>     lua_setuservalue(L, -2);
>> }
>> void* getslice(lua_State *L, int idx) {
>>     ssize_t offset = *(ssize_t*)lua_touserdata(L, idx);
>>     void* udata = lua_touserdata(L, lua_getuservalue(L, idx));
>>     return udata+offset;
>> }
>>
>
> I don't want to create an extra object, because it's more expensive. and we
> should cache all the extra objects because the extra object with the same
> slice offset should not be create more than once, if we use them as a table
> key .
>
>>
>> I'm not sure how this would work?
>> You need 64bits to store a userdata address on a 64 bit architecture.
>
>
> We can create the array in global state to store all the userdata pointer,
> and use the index (handle) in lua value, 32bit index is enough. and
> lua_touserdata can be also return the pointer by lookup the array with
> index.

Having an extra layer of indirection via some global index of
userdatas seems a terrible idea.
It would have to be paid by *every* userdata access.

>> > Storing C/C++ objects in GUI or 3d engines could be more simple :
>> > OBJECT.X
>> > and OBJECT.X.Y can be the same userdata with OBJECT without creating new
>> > userdata, only the slice is different.
>>
>> But you would still need a different object.
>>
>
> We don't need a different object, an associated integer  is enough. In the
> metamethod, we can get the associated integer (It can be offset or a index
> number) first, and then convert it to the sub object's real address with a
> map (in C) in root object.
>
> This can be reduce the number of lua userdata, we don't need create large
> number of userdata for C objects , many sub objects may only access once
> (especially in 3d engine bindings), but we should also create an userdata
> for them now, and should do more for collecting them correctly.

You do need a separate object, or the objects are the same....
I doubt you want rawequal(object.foo1, object.foo2) == true, or
rawequal(object, object.foo1) == true.


-1 on this whole idea.