Re: Casual object churn and efficiency.

[Paul Du Bois <paul.dubois@...>]

If you do this, you have to be very careful to avoid problems caused
by reference semantics.

vPos = vPos + vVel * dt   -- vPos is a reference to +'s return value
vVel = vVel + vAccel * dt   -- vPos has just been mut(il)ated

Thus, all users need to do things like:

vPos = vec3(vPos + vVel * dt)   -- or
vPos:set(vPos + vVel * dt)

maybe there is a metamethod for = that lets you try to do the right
thing; I don't know.

At this point, it is almost better to use explicit 3-arg function calls:

vPos = VecAdd(vPos, vPos, VecMul(nil, vVel, dt))

p

On Mon, 28 Jun 2004 15:43:18 -0400, Marius Gheorghe
<mgheorghe@cabletest.com> wrote:
> 
> Jamie Webb Wrote:
> > As it is, it wouldn't work if you used the same function more
> > than once in an
> > expression.
> 
> I actually beg to differ (provided we are not dealing with preemptive
> multithreading). Unfortunately, my originally proposed code had an obvious
> typo, please find the correction below:
> 
> local vector3_add_rtn = {}
> local function vector3_add(self, o)
>   for i=1,3 do
>      vector3_add_rtn[i] = self[i] + o[i] -- Line in question
>   end
>   return vector3_add_rtn; -- Correction performed here
> end
> 
> The idea is that only two vectors can be added at any given time even in
> complex expressions. In cases where we have multiple vectors being added
> (i.e. v1 + v2 + v3 etc.) we will have intermediate results stored in the
> upvalue. The next invocation of the __add metamethod will have this
> temporary result as one of the arguments. Therefore the assignment marked
> with the comment 'Line in question' becomes effectively either
> 
> vector3_add_rtn[i] = vector3_add_rtn[i] + o[i]
> 
> or
> 
> vector3_add_rtn[i] = self[i] + vector3_add_rtn[i]
> 
> I can't see anything wrong with that in a single threaded application (using
> Lua coroutines should still be OK).
> 
> Marius
> 
>