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Libs that return a pointer

BlitzMax Forums/BlitzMax Programming/Libs that return a pointer

JoshK(Posted 2006) [#1]
This ODE function returns a pointer to a 3D vector:
const dReal * dBodyGetPosition (dBodyID);

How do you retrieve the data once you get the byte ptr? I just need to read the memory at the returned value, the returned value + 4, and the returned value + 8.

Here's my code:

position:Byte Ptr=dBodyGetPosition(box)

Now what?


H&K(Posted 2006) [#2]
OK. Guess yes

If all object names are just pointers to the object, cannot you just allocate the returned pointer to an object of the type the pointer is pointing to?


JoshK(Posted 2006) [#3]
How do I do that?

I tried setting up a vector type and making the function return that. It actually worked, though mem offsets are off:

This code displayed "3", meaning that there are two hidden "fields" from the type pointer. I think this is probably a terribly unstable method:
dBodySetPosition box,1,2,3

Type TVector3
	Field x#,y#,z#
EndType

Local position:tvector3'Byte Ptr
position=dBodyGetPosition(box)
Notify position.x


-- EDIT --

This seems to work:
Type TVector3
	Field x#,y#,z#
EndType

world=dWorldCreate()
geom=dCreateBox(0,100,100,100)
box=dbodycreate(world)
dgeomsetbody geom,body
dBodySetPosition box,1,2,3

Local hposition:Byte Ptr
hposition=dBodyGetPosition(box)
Local position:tvector3
(Byte Ptr Ptr Varptr position) [ 0 ]=hposition-8



Floyd(Posted 2006) [#4]
I was going to suggest this:

Local fp:Float Ptr

fp = Float Ptr dBodyGetPosition(box)

' now x is fp[0]   y is fp[1]   z is fp[2]

Totally untested, of course.


H&K(Posted 2006) [#5]
It seem a bit strange to me, are you sure that its a pointer to JUST a vector? It doesnt contain its magnitude or the like? And I aways thought that the pointers pointed to the begining of the object, not the end of it.


ImaginaryHuman(Posted 2006) [#6]
If it's meant to be returning 32-bit floats then just turn the byte pointer into a float pointer, as Floyd suggested.


Dreamora(Posted 2006) [#7]
If it returns a pointer to a struct of numerics, use arrays, thats the far easier way.
If something for example returns a float* vec[3] then define the return as float[]

Your Ptr Ptr hack might look "usefull" but it is highly critical and can break at any point.


JoshK(Posted 2006) [#8]
Nevermind, it works.