Did not generate C/C++ expression; you may not need `cexpr` here

[mantielero]

I was playing with OpenSceneGraph. I wrote the following code:

{.passL: "-losg -losgViewer", 
  passC:"-I/usr/include/osg" }
import cinterop

csource "Vec3f":
  cnamespace osg:  
    type
      Vec3f* = object of CClass


proc newVec3[X,Y,Z:SomeNumber](x:X, y:Y, z:Z):Vec3f =
  Vec3f.init(x.cfloat, y.cfloat, z.cfloat)


proc x*(this:Vec3f):cfloat =
  cexpr[cfloat]^this.x()

var p = newVec3(1, 2.0, 3.0)

echo p.x()

This code compiles, but I get a warning:

Warning: did not generate C/C++ expression; you may not need `cexpr` here

The code compiles, but produces a runtime error:

Error: call depth limit reached in a debug build (2000 function calls). You can change it with -d:nimCallDepthLimit=<int> but really try to avoid deep recursions instead.
Error: execution of an external program failed: '/home/jose/src/osg.nim/prueba2 '

The source code for the header is: Vec3f

[Ryan Blonna]

That looks like an infinite recursion:

proc x*(this:Vec3f):cfloat =
  cexpr[cfloat]^this.x()

cexpr will pick up a symbol if it exists and will print that warning; it will generate the symbol if it doesn't.

If you want to wrap cinterop with your own procs, you need to use different names to avoid recursion. You can either rename your x function, or use normal importcpp instead of cexpr:

proc x*(this:Vec3f):cfloat {.importcpp:"(#.$1())".}

Check out the Nim manual on importcpp, it's pretty cool.

[mantielero]

Understood. Thanks a lot.