Trouble binding a class - should be simple

Hello,

I am trying to expose part of the btVector3 class from BulletPhysics to Lua:

http://www.bulletphysics.com/Bullet/BulletFull/classbtVector3.html

My first attempt:

// C++
   void RegisterPhysicsTypesWithLua( lua_State* lua )
   {
     module( lua )
     [
       class_<btVector3>( "btVector3" )
       .def( "getX",&btVector3::getX )
       .def( "getY",&btVector3::getY )
       .def( "getZ",&btVector3::getZ )
       .def( "setX",&btVector3::setX )
       .def( "setY",&btVector3::setY )
       .def( "setZ",&btVector3::setZ )
     ];
   }

--Lua
print("Testing btVector3 in lua")
v = btVector3()
print("btVector3 created")

print( "x = " .. v.getX() )  -- should print x = 0.0
v.setX(99.9)
print( "x = " .. v.getX() )  -- should print x = 99.9

------------------------------------------
This first attempt produces the error:
"Attempt to call a nil value"


So I added a constructor and tried again:

// C++
   void RegisterPhysicsTypesWithLua( lua_State* lua )
   {
     module( lua )
     [
       class_<btVector3>( "btVector3" )
       .def( constructor<  float, float, float>() )
       .def( "getX",&btVector3::getX )
       .def( "getY",&btVector3::getY )
       .def( "getZ",&btVector3::getZ )
       .def( "setX",&btVector3::setX )
       .def( "setY",&btVector3::setY )
       .def( "setZ",&btVector3::setZ )
     ];
   }

--Lua
print("Testing btVector3 in lua")
v = btVector3(1.0,2.0,3.0)
print("btVector3 created")

print( "x = " .. v.getX() )  -- should print x = 1.0
v.setX(99.9)
print( "x = " .. v.getX() )  -- should print x = 99.9

------------------------------------------
This second attempt produces the error:
"No such operator defined"


I have double checked that I am definately calling the
RegisterPhysicsTypesWithLua with the correct LuaState.

I expected this to be reasonably trivial as I have been exposing my own
classes with ease. This is however, the first time I have tried to
expose a 3rd party class. Am I missing something obvious?

Thanks very much

Simon



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