Very fast 3D distance check?

Question

Is there a way to do a quick and dirty 3D distance check where the results are rough, but it is very very fast? I need to do depth sorting. I use STL sort like this

Answer 1

If you worry about performance, you should also take care of the way you send your arguments:

float Get3dDistance( Vec3 c1, Vec3 c2 );

implies two copies of Vec3 structure. Use references instead:

float Get3dDistance( Vec3 const & c1, Vec3 const & c2 );