Which one is the way to go accroding to you opinion?
The approach that I posted the code example for or the approach with global variables?
I am asking because I am unsure whether global variables will really make it at all. I mean I am not familiar to the internals of NLopt, but I see a potential problem here if I decide to call SomeComplexCalculations( x) in myfunc only since this implies that myconstraint is just called with the same vector x until myfunc is called again. However if myconstrained was called more than once with two different x during the optimiziation process and myfunc was only called after a suitable x that fullfills the condition was found, I would get into trouble.