you are right, the code doesn't look any better, but this is the error message I'm getting with the current version. Do you have a better idea?

In file included from vpp.cpp:37:
../ql/experimental/processes/gemanroncoroniprocess.hpp:63:14: warning: 
      'QuantLib::GemanRoncoroniProcess::evolve' hides overloaded virtual function [-Woverloaded-virtual]
        Real evolve(Time t0, Real x0, Time dt, Real dw, const Array& du) const;
             ^
../ql/stochasticprocess.hpp:240:27: note: hidden overloaded virtual function
      'QuantLib::StochasticProcess1D::evolve' declared here
        Disposable<Array> evolve(Time t0, const Array& x0,