def gurobi_objective
      dblptr = FFI::MemoryPointer.new :pointer
      ret = Gurobi.GRBgetdblattr @ptr, Gurobi::GRB_DBL_ATTR_OBJVAL, dblptr
      fail if ret != 0
      dblptr.read_double
    end
def gurobi_objective
      dblptr = FFI::MemoryPointer.new :pointer
      ret = Gurobi.GRBgetdblattr @ptr, Gurobi::GRB_DBL_ATTR_OBJVAL, dblptr
      fail if ret != 0
      dblptr.read_double
    end
The value of the objective function