埃菲尔的多重继承

Question

Hoi,The multiple inheritance in eiffel really confused me ,can anybody tell me what class French_Us_Driver inherit from all its parent-class.

 class Driver

   feature(ANY)

      violation  

    end
 end --end Driver

 class French_Driver

    inherite

       Driver

     rename

      violation as French_violatin

  end

  end -- end French_Driver

  class US_Driver

   inherit

    Driver

   rename 

    violation as Us_violation
  end

 end --end Us_Driver

 class French_Us__Driver

   inherit

     French_Driver

     Us_Driver
   end

 end --French_Us_Driver

Now has French_Us_Driver features : Us_violation , French_violation and violation

or : Us_violation , French_violation

Thanks

Answer 1

The class French_Driver only renames a feature violation , it does not add any new one. So, we can call only French_violation on it and not violation (there is no such name anymore in this class because of renaming). The same is true for the class Us_Driver .

As a result the class French_Us_Driver inherits a feature French_violation from French_Driver and a feature Us_violation from Us_Driver , so there are two features in total: French_violation and Us_violation .

The story would stop here if there were no common ancestor where the feature violation originates from. Because the class French_Us_Driver now has 2 versions of the feature violation inherited from the class Driver (one version is named French_violation and the other one - Us_violation ), it's not clear which one should be used when calling a feature violation in the class Driver when the type of the object is French_Us_Driver . The conflict should be resolved by adding a select clause to one of the parent clauses, for example:

class French_Us_Driver inherit
   French_Driver
      select French_violation end
   Us_Driver
end

Then, when a feature violation is called from the class Driver on an object of type French_Us_Driver , the feature French_violation will actually be called.

Finally, it's possible to merge two versions of the feature into one by giving them the same name which does not necessary match the name of the origin (the code below omits the other details of feature redeclaration and assumes for simplicity that all the features are deferred):

class French_Us_Driver inherit
   French_Driver
      rename French_violation as French_Us_violation end
   Us_Driver
      rename Us_violation as French_Us_violation end
end

Answer 2

Don't just select at random. Here is a considered use of select that may be what you want. (I will up vote the other answer as is correct except gives no help on how to use select.)

class 
    French_Us_Driver 
inherit
   Driver 
      select 
          violation 
      end
   French_Driver end
   Us_Driver end
feature
    violation
        inspect country
            when france then French_violation
            when usa    then Us_violation
        end --inspect
    end --violation
end --class