Avoid down-casting value along parallel hierarchies

Question

I have two class hierarchies:

BaseUnit <- S1Unit , BaseUnit <- S2Unit, ..S3Unit, S4Unit

and

Base <- S1 , Base <- S2

In Base I stored the unit's value.

class Base {
  protected: BaseUnit unit; // actually this is an upcast of S1Unit, S2Unit
  // of course I do several things with unit on methods of Base 
}

which is based on S1Unit, S2Unit ..

S1::S1(S1Unit s1unit) : unit(s1unit) { .. }

However, if I now would like to return the specialized unit in S1, how would I best do this?

S1Unit S1::getUnit() const { return this->unit; /* how to cast down? */ }

I could implement S1Unit a constructor of BaseUnit S1Unit::S1Unit(BaseUnit) which works fine, but this kind of downcast copy constructor somehow is not very nice. All answers I have checked show examples using pointers.

What would be a better solution? Any alternatives?

`BaseUnit <- S1Unit , BaseUnit <- S2Unit, ..S3Unit, S4Unit` - I cannot parse this syntax. — Joseph Mansfield, Mar 25 '13 at 17:55
The member `Base::unit` is nothing but a `BaseUnit`. You can't cast it down to anything because it's not anything else. If you want polymorphism, you need to use pointers or references. — Joseph Mansfield, Mar 25 '13 at 17:57
And what about the rest of it? They're all inherited from `BaseUnit`? — Joseph Mansfield, Mar 25 '13 at 18:01
Maybe you should explain this again. Just start from the beginning and write it using other words and flow. :) I didn't understand it. — , Mar 25 '13 at 18:06

score 1 · Accepted Answer · answered Mar 25 '13 at 18:04

1

The functionality that differs between your classes should be exposed via virtual functions.

Then, you don't need to cast down the object and you can just return unit. The member functions called will be the correct ones for the objects you've created.

answered Mar 25 '13 at 18:04

Drew Dormann

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Avoid down-casting value along parallel hierarchies

1 Answers1