Question

Can anybody give me a real world example of a case when dynamic_cast is needed and can’t be worked around at all? Examples I can think of can generally be worked around with double dispatch.

If the constraint is too strong, an example where dynamic_cast is generally the way to go would also be nice.

I’d like to see real examples instead of “it’s normally used to cast between types up and down a type tree”.

Answer 1

Double dispatch requires that the types that are interacting have intimate knowledge of each other’s innards, as it requires that one class call methods on the other class. dynamic_cast works when you cannot modify the innards of a class, or do not wish to break encapsulation of the classes in question.

That is, double dispatch is invasive on the classes involved, while dynamic_cast works without knowledge of the cast in the classes.

You can also use dynamic_cast if you don’t know the target method overload which will be invoked. For an example, see this question I posted yesterday.

Finally, double dispatch does not come without it’s own headaches

The base class Shape must know about all the derived classes, resulting in circular dependencies. If you derive a new class from Shape (say Triangle), you must update the interface of Shape and the interface/implementation of all the other derived classes. In some cases this is not even an option: you may not have the source code for Shape, or not be willing or permitted to modify it.