What are some 'good use' examples of dynamic casting?

Question

We often hear/read that one should avoid dynamic casting. I was wondering what would be \'good use\' examples of it, according to you?

Edit:

Yes, I\'m aware

Answer 1

Here's something I do often, it's not pretty, but it's simple and useful.

I often work with template containers that implement an interface, imagine something like

template
class MyVector : public ContainerInterface
...

Where ContainerInterface has basic useful stuff, but that's all. If I want a specific algorithm on vectors of integers without exposing my template implementation, it is useful to accept the interface objects and dynamic_cast it down to MyVector in the implementation. Example:

// function prototype (public API, in the header file)
void ProcessVector( ContainerInterface& vecIfce );

// function implementation (private, in the .cpp file)
void ProcessVector( ContainerInterface& vecIfce)
{
    MyVector& vecInt = dynamic_cast >(vecIfce); 
    // the cast throws bad_cast in case of error but you could use a
    // more complex method to choose which low-level implementation
    // to use, basically rolling by hand your own polymorphism.

    // Process a vector of integers
    ...
}

I could add a Process() method to the ContainerInterface that would be polymorphically resolved, it would be a nicer OOP method, but I sometimes prefer to do it this way. When you have simple containers, a lot of algorithms and you want to keep your implementation hidden, dynamic_cast offers an easy and ugly solution.

You could also look at double-dispatch techniques.

HTH